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The General Switches and Parameters

The PYPARS common block contains the status code and parameters that regulate the performance of the program. All of them are provided with sensible default values, so that a novice user can neglect them, and only gradually explore the full range of possibilities. Some of the switches and parameters in PYPARS will be described later, in the shower and beam-remnants sections.


\fbox{\texttt{COMMON/PYPARS/MSTP(200),PARP(200),MSTI(200),PARI(200)}}

Purpose:
to give access to status code and parameters that regulate the performance of the program. If the default values, denoted below by (D = ...), are not satisfactory, they must in general be changed before the PYINIT call. Exceptions, i.e. variables that can be changed for each new event, are denoted by (C).

MSTP(1) :
(D = 3) maximum number of generations. Automatically set $\leq 4$.

MSTP(2) :
(D = 1) calculation of $\alpha_{\mathrm{s}}$ at hard interaction, in the routine PYALPS.
= 0 :
$\alpha_{\mathrm{s}}$ is fixed at value PARU(111).
= 1 :
first-order running $\alpha_{\mathrm{s}}$.
= 2 :
second-order running $\alpha_{\mathrm{s}}$.

MSTP(3) :
(D = 2) selection of $\Lambda$ value in $\alpha_{\mathrm{s}}$ for MSTP(2) $\geq 1$.
= 1 :
$\Lambda$ is given by PARP(1) for hard interactions, by PARP(61) for space-like showers, by PARP(72) for time-like showers not from a resonance decay, and by PARJ(81) for time-like ones from a resonance decay (including e.g. $\gamma/\mathrm{Z}^0 \to \mathrm{q}\overline{\mathrm{q}}$ decays, i.e. conventional $\mathrm{e}^+\mathrm{e}^-$ physics). This $\Lambda$ is assumed to be valid for 5 flavours; for the hard interaction the number of flavours assumed can be changed by MSTU(112).
= 2 :
$\Lambda$ value is chosen according to the parton-distribution-function parameterizations. The choice is always based on the proton parton-distribution set selected, i.e. is unaffected by pion and photon parton-distribution selection. All the $\Lambda$ values are assumed to refer to 4 flavours, and MSTU(112) is set accordingly. This $\Lambda$ value is used both for the hard scattering and the initial- and final-state radiation. The ambiguity in the choice of the $Q^2$ argument still remains (see MSTP(32), MSTP(64) and MSTJ(44)). This $\Lambda$ value is used also for MSTP(57) = 0, but the sensible choice here would be to use MSTP(2) = 0 and have no initial- or final-state radiation. This option does not change the PARJ(81) value of time-like parton showers in resonance decays, so that LEP experience on this specific parameter is not overwritten unwittingly. Therefore PARJ(81) can be updated completely independently.
= 3 :
as = 2, except that here also PARJ(81) is overwritten in accordance with the $\Lambda$ value of the proton parton-distribution-function set.

MSTP(4) :
(D = 0) treatment of the Higgs sector, predominantly the neutral one.
= 0 :
the $\mathrm{h}^0$ is given the Standard Model Higgs couplings, while $\H ^0$ and $\mathrm{A}^0$ couplings should be set by you in PARU(171) - PARU(175) and PARU(181) - PARU(185), respectively.
= 1 :
you should set couplings for all three Higgs bosons, for the $\mathrm{h}^0$ in PARU(161) - PARU(165), and for the $\H ^0$ and $\mathrm{A}^0$ as above.
= 2 :
the mass of $\mathrm{h}^0$ in PMAS(25,1) and the $\tan\beta$ value in PARU(141) are used to derive $\H ^0$, $\mathrm{A}^0$ and $\H ^{\pm}$ masses, and $\mathrm{h}^0$, $\H ^0$, $\mathrm{A}^0$ and $\H ^{\pm}$ couplings, using the relations of the Minimal Supersymmetric extension of the Standard Model at Born level [Gun90]. Existing masses and couplings are overwritten by the derived values. See section [*] for discussion on parameter constraints.
= 3:
as = 2, but using relations at the one-loop level. This option is not yet implemented as such. However, if you initialize the SUSY machinery with IMSS(1) = 1, then the SUSY parameters will be used to calculate also Higgs masses and couplings. These are stored in the appropriate slots, and the value of MSTP(4) is overwritten to 1.

MSTP(7) :
(D = 0) choice of heavy flavour in subprocesses 81-85. Does not apply for MSEL = 4 - 8, where the MSEL value always takes precedence.
= 0 :
for processes 81-84 (85) the `heaviest' flavour allowed for gluon (photon) splitting into a quark-antiquark (fermion-antifermion) pair, as set in the MDME array. Note that `heavy' is defined as the one with largest KF code, so that leptons take precedence if they are allowed.
= 1 - 8 :
pick this particular quark flavour; e.g., MSTP(7) = 6 means that top will be produced.
= 11 - 18 :
pick this particular lepton flavour. Note that neutrinos are not possible, i.e. only 11, 13, 15 and 17 are meaningful alternatives. Lepton pair production can only occur in process 85, so if any of the other processes have been switched on they are generated with the same flavour as would be obtained in the option MSTP(7) = 0.

MSTP(8) :
(D = 0) choice of electroweak parameters to use in the decay widths of resonances ($\mathrm{W}$, $\mathrm{Z}$, $\mathrm{h}$, ...) and cross sections (production of $\mathrm{W}$'s, $\mathrm{Z}$'s, $\mathrm{h}$'s, ...).
= 0 :
everything is expressed in terms of a running $\alpha_{\mathrm{em}}(Q^2)$ and a fixed $\sin^2 \! \theta_W $, i.e. $G_{\mathrm{F}}$ is nowhere used.
= 1 :
a replacement is made according to $\alpha_{\mathrm{em}}(Q^2) \to \sqrt{2} G_{\mathrm{F}} m_{\mathrm{W}}^2 \sin^2 \! \theta_W / \pi$ in all widths and cross sections. If $G_{\mathrm{F}}$ and $m_{\mathrm{Z}}$ are considered as given, this means that $\sin^2 \! \theta_W $ and $m_{\mathrm{W}}$ are the only free electroweak parameter.
= 2 :
a replacement is made as for = 1, but additionally $\sin^2 \! \theta_W $ is constrained by the relation $\sin^2 \! \theta_W = 1 - m_{\mathrm{W}}^2/m_{\mathrm{Z}}^2$. This means that $m_{\mathrm{W}}$ remains as a free parameter, but that the $\sin^2 \! \theta_W $ value in PARU(102) is never used, except in the vector couplings in the combination $v = a - 4 \sin^2 \! \theta_W e$. This latter degree of freedom enters e.g. for forward-backward asymmetries in $\mathrm{Z}^0$ decays.
Note:
this option does not affect the emission of real photons in the initial and final state, where $\alpha_{\mathrm{em}}$ is always used. However, it does affect also purely electromagnetic hard processes, such as $\mathrm{q}\overline{\mathrm{q}}\to \gamma \gamma$.

MSTP(9) :
(D = 0) inclusion of top (and fourth generation) as allowed remnant flavour $\mathrm{q}'$ in processes that involve $\mathrm{q}\to \mathrm{q}' + \mathrm{W}$ branchings as part of the overall process, and where the matrix elements have been calculated under the assumption that $\mathrm{q}'$ is massless.
= 0 :
no.
= 1 :
yes, but it is possible, as before, to switch off individual channels by the setting of MDME switches. Mass effects are taken into account, in a crude fashion, by rejecting events where kinematics becomes inconsistent when the $\mathrm{q}'$ mass is included.

MSTP(11) :
(D = 1) use of electron parton distribution in $\mathrm{e}^+\mathrm{e}^-$ and $\mathrm{e}\mathrm{p}$ interactions.
= 0 :
no, i.e. electron carries the whole beam energy.
= 1 :
yes, i.e. electron carries only a fraction of beam energy in agreement with next-to-leading electron parton-distribution function, thereby including the effects of initial-state bremsstrahlung.

MSTP(12) :
(D = 0) use of $\mathrm{e}^-$ (`sea', i.e. from $\mathrm{e}\to \gamma \to \mathrm{e}$), $\mathrm{e}^+$, quark and gluon distribution functions inside an electron.
= 0 :
off.
= 1 :
on, provided that MSTP(11) $\geq 1$. Quark and gluon distributions are obtained by numerical convolution of the photon content inside an electron (as given by the bremsstrahlung spectrum of MSTP(11) = 1) with the quark and gluon content inside a photon. The required numerical precision is set by PARP(14). Since the need for numerical integration makes this option somewhat more time-consuming than ordinary parton-distribution evaluation, one should only use it when studying processes where it is needed.
Note:
for all traditional photoproduction/DIS physics this option is superseded by the 'gamma/lepton' option for PYINIT calls, but can still be of use for some less standard processes.

MSTP(13) :
(D = 1) choice of $Q^2$ range over which electrons are assumed to radiate photons; affects normalization of $\mathrm{e}^-$ (sea), $\mathrm{e}^+$, $\gamma$, quark and gluon distributions inside an electron for MSTP(12) = 1.
= 1 :
range set by $Q^2$ argument of parton-distribution-function call, i.e. by $Q^2$ scale of the hard interaction. Therefore parton distributions are proportional to $\ln(Q^2/m_e^2)$.
= 2 :
range set by the user-determined $Q_{\mathrm{max}}^2$, given in PARP(13). Parton distributions are assumed to be proportional to $\ln((Q_{\mathrm{max}}^2/m_e^2)(1-x)/x^2)$. This is normally most appropriate for photoproduction, where the electron is supposed to go undetected, i.e. scatter less than $Q_{\mathrm{max}}^2$.
Note:
the choice of effective range is especially touchy for the quark and gluon distributions. An (almost) on-the-mass-shell photon has a VMD piece that dies away for a virtual photon. A simple convolution of distribution functions does not take this into account properly. Therefore the contribution from $Q$ values above the $\rho$ mass should be suppressed. A choice of $Q_{\mathrm{max}} \approx 1$ GeV is then appropriate for a photoproduction limit description of physics. See also note for MSTP(12).

MSTP(14) :
(D = 30) structure of incoming photon beam or target. Historically, numbers up to 10 were set up for real photons, and subsequent ones have been added also to allow for virtual photon beams. The reason is that the earlier options specify e.g. direct$\times$VMD, summing over the possibilities of which photon is direct and which VMD. This is allowed when the situation is symmetric, i.e. for two incoming real photons, but not if one is virtual. Some of the new options agree with previous ones, but are included to allow a more consistent pattern. Further options above 25 have been added also to include DIS processes.
= 0 :
a photon is assumed to be point-like (a direct photon), i.e. can only interact in processes which explicitly contain the incoming photon, such as $\mathrm{f}_i \gamma \to \mathrm{f}_i \mathrm{g}$ for $\gamma\mathrm{p}$ interactions. In $\gamma\gamma$ interactions both photons are direct, i.e the main process is $\gamma \gamma \to \mathrm{f}_i \overline{\mathrm{f}}_i$.
= 1 :
a photon is assumed to be resolved, i.e. can only interact through its constituent quarks and gluons, giving either high-$p_{\perp}$ parton-parton scatterings or low-$p_{\perp}$ events. Hard processes are calculated with the use of the full photon parton distributions. In $\gamma\gamma$ interactions both photons are resolved.
= 2 :
a photon is assumed resolved, but only the VMD piece is included in the parton distributions, which therefore mainly are scaled-down versions of the $\rho^0 / \pi^0$ ones. Both high-$p_{\perp}$ parton-parton scatterings and low-$p_{\perp}$ events are allowed. In $\gamma\gamma$ interactions both photons are VMD-like.
= 3 :
a photon is assumed resolved, but only the anomalous piece of the photon parton distributions is included. (This event class is called either anomalous or GVMD; we will use both interchangeably, though the former is more relevant for high-$p_{\perp}$ phenomena and the latter for low-$p_{\perp}$ ones.) In $\gamma\gamma$ interactions both photons are anomalous.
= 4 :
in $\gamma\gamma$ interactions one photon is direct and the other resolved. A typical process is thus $\mathrm{f}_i \gamma \to \mathrm{f}_i \mathrm{g}$. Hard processes are calculated with the use of the full photon parton distributions for the resolved photon. Both possibilities of which photon is direct are included, in event topologies and in cross sections. This option cannot be used in configurations with only one incoming photon.
= 5 :
in $\gamma\gamma$ interactions one photon is direct and the other VMD-like. Both possibilities of which photon is direct are included, in event topologies and in cross sections. This option cannot be used in configurations with only one incoming photon.
= 6 :
in $\gamma\gamma$ interactions one photon is direct and the other anomalous. Both possibilities of which photon is direct are included, in event topologies and in cross sections. This option cannot be used in configurations with only one incoming photon.
= 7 :
in $\gamma\gamma$ interactions one photon is VMD-like and the other anomalous. Only high-$p_{\perp}$ parton-parton scatterings are allowed. Both possibilities of which photon is VMD-like are included, in event topologies and in cross sections. This option cannot be used in configurations with only one incoming photon.
= 10 :
the VMD, direct and anomalous/GVMD components of the photon are automatically mixed. For $\gamma\mathrm{p}$ interactions, this means an automatic mixture of the three classes 0, 2 and 3 above [Sch93,Sch93a], for $\gamma\gamma$ ones a mixture of the six classes 0, 2, 3, 5, 6 and 7 above [Sch94a]. Various restrictions exist for this option, as discussed in section [*].
= 11 :
direct$\times$direct (see note 5); intended for virtual photons.
= 12 :
direct$\times$VMD (i.e. first photon direct, second VMD); intended for virtual photons.
= 13 :
direct$\times$anomalous; intended for virtual photons.
= 14 :
VMD$\times$direct; intended for virtual photons.
= 15 :
VMD$\times$VMD; intended for virtual photons.
= 16 :
VMD$\times$anomalous; intended for virtual photons.
= 17 :
anomalous$\times$direct; intended for virtual photons.
= 18 :
anomalous$\times$VDM; intended for virtual photons.
= 19 :
anomalous$\times$anomalous; intended for virtual photons.
= 20 :
a mixture of the nine above components, 11-19, in the same spirit as = 10 provides a mixture for real gammas (or a virtual gamma on a hadron). For gamma-hadron, this option coincides with = 10.
= 21 :
direct$\times$direct (see note 5).
= 22 :
direct$\times$resolved.
= 23 :
resolved$\times$direct.
= 24 :
resolved$\times$resolved.
= 25 :
a mixture of the four above components, offering a simpler alternative to = 20 in cases where the parton distributions of the photon have not been split into VMD and anomalous components. For $\gamma$-hadron, only two components need be mixed.
= 26 :
DIS$\times$VMD/$\mathrm{p}$.
= 27 :
DIS$\times$anomalous.
= 28 :
VMD/$\mathrm{p}$$\times$DIS.
= 29 :
anomalous$\times$DIS.
= 30 :
a mixture of all the 4 (for $\gamma^*\mathrm{p}$) or 13 (for $\gamma^*\gamma^*$) components that are available, i.e. (the relevant ones of) 11-19 and 26-29 above; is as = 20 with the DIS processes mixed in.
Note 1:
the MSTP(14) options apply for a photon defined by a 'gamma' or 'gamma/lepton' beam in the PYINIT call, but not to those photons implicitly obtained in a 'lepton' beam with the MSTP(12) = 1 option. This latter approach to resolved photons is more primitive and is no longer recommended for QCD processes.
Note 2:
for real photons our best understanding of how to mix event classes is provided by the option 10 above, which also can be obtained by combining three (for $\gamma\mathrm{p}$) or six (for $\gamma\gamma$) separate runs. In a simpler alternative the VMD and anomalous classes are joined into a single resolved class. Then $\gamma\mathrm{p}$ physics only requires two separate runs, with 0 and 1, and $\gamma\gamma$ physics requires three, with 0, 1 and 4.
Note 3:
most of the new options from 11 onwards are not needed and therefore not defined for $\mathrm{e}\mathrm{p}$ collisions. The recommended 'best' value thus is MSTP(14) = 30, which also is the new default value.
Note 4:
as a consequence of the appearance of new event classes, the MINT(122) and MSTI(9) codes are not the same for $\gamma^*\gamma^*$ events as for $\gamma\mathrm{p}$, $\gamma^*\mathrm{p}$ or $\gamma\gamma$ ones. Instead the code is $3(i_1 - 1) + i_2$, where $i$ is 1 for direct, 2 for VMD and 3 for anomalous/GVMD and indices refer to the two incoming photons. For $\gamma^*\mathrm{p}$ code 4 is DIS, and for $\gamma^*\gamma^*$ codes 10-13 corresponds to the MSTP(14) codes 26-29. As before, MINT(122) and MSTI(9) are only defined when several processes are to be mixed, not when generating one at a time. Also the MINT(123) code is modified (not shown here).
Note 5:
the direct$\times$direct event class excludes lepton pair production when run with the default MSEL = 1 option (or MSEL = 2), in order not to confuse users. You can obtain lepton pairs as well, e.g. by running with MSEL = 0 and switching on the desired processes by hand.
Note 6:
for all non-QCD processes, a photon is assumed unresolved when MSTP(14) = 10, 20 or 25. In principle, both the resolved and direct possibilities ought to be explored, but this mixing is not currently implemented, so picking direct at least will explore one of the two main alternatives rather than none. Resolved processes can be accessed by the more primitive machinery of having a lepton beam and MSTP(12) = 1.

MSTP(15) :
(D = 0) possibility to modify the nature of the anomalous photon component (as used with the appropriate MSTP(14) options), in particular with respect to the scale choices and cut-offs of hard processes. These options are mainly intended for comparative studies and should not normally be touched. Some of the issues are discussed in [Sch93a], while others have only been used for internal studies and are undocumented.
= 0 :
none, i.e. the same treatment as for the VMD component.
= 1 :
evaluate the anomalous parton distributions at a scale $Q^2/$PARP(17)$^2$.
= 2 :
as = 1, but instead of PARP(17) use either PARP(81)/PARP(15) or PARP(82)/PARP(15), depending on MSTP(82) value.
= 3 :
evaluate the anomalous parton distribution functions of the photon as $f^{\gamma,\mathrm{anom}}(x, Q^2, p_0^2) -
f^{\gamma,\mathrm{anom}}(x, Q^2, r^2 Q^2)$ with $r = $PARP(17).
= 4 :
as = 3, but instead of PARP(17) use either PARP(81)/PARP(15) or PARP(82)/PARP(15), depending on MSTP(82) value.
= 5 :
use larger $p_{\perp\mathrm{min}}$ for the anomalous component than for the VMD one, but otherwise no difference.

MSTP(16) :
(D = 1) choice of definition of the fractional momentum taken by a photon radiated off a lepton. Enters in the flux factor for the photon rate, and thereby in cross sections.
= 0 :
$x$, i.e. energy fraction in the rest frame of the event.
= 1 :
$y$, i.e. light-cone fraction.

MSTP(17) :
(D = 4) possibility of a extra factor for processes involving resolved virtual photons, to approximately take into account the effects of longitudinal photons. Given on the form
$R = 1 + \mbox{\texttt{PARP(165)}} \, r(Q^2,\mu^2) \,
f_L(y,Q^2)/f_T(y,Q^2)$.
Here the 1 represents the basic transverse contribution, PARP(165) is some arbitrary overall factor, and $f_L/f_T$ the (known) ratio of longitudinal to transverse photon flux factors. The arbitrary function $r$ depends on the photon virtuality $Q^2$ and the hard scale $\mu^2$ of the process. See [Fri00] for a discussion of the options.
= 0 :
No contribution, i.e. $r=0$.
= 1 :
$r = 4 \mu^2 Q^2 / (\mu^2 + Q^2)^2$.
= 2 :
$r = 4 Q^2 / (\mu^2 + Q^2)$.
= 3 :
$r = 4 Q^2 / (m_{\rho}^2 + Q^2)$.
= 4 :
$r = 4 m_V^2 Q^2 / (m_V^2 + Q^2)^2$, where $m_V$ is the vector meson mass for VMD and $2k_{\perp}$ for GVMD states. Since there is no $\mu$ dependence here (as well as for = 3 and = 5) also minimum-bias cross sections are affected, where $\mu$ would be vanishing. Currently the actual vector meson mass in the VMD case is replaced by $m_{\rho}$, for simplicity.
= 5 :
$r = 4 Q^2 / (m_V^2 + Q^2)$, with $m_V$ and comments as above.
Note:
for a photon given by the 'gamma/lepton' option in the PYINIT call, the $y$ spectrum is dynamically generated and $y$ is thus known from event to event. For a photon beam in the PYINIT call, $y$ is unknown from the onset, and has to be provided by you if any longitudinal factor is to be included. So long as these values, in PARP(167) and PARP(168), are at their default values, 0, it is assumed they have not been set and thus the MSTP(17) and PARP(165) values are inactive.

MSTP(18) :
(D = 3) choice of $p_{\perp\mathrm{min}}$ for direct processes.
= 1 :
same as for VMD and GVMD states, i.e. the $p_{\perp\mathrm{min}}(W^2)$ scale. Primarily intended for real photons.
= 2 :
$p_{\perp\mathrm{min}}$ is chosen to be PARP(15), i.e. the original old behaviour proposed in [Sch93,Sch93a]. In that case, also parton distributions, jet cross sections and $\alpha_{\mathrm{s}}$ values were dampened for small $p_{\perp}$, so it may not be easy to obtain full backwards compatibility with this option.
= 3 :
as = 1, but if the $Q$ scale of the virtual photon is above the VMD/GVMD $p_{\perp\mathrm{min}}(W^2)$, $p_{\perp\mathrm{min}}$ is chosen equal to $Q$. This is part of the strategy to mix in DIS processes at $p_{\perp}$ below $Q$, e.g. in MSTP(14) = 30.

MSTP(19) :
(D = 4) choice of partonic cross section in the DIS process 99.
= 0 :
QPM answer $4 \pi^2 \alpha_{\mathrm{em}}/Q^2 \,
\sum_{\mathrm{q}} e_{\mathrm{q}}^2 (x q(x,Q^2) + x \overline{q}(x,Q^2))$ (with parton distributions frozen below the lowest $Q$ allowed in the parameterization). Note that this answer is divergent for $Q^2 \to 0$ and thus violates gauge invariance.
= 1 :
QPM answer is modified by a factor $Q^2/(Q^2+m_{\rho}^2)$ to provide a finite cross section in the $Q^2 \to 0$ limit. A minimal regularization recipe.
= 2 :
QPM answer is modified by a factor $Q^4/(Q^2 + m_{\rho}^2)^2$ to provide a vanishing cross section in the $Q^2 \to 0$ limit. Appropriate if one assumes that the normal photoproduction description gives the total cross section for $Q^2 = 0$, without any DIS contribution.
= 3 :
as = 2, but additionally suppression by a parameterized factor $f(W^2,Q^2)$ (different for $\gamma^*\mathrm{p}$ and $\gamma^*\gamma^*$) that avoids double-counting the direct-process region where $p_{\perp}> Q$. Shower evolution for DIS events is then also restricted to be at scales below $Q$, whereas evolution all the way up to $W$ is allowed in the other options above.
= 4 :
as = 3, but additionally include factor $1/(1-x)$ for conversion from $F_2$ to $\sigma$. This is formally required, but is only relevant for small $W^2$ and therefore often neglected.

MSTP(20) :
(D = 3) suppression of resolved (VMD or GVMD) cross sections, introduced to compensate for an overlap with DIS processes in the region of intermediate $Q^2$ and rather small $W^2$.
= 0 :
no; used as is.
> 1 :
yes, by a factor $(W^2/(W^2 + Q_1^2 + Q_2^2))^{\mbox{\texttt{MSTP(20)}}}$ (where $Q_i^2 = 0$ for an incoming hadron).
Note:
the suppression factor is joined with the dipole suppression stored in VINT(317) and VINT(318).

MSTP(21) :
(D = 1) nature of fermion-fermion scatterings simulated in process 10 by $t$-channel exchange.
= 0 :
all off (!).
= 1 :
full mixture of $\gamma^* / \mathrm{Z}^0$ neutral current and $\mathrm{W}^{\pm}$ charged current.
= 2 :
$\gamma$ neutral current only.
= 3 :
$\mathrm{Z}^0$ neutral current only.
= 4 :
$\gamma^* / \mathrm{Z}^0$ neutral current only.
= 5 :
$\mathrm{W}^{\pm}$ charged current only.

MSTP(22) :
(D = 0) special override of normal $Q^2$ definition used for maximum of parton-shower evolution, intended for Deeply Inelastic Scattering in lepton-hadron events, see section [*].

MSTP(23) :
(D = 1) for Deeply Inelastic Scattering processes (10 and 83), this option allows the $x$ and $Q^2$ of the original hard scattering to be retained by the final electron when showers are considered (with warnings as below; partly obsolete).
= 0 :
no correction procedure, i.e. $x$ and $Q^2$ of the scattered electron differ from the originally generated $x$ and $Q^2$.
= 1 :
post facto correction, i.e. the change of electron momentum, by initial and final QCD radiation, primordial $k_{\perp}$ and beam-remnant treatment, is corrected for by a shuffling of momentum between the electron and hadron side in the final state. Only process 10 is corrected, while process 83 is not.
= 2 :
as = 1, except that both process 10 and 83 are treated. This option is dangerous, especially for top, since it may well be impossible to `correct' in process 83: the standard DIS kinematics definitions are based on the assumption of massless quarks. Therefore infinite loops are not excluded.
Note 1:
the correction procedure will fail for a fraction of the events, which are thus rejected (and new ones generated in their place). The correction option is not unambiguous, and should not be taken too seriously. For very small $Q^2$ values, the $x$ is not exactly preserved even after this procedure.
Note 2:
this switch does not affect the recommended DIS description obtained with a 'gamma/lepton' beam/target in PYINIT, where $x$ and $Q^2$ are always conserved.

MSTP(25) :
(D = 0) angular decay correlations in Higgs decays to $\mathrm{W}^+ \mathrm{W}^-$ or $\mathrm{Z}^0 \mathrm{Z}^0$ to four fermions [Skj93].
= 0 :
assuming the Higgs decay is pure scalar for $\mathrm{h}^0$ and $\H ^0$ and pure pseudoscalar for $\mathrm{A}^0$.
= 1 :
assuming the Higgs decay is always pure scalar (CP-even).
= 2 :
assuming the Higgs decay is always pure pseudoscalar (CP-odd).
= 3 :
assuming the Higgs decay is a mixture of the two (CP-even and CP-odd), including the CP-violating interference term. The parameter $\eta$, PARP(25) sets the strength of the CP-odd admixture, with the interference term being proportional to $\eta$ and the CP-odd one to $\eta^2$.
Note :
since the decay of an $\mathrm{A}^0$ to $\mathrm{W}^+ \mathrm{W}^-$ or $\mathrm{Z}^0 \mathrm{Z}^0$ is vanishing at the Born level, and no loop diagrams are included, currently this switch is only relevant for $\mathrm{h}^0$ and $\H ^0$. It is mainly intended to allow `straw man' studies of the quantum numbers of a Higgs state, decoupled from the issue of branching ratios.

MSTP(31) :
(D = 1) parameterization of total, elastic and diffractive cross sections.
= 0 :
everything is to be set by you yourself in the PYINT7 common block. For photoproduction, additionally you need to set VINT(281). Normally you would set these values once and for all before the PYINIT call, but if you run with variable energies (see MSTP(171)) you can also set it before each new PYEVNT call.
= 1 :
Donnachie-Landshoff for total cross section [Don92], and Schuler-Sjöstrand for elastic and diffractive cross sections [Sch94,Sch93a].

MSTP(32) :
(D = 8) $Q^2$ definition in hard scattering for $2 \to 2$ processes. For resonance production $Q^2$ is always chosen to be $\hat{s} = m_R^2$, where $m_R$ is the mass of the resonance. For gauge boson scattering processes $VV \to VV$ the $\mathrm{W}$ or $\mathrm{Z}^0$ squared mass is used as scale in parton distributions. See PARP(34) for a possibility to modify the choice below by a multiplicative factor.
The newer options 6-10 are specifically intended for processes with incoming virtual photons. These are ordered from a `minimal' dependence on the virtualities to a `maximal' one, based on reasonable kinematics considerations. The old default value MSTP(32) = 2 forms the starting point, with no dependence at all, and the new default is some intermediate choice. Notation is that $P_1^2$ and $P_2^2$ are the virtualities of the two incoming particles, $p_{\perp}$ the transverse momentum of the scattering process, and $m_3$ and $m_4$ the masses of the two outgoing partons. For a direct photon, $P^2$ is the photon virtuality and $x = 1$. For a resolved photon, $P^2$ still refers to the photon, rather than the unknown virtuality of the reacting parton in the photon, and $x$ is the momentum fraction taken by this parton.
= 1 :
$Q^2 = 2 \hat{s} \hat{t} \hat{u} / (\hat{s}^2 +
\hat{t}^2 + \hat{u}^2)$.
= 2 :
$Q^2 = (m_{\perp 3}^2 + m_{\perp 4}^2)/2 =
p_{\perp}^2 + (m_3^2 + m_4^2)/2$.
= 3 :
$Q^2 = \min(-\hat{t}, -\hat{u})$.
= 4 :
$Q^2 = \hat{s}$.
= 5 :
$Q^2 = -\hat{t}$.
= 6 :
$Q^2 = (1 + x_1 P_1^2/\hat{s} + x_2 P_2^2/\hat{s})
(p_{\perp}^2 + m_3^2/2 + m_4^2/2)$.
= 7 :
$Q^2 = (1 + P_1^2/\hat{s} + P_2^2/\hat{s})
(p_{\perp}^2 + m_3^2/2 + m_4^2/2)$.
= 8 :
$Q^2 = p_{\perp}^2 + (P_1^2 + P_2^2 + m_3^2 + m_4^2)/2$.
= 9 :
$Q^2 = p_{\perp}^2 + P_1^2 + P_2^2 +m_3^2 + m_4^2$.
= 10 :
$Q^2 = s$ (the full energy-squared of the process).
= 11 :
$Q^2 = (m_3 + m_4)^2/4$.
= 12 :
$Q^2$ is set by the user as fixed numbers, factorization scale in PARP(193) and renormalization scale in PARP(194).
= 13 :
$Q^2 = p_{\perp}^2$, i.e. without any dependence on masses.
Note:
options 6 and 7 are motivated by assuming that one wants a scale that interpolates between $\hat{t}$ for small $\hat{t}$ and $\hat{u}$ for small $\hat{u}$, such as $Q^2 = - \hat{t}\hat{u}/(\hat{t}+\hat{u})$. When kinematics for the $2 \to 2$ process is constructed as if an incoming photon is massless when it is not, it gives rise to a mismatch factor $1 + P^2/\hat{s}$ (neglecting the other masses) in this $Q^2$ definition, which is then what is used in option 7 (with the neglect of some small cross-terms when both photons are virtual). When a virtual photon is resolved, the virtuality of the incoming parton can be anything from $xP^2$ and upwards. So option 6 uses the smallest kinematically possible value, while 7 is more representative of the typical scale. Option 8 and 9 are more handwaving extensions of the default option, with 9 specially constructed to ensure that the $Q^2$ scale is always bigger than $P^2$.

MSTP(33) :
(D = 0) inclusion of $K$ factors in hard cross sections for parton-parton interactions (i.e. for incoming quarks and gluons).
= 0 :
none, i.e. $K = 1$.
= 1 :
a common $K$ factor is used, as stored in PARP(31).
= 2 :
separate factors are used for ordinary (PARP(31)) and colour annihilation graphs (PARP(32)).
= 3 :
A $K$ factor is introduced by a shift in the $\alpha_{\mathrm{s}}$ $Q^2$ argument, $\alpha_{\mathrm{s}}= \alpha_{\mathrm{s}}($PARP(33)$Q^2)$.

MSTP(34) :
(D = 1) use of interference term in matrix elements for QCD processes, see section [*].
= 0 :
excluded (i.e. string-inspired matrix elements).
= 1 :
included (i.e. conventional QCD matrix elements).
Note:
for the option MSTP(34) = 1, i.e. interference terms included, these terms are divided between the different possible colour configurations according to the pole structure of the (string-inspired) matrix elements for the different colour configurations.

MSTP(35) :
(D = 0) threshold behaviour for heavy-flavour production, i.e. ISUB = 81, 82, 84, 85, and also for $\mathrm{Z}$ and $\mathrm{Z}'$ decays. The non-standard options are mainly intended for top, but can be used, with less theoretical reliability, also for charm and bottom (for $\mathrm{Z}$ and $\mathrm{Z}'$ only top and heavier flavours are affected). The threshold factors are given in eqs. ([*]) and ([*]).
= 0 :
naïve lowest-order matrix-element behaviour.
= 1 :
enhancement or suppression close to threshold, according to the colour structure of the process. The $\alpha_{\mathrm{s}}$ value appearing in the threshold factor (which is not the same as the $\alpha_{\mathrm{s}}$ of the lowest-order $2 \to 2$ process) is taken to be fixed at the value given in PARP(35). The threshold factor used in an event is stored in PARI(81).
= 2 :
as = 1, but the $\alpha_{\mathrm{s}}$ value appearing in the threshold factor is taken to be running, with argument $Q^2 = m_{\mathrm{Q}} \sqrt{ (\hat{m} - 2m_{\mathrm{Q}})^2 + \Gamma_{\mathrm{Q}}^2}$. Here $m_{\mathrm{Q}}$ is the nominal heavy-quark mass, $\Gamma_{\mathrm{Q}}$ is the width of the heavy-quark-mass distribution, and $\hat{m}$ is the invariant mass of the heavy-quark pair. The $\Gamma_{\mathrm{Q}}$ value has to be stored by you in PARP(36). The regularization of $\alpha_{\mathrm{s}}$ at low $Q^2$ is given by MSTP(36).

MSTP(36) :
(D = 2) regularization of $\alpha_{\mathrm{s}}$ in the limit $Q^2 \to 0$ for the threshold factor obtainable in the MSTP(35) = 2 option; see MSTU(115) for a list of the possibilities.

MSTP(37) :
(D = 1) inclusion of running quark masses in Higgs production ( $\mathrm{q}\overline{\mathrm{q}}\to \mathrm{h}^0$) and decay ( $\mathrm{h}^0 \to \mathrm{q}\overline{\mathrm{q}}$) couplings, obtained by calls to the PYMRUN function. Also included for charged Higgs and technipion production and decay.
= 0 :
not included, i.e. fixed quark masses are used according to the values in the PMAS array.
= 1 :
included, with running starting from the value given in the PMAS array, at a $Q_0$ scale of PARP(37) times the quark mass itself, up to a $Q$ scale given by the Higgs mass. This option only works when $\alpha_{\mathrm{s}}$ is allowed to run (so one can define a $\Lambda$ value). Therefore it is only applied if additionally MSTP(2) $\geq 1$.

MSTP(38) :
(D = 5) handling of quark loop masses in the box graphs $\mathrm{g}\mathrm{g}\to \gamma \gamma$ and $\mathrm{g}\mathrm{g}\to \mathrm{g}\gamma$, and in the Higgs production loop graphs $\mathrm{q}\overline{\mathrm{q}}\to \mathrm{g}\mathrm{h}^0$, $\mathrm{q}\mathrm{g}\to \mathrm{q}\mathrm{h}^0$ and $\mathrm{g}\mathrm{g}\to \mathrm{g}\mathrm{h}^0$, and their equivalents with $\H ^0$ or $\mathrm{A}^0$ instead of $\mathrm{h}^0$.
= 0 :
for $\mathrm{g}\mathrm{g}\to \gamma \gamma$ and $\mathrm{g}\mathrm{g}\to \mathrm{g}\gamma$ the program will for each flavour automatically choose the massless approximation for light quarks and the full massive formulae for heavy quarks, with a dividing line between light and heavy quarks that depends on the actual $\hat{s}$ scale. For Higgs production, all quark loop contributions are included with the proper masses. This option is then correct only in the Standard Model Higgs scenario, and should not be used e.g. in the MSSM.
$\geq$1 :
for $\mathrm{g}\mathrm{g}\to \gamma \gamma$ and $\mathrm{g}\mathrm{g}\to \mathrm{g}\gamma$ the program will use the massless approximation throughout, assuming the presence of MSTP(38) effectively massless quark species (however, at most 8). Normally one would use = 5 for the inclusion of all quarks up to bottom, and = 6 to include top as well. For Higgs production, the approximate expressions derived in the $m_{\t } \to \infty$ limit are used, rescaled to match the correct $\mathrm{g}\mathrm{g}\to \mathrm{h}^0/\H ^0/\mathrm{A}^0$ cross sections. This procedure should work, approximately, also for non-standard Higgs particles.
Warning:
for = 0, numerical instabilities may arise in $\mathrm{g}\mathrm{g}\to \gamma \gamma$ and $\mathrm{g}\mathrm{g}\to \mathrm{g}\gamma$ for scattering at small angles. You are therefore recommended in this case to set CKIN(27) and CKIN(28) so as to exclude the range of scattering angles that is not of interest anyway. Numerical problems may also occur for Higgs production with = 0, and additionally the lengthy expressions make the code error-prone.

MSTP(39) :
(D = 2) choice of $Q^2$ scale for parton distributions and initial-state parton showers in processes $\mathrm{g}\mathrm{g}\to \mathrm{Q}\overline{\mathrm{Q}}\mathrm{h}$ or $\mathrm{q}\overline{\mathrm{q}}\to \mathrm{Q}\overline{\mathrm{Q}}\mathrm{h}$.
= 1 :
$m_{\mathrm{Q}}^2$.
= 2 :
$\max(m_{\perp\mathrm{Q}}^2,m_{\perp\overline{\mathrm{Q}}}^2 ) =
m_{\mathrm{Q}}^2 + \max(p_{\perp\mathrm{Q}}^2 , p_{\perp\overline{\mathrm{Q}}}^2)$.
= 3 :
$m_{\mathrm{h}}^2$, where $m_{\mathrm{h}}$ is the actual Higgs mass of the event, fluctuating from one event to the next.
= 4 :
$\hat{s} = (p_{\mathrm{h}} + p_{\mathrm{Q}} + p_{\overline{\mathrm{Q}}})^2$.
= 5 :
$m_{\mathrm{h}}^2$, where $m_{\mathrm{h}}$ is the nominal, fixed Higgs mass.
= 6 :
$(m_3 + m_5)^2/4$.
= 7 :
$(m_{\perp 3}^2 + m_{\perp 4}^2)/2$.
= 8 :
set by the user as fixed numbers, factorization scale in PARP(193) and renormalization scale in PARP(194).

MSTP(40) :
(D = 0) option for Coulomb correction in process 25, $\mathrm{W}^+ \mathrm{W}^-$ pair production, see [Kho96]. The value of the Coulomb correction factor for each event is stored in VINT(95).
= 0 :
`no Coulomb'. Is the often-used reference point.
= 1 :
`unstable Coulomb', gives the correct first-order expression valid in the non-relativistic limit. Is the reasonable option to use as a `best bet' description of LEP 2 physics.
= 2 :
`second-order Coulomb' gives the correct second-order expression valid in the non-relativistic limit. In principle this is even better than = 1, but the differences are negligible and computer time does go up because of the need for a numerical integration in the weight factor.
= 3 :
`dampened Coulomb', where the unstable Coulomb expression has been modified by a $(1-\beta)^2$ factor in front of the arctan term. This is intended as an alternative to = 1 within the band of our uncertainty in the relativistic limit.
= 4 :
`stable Coulomb', i.e. effects are calculated as if the $\mathrm{W}$'s were stable. Is incorrect, and mainly intended for comparison purposes.
Note :
unfortunately the $\mathrm{W}$'s at LEP 2 are not in the non-relativistic limit, so the separation of Coulomb effects from other radiative corrections is not gauge invariant. The options above should therefore be viewed as indicative only, not as the ultimate answer.

MSTP(41) :
(D = 2) master switch for all resonance decays ($\mathrm{Z}^0$, $\mathrm{W}^{\pm}$, $\t $, $\mathrm{h}^0$, $\mathrm{Z}'^0$, $\mathrm{W}'^{\pm}$, $\H ^0$, $\mathrm{A}^0$, $\H ^{\pm}$, $\L _{\mathrm{Q}}$, $\mathrm{R}^0$, $\d ^*$, $\u ^*$, ...).
= 0 :
all off.
= 1 :
all on.
= 2 :
on or off depending on their individual MDCY values.
Note:
also for MSTP(41) = 1 it is possible to switch off the decays of specific resonances by using the MDCY(KC,1) switches in PYTHIA. However, since the MDCY values are overwritten in the PYINIT call when MSTP(41) = 1 (or 0), individual resonances should then be switched off after the PYINIT call.
Warning:
for top, leptoquark and other colour-carrying resonances it is dangerous to switch off decays if one later on intends to let them decay (with PYEXEC); see section [*].

MSTP(42) :
(D = 1) mass treatment in $2 \to 2$ processes, where the final-state resonances have finite width (see PARP(41)). (Does not apply for the production of a single $s$-channel resonance, where the mass appears explicitly in the cross section of the process, and thus is always selected with width.)
= 0 :
particles are put on the mass shell.
= 1 :
mass generated according to a Breit-Wigner.

MSTP(43) :
(D = 3) treatment of $\mathrm{Z}^0/\gamma^*$ interference in matrix elements. So far implemented in subprocesses 1, 15, 19, 22, 30 and 35; in other processes what is called a $\mathrm{Z}^0$ is really a $\mathrm{Z}^0$ only, without the $\gamma^*$ piece.
= 1 :
only $\gamma^*$ included.
= 2 :
only $\mathrm{Z}^0$ included.
= 3 :
complete $\mathrm{Z}^0/\gamma^*$ structure (with interference) included.

MSTP(44) :
(D = 7) treatment of $\mathrm{Z}'^0/\mathrm{Z}^0/\gamma^*$ interference in matrix elements.
= 1 :
only $\gamma^*$ included.
= 2 :
only $\mathrm{Z}^0$ included.
= 3 :
only $\mathrm{Z}'^0$ included.
= 4 :
only $\mathrm{Z}^0/\gamma^*$ (with interference) included.
= 5 :
only $\mathrm{Z}'^0/\gamma^*$ (with interference) included.
= 6 :
only $\mathrm{Z}'^0/\mathrm{Z}^0$ (with interference) included.
= 7 :
complete $\mathrm{Z}'^0/\mathrm{Z}^0/\gamma^*$ structure (with interference) included.

MSTP(45) :
(D = 3) treatment of $\mathrm{W}\mathrm{W}\to \mathrm{W}\mathrm{W}$ structure (ISUB = 77).
= 1 :
only $\mathrm{W}^+\mathrm{W}^+ \to \mathrm{W}^+\mathrm{W}^+$ and $\mathrm{W}^-\mathrm{W}^- \to \mathrm{W}^-\mathrm{W}^-$ included.
= 2 :
only $\mathrm{W}^+\mathrm{W}^- \to \mathrm{W}^+\mathrm{W}^-$ included.
= 3 :
all charge combinations $\mathrm{W}\mathrm{W}\to \mathrm{W}\mathrm{W}$ included.

MSTP(46) :
(D = 1) treatment of $V V \to V' V'$ structures (ISUB = 71-77), where $V$ represents a longitudinal gauge boson.
= 0 :
only $s$-channel Higgs exchange included (where existing). With this option, subprocesses 71-72 and 76-77 will essentially be equivalent to subprocesses 5 and 8, respectively, with the proper decay channels (i.e. only $\mathrm{Z}^0 \mathrm{Z}^0$ or $\mathrm{W}^+ \mathrm{W}^-$) set via MDME. The description obtained for subprocesses 5 and 8 in this case is more sophisticated, however; see section [*].
= 1 :
all graphs contributing to $V V \to V' V'$ processes are included.
= 2 :
only graphs not involving Higgs exchange (either in $s$, $t$ or $u$ channel) are included; this option then gives the naïve behaviour one would expect if no Higgs exists, including unphysical unitarity violations at high energies.
= 3 :
the strongly interacting Higgs-like model of Dobado, Herrero and Terron [Dob91] with Padé unitarization. Note that to use this option it is necessary to set the Higgs mass to a large number like 20 TeV (i.e. PMAS(25,1) = 20000). The parameter $\nu$ is stored in PARP(44), but should not have to be changed.
= 4 :
as = 3, but with K-matrix unitarization [Dob91].
= 5 :
the strongly interacting QCD-like model of Dobado, Herrero and Terron [Dob91] with Padé unitarization. The parameter $\nu$ is stored in PARP(44), but should not have to be changed. The effective techni-$\rho$ mass in this model is stored in PARP(45); by default it is 2054 GeV, which is the expected value for three technicolors, based on scaling up the ordinary $\rho$ mass appropriately.
= 6 :
as = 5, but with K-matrix unitarization [Dob91]. While PARP(45) still is a parameter of the model, this type of unitarization does not give rise to a resonance at a mass of PARP(45).

MSTP(47) :
(D = 1) (C) angular orientation of decay products of resonances ($\mathrm{Z}^0$, $\mathrm{W}^{\pm}$, $\t $, $\mathrm{h}^0$, $\mathrm{Z}'^0$, $\mathrm{W}'^{\pm}$, etc.), either when produced singly or in pairs (also from an $\mathrm{h}^0$ decay), or in combination with a single quark, gluon or photon.
= 0 :
independent decay of each resonance, isotropic in c.m. frame of the resonance.
= 1 :
correlated decay angular distributions according to proper matrix elements, to the extent these are implemented.

MSTP(48) :
(D = 0) (C) switch for the treatment of $\gamma^* / \mathrm{Z}^0$ decay for process 1 in $\mathrm{e}^+\mathrm{e}^-$ events.
= 0 :
normal machinery.
= 1 :
if the decay of the $\mathrm{Z}^0$ is to either of the five lighter quarks, $\d $, $\u $, $\mathrm{s}$, $\c $ or $\b $, the special treatment of $\mathrm{Z}^0$ decay is accessed, including matrix element options, according to section [*].
This option is based on the machinery of the PYEEVT and associated routines when it comes to the description of QCD multijet structure and the angular orientation of jets, but relies on the normal PYEVNT machinery for everything else: cross section calculation, initial-state photon radiation, flavour composition of decays (i.e. information on channels allowed), etc.
The initial state has to be $\mathrm{e}^+\mathrm{e}^-$; forward-backward asymmetries would not come out right for quark-annihilation production of the $\gamma^* / \mathrm{Z}^0$ and therefore the machinery defaults to the standard one in such cases.
You can set the behaviour for the MSTP(48) option using the normal matrix element related switches. This especially means MSTJ(101) for the selection of first- or second-order matrix elements (= 1 and = 2, respectively). Further selectivity is obtained with the switches and parameters MSTJ(102) (for the angular orientation part only), MSTJ(103) (except the production threshold factor part), MSTJ(106), MSTJ(108) - MSTJ(111), PARJ(121), PARJ(122), and PARJ(125) - PARJ(129). Information can be read from MSTJ(120), MSTJ(121), PARJ(150), PARJ(152) - PARJ(156), PARJ(168), PARJ(169), PARJ(171).
The other $\mathrm{e}^+\mathrm{e}^-$ switches and parameters should not be touched. In most cases they are simply not accessed, since the related part is handled by the PYEVNT machinery instead. In other cases they could give incorrect or misleading results. Beam polarization as set by PARJ(131) - PARJ(134), for instance, is only included for the angular orientation, but is missing for the cross section information. PARJ(123) and PARJ(124) for the $\mathrm{Z}^0$ mass and width are set in the PYINIT call based on the input mass and calculated widths.
The cross section calculation is unaffected by the matrix element machinery. Thus also for negative MSTJ(101) values, where only specific jet multiplicities are generated, the PYSTAT cross section is the full one.

MSTP(49) :
(D = 1) assumed variation of the Higgs width to massive gauge boson pairs, i.e. $\mathrm{W}^+ \mathrm{W}^-$, $\mathrm{Z}^0 \mathrm{Z}^0$ and $\mathrm{W}^{\pm}\mathrm{Z}^0$, as a function of the actual mass $\hat{m} = \sqrt{\hat{s}}$ and the nominal mass $m_{\mathrm{h}}$. The switch applies both to $\mathrm{h}^0$, $\H ^0$, $\mathrm{A}^0$ and $\H ^{\pm}$ decays.
= 0 :
the width is proportional to $\hat{m}^3$; thus the high-mass tail of the Breit-Wigner is enhanced.
= 1 :
the width is proportional to $m_{\mathrm{h}}^2 \hat{m}$. For a fixed Higgs mass $m_{\mathrm{h}}$ this means a width variation across the Breit-Wigner more in accord with other resonances (such as the $\mathrm{Z}^0$). This alternative gives more emphasis to the low-mass tail, where the parton distributions are peaked (for hadron colliders). This option is favoured by resummation studies [Sey95a].
Note 1:
the partial width of a Higgs to a fermion pair is always taken to be proportional to the actual Higgs mass $\hat{m}$, irrespectively of MSTP(49). Also the width to a gluon or photon pair (via loops) is unaffected.
Note 2:
this switch does not affect processes 71-77, where a fixed Higgs width is used in order to control cancellation of divergences.

MSTP(50) :
(D = 0) Switch to allow or not longitudinally polarized incoming beams, with the two polarizations stored in PARJ(131) and PARJ(132), respectively. Most cross section expressions with polarization reduce to the unpolarized behaviour for the default PARJ(131) = PARJ(132) = 0., and then this switch is superfluous and not implemented. Currently MSTP(50) is only used in process 25, $\mathrm{f}\overline{\mathrm{f}}\to \mathrm{W}^+ \mathrm{W}^-$, for reasons explained in section [*].
= 0 :
no polarization effects, no matter what PARJ(131) and PARJ(132) values are set.
= 1 :
include polarization information in the cross section of the process and for angular correlations.

MSTP(51) :
(D = 7) choice of proton parton-distribution set; see also MSTP(52).
= 1 :
CTEQ 3L (leading order).
= 2 :
CTEQ 3M ( $\overline{\mathrm{MS}}$).
= 3 :
CTEQ 3D (DIS).
= 4 :
GRV 94L (leading order).
= 5 :
GRV 94M ( $\overline{\mathrm{MS}}$).
= 6 :
GRV 94D (DIS).
= 7 :
CTEQ 5L (leading order).
= 8 :
CTEQ 5M1 ( $\overline{\mathrm{MS}}$; slightly updated version of CTEQ 5M).
= 11 :
GRV 92L (leading order).
= 12 :
EHLQ set 1 (leading order; 1986 updated version).
= 13 :
EHLQ set 2 (leading order; 1986 updated version).
= 14 :
Duke-Owens set 1 (leading order).
= 15 :
Duke-Owens set 2 (leading order).
= 16 :
simple ansatz with all parton distributions of the form $c/x$, with $c$ some constant; intended for internal debug use only.
Note 1:
distributions 11-15 are obsolete and should not be used for current physics studies. They are only implemented to have some sets in common between PYTHIA 5 and 6, for cross-checks.
Note 2:
since all parameterizations have some region of applicability, the parton distributions are assumed frozen below the lowest $Q^2$ covered by the parameterizations. In some cases, evolution is also frozen above the maximum $Q^2$.

MSTP(52) :
(D = 1) choice of proton parton-distribution-function library.
= 1 :
the internal PYTHIA one, with parton distributions according to the MSTP(51) above.
= 2 :
the PDFLIB one [Plo93], with the PDFLIB (version 4) NGROUP and NSET numbers to be given as MSTP(51) = 1000$\times$NGROUP + NSET, or similarly for the LHAPDF one [Gie02].
Note 1:
to make use of option 2, it is necessary to link PDFLIB/LHAPDF. Additionally, on most computers, the three dummy routines PDFSET, STRUCTM and (for virtual photons) STRUCTP at the end of the PYTHIA file should be removed or commented out.
Warning:
for external parton distribution libraries, PYTHIA does not check whether MSTP(51) corresponds to a valid code, or if special $x$ and $Q^2$ restrictions exist for a given set, such that crazy values could be returned. This puts an extra responsibility on you.
Note 2:
when PDFLIB/LHAPDF is used, PYTHIA can initialize either with a four- or a five-flavour $\Lambda$, depending on how NFL in the /W50511/ commonblock is set, extracting either QCDL4 or QCDL5 from the /W50512/ commonblock.

MSTP(53) :
(D = 3) choice of pion parton-distribution set; see also MSTP(54).
= 1 :
Owens set 1.
= 2 :
Owens set 2.
= 3 :
GRV LO (updated version).

MSTP(54) :
(D = 1) choice of pion parton-distribution-function library.
= 1 :
the internal PYTHIA one, with parton distributions according to the MSTP(53) above.
= 2 :
the PDFLIB one [Plo93], with the PDFLIB (version 4) NGROUP and NSET numbers to be given as MSTP(53) = 1000$\times$NGROUP + NSET, or similarly for the LHAPDF one [Gie02].
Note:
to make use of option 2, it is necessary to link PDFLIB/LHAPDF. Additionally, on most computers, the three dummy routines PDFSET, STRUCTM and STRUCTP at the end of the PYTHIA file should be removed or commented out.
Warning:
for external parton distribution libraries, PYTHIA does not check whether MSTP(53) corresponds to a valid code, or if special $x$ and $Q^2$ restrictions exist for a given set, such that crazy values could be returned. This puts an extra responsibility on you.

MSTP(55)
: (D = 5) choice of the parton-distribution set of the photon; see also MSTP(56) and MSTP(60).
= 1 :
Drees-Grassie.
= 5 :
SaS 1D (in DIS scheme, with $Q_0=0.6$ GeV).
= 6 :
SaS 1M (in $\overline{\mbox{\textsc{ms}}}$ scheme, with $Q_0=0.6$ GeV).
= 7 :
SaS 2D (in DIS scheme, with $Q_0=2$ GeV).
= 8 :
SaS 2M (in $\overline{\mbox{\textsc{ms}}}$ scheme, with $Q_0=2$ GeV).
= 9 :
SaS 1D (in DIS scheme, with $Q_0=0.6$ GeV).
= 10 :
SaS 1M (in $\overline{\mbox{\textsc{ms}}}$ scheme, with $Q_0=0.6$ GeV).
= 11 :
SaS 2D (in DIS scheme, with $Q_0=2$ GeV).
= 12 :
SaS 2M (in $\overline{\mbox{\textsc{ms}}}$ scheme, with $Q_0=2$ GeV).
Note 1:
sets 5-8 use the parton distributions of the respective set, and nothing else. These are appropriate for most applications, e.g. jet production in $\gamma\mathrm{p}$ and $\gamma\gamma$ collisions. Sets 9-12 instead are appropriate for $\gamma^*\gamma$ processes, i.e. DIS scattering on a photon, as measured in $F_2^{\gamma}$. Here the anomalous contribution for $\c $ and $\b $ quarks are handled by the Bethe-Heitler formulae, and the direct term is artificially lumped with the anomalous one, so that the event simulation more closely agrees with what will be experimentally observed in these processes. The agreement with the $F_2^{\gamma}$ parameterization is still not perfect, e.g. in the treatment of heavy flavours close to threshold.
Note 2:
sets 5-12 contain both VMD pieces and anomalous pieces, separately parameterized. Therefore the respective piece is automatically called, whatever MSTP(14) value is used to select only a part of the allowed photon interactions. For other sets (set 1 above or PDFLIB/LHAPDF sets), usually there is no corresponding subdivision. Then an option like MSTP(14) = 2 (VMD part of photon only) is based on a rescaling of the pion distributions, while MSTP(14) = 3 gives the SaS anomalous parameterization.
Note 3:
formally speaking, the $k_0$ (or $p_0$) cut-off in PARP(15) need not be set in any relation to the $Q_0$ cut-off scales used by the various parameterizations. Indeed, due to the familiar scale choice ambiguity problem, there could well be some offset between the two. However, unless you know what you are doing, it is recommended that you let the two agree, i.e. set PARP(15) = 0.6 for the SaS 1 sets and = 2. for the SaS 2 sets.

MSTP(56) :
(D = 1) choice of photon parton-distribution-function library.
= 1 :
the internal PYTHIA one, with parton distributions according to the MSTP(55) above.
= 2 :
the PDFLIB one [Plo93], with the PDFLIB (version 4) NGROUP and NSET numbers to be given as MSTP(55) = 1000$\times$NGROUP + NSET, or similarly for the LHAPDF one [Gie02]. When the VMD and anomalous parts of the photon are split, like for MSTP(14) = 10, it is necessary to specify pion set to be used for the VMD component, in MSTP(53) and MSTP(54), while MSTP(55) here is irrelevant.
= 3 :
when the parton distributions of the anomalous photon are requested, the homogeneous solution is provided, evolved from a starting value PARP(15) to the requested $Q$ scale. The homogeneous solution is normalized so that the net momentum is unity, i.e. any factors of $\alpha_{\mathrm{em}}/2\pi$ and charge have been left out. The flavour of the original $\mathrm{q}$ is given in MSTP(55) (1, 2, 3, 4 or 5 for $\d $, $\u $, $\mathrm{s}$, $\c $ or $\b $); the value 0 gives a mixture according to squared charge, with the exception that $\c $ and $\b $ are only allowed above the respective mass threshold ( $Q > m_{\mathrm{q}}$). The four-flavour $\Lambda$ value is assumed given in PARP(1); it is automatically recalculated for 3 or 5 flavours at thresholds. This option is not intended for standard event generation, but is useful for some theoretical studies.
Note:
to make use of option 2, it is necessary to link PDFLIB/LHAPDF. Additionally, on most computers, the three dummy routines PDFSET, STRUCTM and STRUCTP at the end of the PYTHIA file should be removed or commented out.
Warning 1:
for external parton-distribution libraries, PYTHIA does not check whether MSTP(55) corresponds to a valid code, or if special $x$ and $Q^2$ restrictions exist for a given set, such that crazy values could be returned. This puts an extra responsibility on you.
Warning 2:
so much of the machinery for virtual photons is based on a subdivision of the photon according to the SaS prescription that a usage of PDFLIB cannot be recommended for such; in some cases unphysical results may arise from mismatches between what PDFLIB delivers and what is assumed internally.

MSTP(57) :
(D = 1) choice of $Q^2$ dependence in parton-distribution functions.
= 0 :
parton distributions are evaluated at nominal lower cut-off value $Q_0^2$, i.e. are made $Q^2$-independent.
= 1 :
the parameterized $Q^2$ dependence is used.
= 2 :
the parameterized parton-distribution behaviour is kept at large $Q^2$ and $x$, but modified at small $Q^2$ and/or $x$, so that parton distributions vanish in the limit $Q^2 \to 0$ and have a theoretically motivated small-$x$ shape [Sch93a]. This option is only valid for the $\mathrm{p}$ and $\mathrm{n}$. It is obsolete within the current 'gamma/lepton' framework.
= 3 :
as = 2, except that also the $\pi^{\pm}$ is modified in a corresponding manner. A VMD photon is not mapped to a pion, but is treated with the same photon parton distributions as for other MSTP(57) values, but with properly modified behaviour for small $x$ or $Q^2$. This option is obsolete within the current 'gamma/lepton' framework.

MSTP(58) :
(D = min(5, 2$\times$MSTP(1))) maximum number of quark flavours used in parton distributions, and thus also for initial-state space-like showers. If some distributions (notably $\t $) are absent in the parameterization selected in MSTP(51), these are obviously automatically excluded.

MSTP(59) :
(D = 1) choice of electron-inside-electron parton distribution.
= 1 :
the recommended standard for LEP 1, next-to-leading exponentiated, see [Kle89], p. 34.
= 2 :
the recommended `$\beta$' scheme for LEP 2, also next-to-leading exponentiated, see [Bee96], p. 130.

MSTP(60) :
(D = 7) extension of the SaS real-photon distributions to off-shell photons, especially for the anomalous component. See [Sch96] for an explanation of the options. The starting point is the expression in eq. ([*]), which requires a numerical integration of the anomalous component, however, and therefore is not convenient. Approximately, the dipole damping factor can be removed and compensated by a suitably shifted lower integration limit, whereafter the integral simplifies. Different `goodness' criteria for the choice of the shifted lower limit is represented by the options 2-7 below.
= 1 :
dipole dampening by integration; very time-consuming.
= 2 :
$P_0^2 = \max( Q_0^2, P^2 )$.
= 3 :
${P'}_0^2 = Q_0^2 + P^2$.
= 4 :
$P_{\mathrm{eff}}$ that preserves momentum sum.
= 5 :
$P_{\mathrm{int}}$ that preserves momentum and average evolution range.
= 6 :
$P_{\mathrm{eff}}$, matched to $P_0$ in $P^2 \to Q^2$ limit.
= 7 :
$P_{\mathrm{int}}$, matched to $P_0$ in $P^2 \to Q^2$ limit.

MSTP(61) :
(D = 2) (C) master switch for initial-state QCD and QED radiation.
= 0 :
off.
= 1 :
on.
= 1 :
on for QCD radiation in hadronic events and QED radiation in leptonic ones.
= 2 :
on for QCD and QED radiation in hadronic events and QED radiation in leptonic ones.

MSTP(62) - MSTP(70) :
(C) further switches for initial-state radiation, see section [*].

MSTP(71) :
(D = 1) (C) master switch for final-state QCD and QED radiation.
= 0 :
off.
= 1 :
on.
Note:
additional switches (e.g. for conventional/coherent showers) are available in MSTJ(38) - MSTJ(50) and PARJ(80) - PARJ(90), see section [*].

MSTP(72):
(C) further switch for initial-state radiation, see section [*].

MSTP(81) :
(D = 1) master switch for multiple interactions.
= 0 :
off.
= 1 :
on.

MSTP(82) - MSTP(86) :
further switches for multiple interactions, see section [*].

MSTP(91) - MSTP(95) :
switches for beam-remnant treatment, see section [*].

MSTP(101) :
(D = 3) (C) structure of diffractive system.
= 1 :
forward moving diquark + interacting quark.
= 2 :
forward moving diquark + quark joined via interacting gluon (`hairpin' configuration).
= 3 :
a mixture of the two options above, with a fraction PARP(101) of the former type.

MSTP(102) :
(D = 1) (C) decay of a $\rho^0$ meson produced by `elastic' scattering of an incoming $\gamma$, as in $\gamma \mathrm{p}\to \rho^0 \mathrm{p}$, or the same with the hadron diffractively excited.
= 0 :
the $\rho^0$ is allowed to decay isotropically, like any other $\rho^0$.
= 1 :
the decay $\rho^0 \to \pi^+ \pi^-$ is done with an angular distribution proportional to $\sin^2 \theta$ in its rest frame, where the $z$ axis is given by the direction of motion of the $\rho^0$. The $\rho^0$ decay is then done as part of the hard process, i.e. also when MSTP(111) = 0.

MSTP(110) :
(D = 0) switch to allow some or all resonance widths to be modified by the factor PARP(110). This is not intended for serious physics studies. The main application is rather to generate events with an artificially narrow resonance width in order to study the detector-related smearing effects on the mass resolution.
> 0 :
rescale the particular resonance with KF = MSTP(110). If the resonance has an antiparticle, this one is affected as well.
= -1 :
rescale all resonances, except $\t $, $\overline{\mathrm{t}}$, $\mathrm{Z}^0$ and $\mathrm{W}^{\pm}$.
= -2 :
rescale all resonances.
Warning:
only resonances with a width evaluated by PYWIDT are affected, and preferentially then those with MWID value 1 or 3. For other resonances the appearance of effects or not depends on how the cross sections have been implemented. So it is important to check that indeed the mass distribution is affected as expected. Also beware that, if a sequential decay chain is involved, the scaling may become more complicated. Furthermore, depending on implementational details, a cross section may or may not scale with PARP(110) (quite apart from differences related to the convolution with parton distributions etc.). All in all, it is then an option to be used only with open eyes, and for very specific applications.

MSTP(111) :
(D = 1) (C) master switch for fragmentation and decay, as obtained with a PYEXEC call.
= 0 :
off.
= 1 :
on.
= -1 :
only choose kinematical variables for hard scattering, i.e. no jets are defined. This is useful, for instance, to calculate cross sections (by Monte Carlo integration) without wanting to simulate events; information obtained with PYSTAT(1) will be correct.

MSTP(112) :
(D = 1) (C) cuts on partonic events; only affects an exceedingly tiny fraction of events. Normally this concerns what happens in the PYPREP routine, if a colour singlet subsystem has a very small invariant mass and attempts to collapse it to a single particle fail, see section [*].
= 0 :
no cuts (can be used only with independent fragmentation, at least in principle).
= 1 :
string cuts (as normally required for fragmentation).

MSTP(113) :
(D = 1) (C) recalculation of energies of partons from their momenta and masses, to be done immediately before and after fragmentation, to partly compensate for some numerical problems appearing at high energies.
= 0 :
not performed.
= 1 :
performed.

MSTP(115) :
(D = 0) (C) choice of colour rearrangement scenario for process 25, $\mathrm{W}^+ \mathrm{W}^-$ pair production, when both $\mathrm{W}$'s decay hadronically. (Also works for process 22, $\mathrm{Z}^0 \mathrm{Z}^0$ production, except when the $\mathrm{Z}$'s are allowed to fluctuate to very small masses.) See section [*] for details.
= 0 :
no reconnection.
= 1 :
scenario I, reconnection inspired by a type I superconductor, with the reconnection probability related to the overlap volume in space and time between the $\mathrm{W}^+$ and $\mathrm{W}^-$ strings. Related parameters are found in PARP(115) - PARP(119), with PARP(117) of special interest.
= 2 :
scenario II, reconnection inspired by a type II superconductor, with reconnection possible when two string cores cross. Related parameter in PARP(115).
= 3 :
scenario II', as model II but with the additional requirement that a reconnection will only occur if the total string length is reduced by it.
= 5 :
the GH scenario, where the reconnection can occur that reduces the total string length ($\lambda$ measure) most. PARP(120) gives the fraction of such event where a reconnection is actually made; since almost all events could allow a reconnection that would reduce the string length, PARP(120) is almost the same as the reconnection probability.
= 11 :
the intermediate scenario, where a reconnection is made at the `origin' of events, based on the subdivision of all radiation of a $\mathrm{q}\overline{\mathrm{q}}$ system as coming either from the $\mathrm{q}$ or the $\overline{\mathrm{q}}$. PARP(120) gives the assumed probability that a reconnection will occur. A somewhat simpleminded model, but not quite unrealistic.
= 12 :
the instantaneous scenario, where a reconnection is allowed to occur before the parton showers, and showering is performed inside the reconnected systems with maximum virtuality set by the mass of the reconnected systems. PARP(120) gives the assumed probability that a reconnection will occur. Is completely unrealistic, but useful as an extreme example with very large effects.

MSTP(121) :
(D = 0) calculation of kinematics selection coefficients and differential cross section maxima for included (by you or default) subprocesses.
= 0 :
not known; to be calculated at initialization.
= 1 :
not known; to be calculated at initialization; however, the maximum value then obtained is to be multiplied by PARP(121) (this may be useful if a violation factor has been observed in a previous run of the same kind).
= 2 :
known; kinematics selection coefficients stored by you in COEF(ISUB,J) (J = 1-20) in common block PYINT2 and maximum of the corresponding differential cross section times Jacobians in XSEC(ISUB,1) in common block PYINT5. This is to be done for each included subprocess ISUB before initialization, with the sum of all XSEC(ISUB,1) values, except for ISUB = 95, stored in XSEC(0,1).

MSTP(122) :
(D = 1) initialization and differential cross section maximization print-out. Also, less importantly, level of information on where in phase space a cross section maximum has been violated during the run.
= 0 :
none.
= 1 :
short message at initialization; only when an error (i.e. not a warning) is generated during the run.
= 2 :
detailed message, including full maximization., at initialization; always during run.

MSTP(123) :
(D = 2) reaction to violation of maximum differential cross section or to occurence of negative differential cross sections (except when allowed for external processes, i.e. when IDWTUP < 0).
= 0 :
stop generation, print message.
= 1 :
continue generation, print message for each subsequently larger violation.
= 2 :
as = 1, but also increase value of maximum.

MSTP(124) :
(D = 1) (C) frame for presentation of event.
= 1 :
as specified in PYINIT.
= 2 :
c.m. frame of incoming particles.
= 3 :
hadronic c.m. frame for DIS events, with warnings as given for PYFRAM.

MSTP(125) :
(D = 1) (C) documentation of partonic process, see section [*] for details.
= 0 :
only list ultimate string/particle configuration.
= 1 :
additionally list short summary of the hard process.
= 2 :
list complete documentation of intermediate steps of parton-shower evolution.

MSTP(126) :
(D = 100) number of lines at the beginning of event record that are reserved for event-history information; see section [*]. This value should never be reduced, but may be increased at a later date if more complicated processes are included.

MSTP(127) :
(D = 0) possibility to continue run even if none of the requested processes have non-vanishing cross sections.
= 0 :
no, the run will be stopped in the PYINIT call.
= 1 :
yes, the PYINIT execution will finish normally, but with the flag MSTI(53) = 1 set to signal the problem. If nevertheless PYEVNT is called after this, the run will be stopped, since no events can be generated. If instead a new PYINIT call is made, with changed conditions (e.g. modified supersymmetry parameters in a SUSY run), it may now become possible to initialize normally and generate events.

MSTP(128) :
(D = 0) storing of copy of resonance decay products in the documentation section of the event record, and mother pointer (K(I,3)) relation of the actual resonance decay products (stored in the main section of the event record) to the documentation copy.
= 0 :
products are stored also in the documentation section, and each product stored in the main section points back to the corresponding entry in the documentation section.
= 1 :
products are stored also in the documentation section, but the products stored in the main section point back to the decaying resonance copy in the main section.
= 2 :
products are not stored in the documentation section; the products stored in the main section point back to the decaying resonance copy in the main section.

MSTP(129) :
(D = 10) for the maximization of $2 \to 3$ processes (ISET(ISUB) = 5) each phase-space point in $\tau$, $y$ and $\tau'$ is tested MSTP(129) times in the other dimensions (at randomly selected points) to determine the effective maximum in the ($\tau$, $y$, $\tau'$) point.

MSTP(131) :
(D = 0) master switch for pile-up events, i.e. several independent hadron-hadron interactions generated in the same bunch-bunch crossing, with the events following one after the other in the event record. See section [*] for details.
= 0 :
off, i.e. only one event is generated at a time.
= 1 :
on, i.e. several events are allowed in the same event record. Information on the processes generated may be found in MSTI(41) - MSTI(50).

MSTP(132) - MSTP(134) :
further switches for pile-up events, see section [*].

MSTP(141) :
(D = 0) calling of PYKCUT in the event-generation chain, for inclusion of user-specified cuts.
= 0 :
not called.
= 1 :
called.

MSTP(142) :
(D = 0) calling of PYEVWT in the event-generation chain, either to give weighted events or to modify standard cross sections. See PYEVWT description in section [*] for further details.
= 0 :
not called.
= 1 :
called; the distribution of events among subprocesses and in kinematics variables is modified by the factor WTXS, set by you in the PYEVWT call, but events come with a compensating weight PARI(10) = 1./WTXS, such that total cross sections are unchanged.
= 2 :
called; the cross section itself is modified by the factor WTXS, set by you in the PYEVWT call.

MSTP(143) :
(D = 0) calling of UPVETO in the event-generation chain, to give the possibly to abort the generation of an event.
= 0 :
not called, so no events are aborted (for this reason).
= 1 :
yes, UPVETO is called, from inside the PYEVNT routine (but not from PYEVNW), and a user can then decide whether to abort the current event or not.

MSTP(145) :
(D = 0) choice of polarization state for NRQCD production of charmonium or bottomonium, processes in the ranges 421-439 and 461-479.
= 0 :
unpolarized squared partonic amplitude.
= 1 :
helicity or density matrix elements, as chosen by MSTP(146) and MSTP(147). Only intended for experts.

MSTP(146) :
(D = 1) choice of polarization reference frame when MSTP(145) = 1.
= 1 :
recoil (recommended since it matches how PYTHIA defines particle directions, which the others do not obviously do).
= 2 :
Gottfried-Jackson.
= 3 :
target.
= 4 :
Collins-Soper.

MSTP(147) :
(D = 0) particular helicity or density matrix component when MSTP(145) = 1.
= 0 :
helicity 0.
= 1 :
helicity $\pm 1$.
= 2 :
helicity $\pm 2$.
= 3 :
density matrix element $\rho_{0,0}$.
= 4 :
density matrix element $\rho_{1,1}$.
= 5 :
density matrix element $\rho_{1,0}$.
= 6 :
density matrix element $\rho_{1,-1}$.

MSTP(148) :
(D = 0) possibility to allow final-state shower evolution of the $\c\overline{\mathrm{c}}[^3S_1^{(8)}]$ and $\b\overline{\mathrm{b}}[^3S_1^{(8)}]$ states produced in the NRQCD production of charmonium or bottomonium. Switching it on may exaggerate shower effects, since not all $\mathrm{Q}\overline{\mathrm{Q}}[^3S_1^{(8)}]$ comes from the fragmentation component where radiation is expected.
= 0 :
off.
= 1 :
on.

MSTP(149) :
(D = 0) if the $\mathrm{Q}\overline{\mathrm{Q}}[^3S_1^{(8)}]$ states are allowed to radiate, MSTP(148) = 1, it determines the kinematics of the $\mathrm{Q}\overline{\mathrm{Q}}[^3S_1^{(8)}] \to \mathrm{Q}\overline{\mathrm{Q}}[^3S_1^{(8)}] + \mathrm{g}$ branching.
= 0 :
always pick the $\mathrm{Q}\overline{\mathrm{Q}}[^3S_1^{(8)}]$ to be the harder, i.e. $z > 0.5$.
= 1 :
allow $z < 0.5$ and $z > 0.5$ equally.

MSTP(151) :
(D = 0) introduce smeared position of primary vertex of events.
= 0 :
no, i.e. the primary vertex of each event is at the origin.
= 1 :
yes, with Gaussian distributions separately in $x$, $y$, $z$ and $t$. The respective widths of the Gaussians have to be given in PARP(151) - PARP(154). Also pile-up events obtain separate primary vertices. No provisions are made for more complicated beam-spot shapes, e.g. with a spread in $z$ that varies as a function of $t$. Note that a large beam spot combined with some of the MSTJ(22) options may lead to many particles not being allowed to decay at all.

MSTP(161) :
(D = 0) unit number of file on which PYUPIN should write its initialization info, and from which UPINIT should read it back in, in cases where the Les Houches Accord is used to store PYTHIA hard processes.

MSTP(162) :
(D = 0) unit number of file on which PYUPEV should write its event info, and from which UPEVNT should read it back in, in cases where the Les Houches Accord is used to store PYTHIA hard processes.

MSTP(171) :
(D = 0) possibility of variable energies from one event to the next. For further details see section [*].
= 0 :
no; i.e. the energy is fixed at the initialization call.
= 1 :
yes; i.e. a new energy has to be given for each new event.
Warning:
variable energies cannot be used in conjunction with the internal generation of a virtual photon flux obtained by a PYINIT call with 'gamma/lepton' argument. The reason is that a variable-energy machinery is now used internally for the $\gamma$-hadron or $\gamma\gamma$ subsystem, with some information saved at initialization for the full energy.

MSTP(172) :
(D = 2) options for generation of events with variable energies, applicable when MSTP(171) = 1.
= 1 :
an event is generated at the requested energy, i.e. internally a loop is performed over possible event configurations until one is accepted. If the requested c.m. energy of an event is below PARP(2) the run is aborted. Cross-section information can not be trusted with this option, since it depends on how you decided to pick the requested energies.
= 2 :
only one event configuration is tried. If that is accepted, the event is generated in full. If not, no event is generated, and the status code MSTI(61) = 1 is returned. You are then expected to give a new energy, looping until an acceptable event is found. No event is generated if the requested c.m. energy is below PARP(2), instead MSTI(61) = 1 is set to signal the failure. In principle, cross sections should come out correctly with this option.

MSTP(173) :
(D = 0) possibility for you to give in an event weight to compensate for a biased choice of beam spectrum.
= 0 :
no, i.e. event weight is unity.
= 1 :
yes; weight to be given for each event in PARP(173), with maximum weight given at initialization in PARP(174).

MSTP(181) :
(R) PYTHIA version number.

MSTP(182) :
(R) PYTHIA subversion number.

MSTP(183) :
(R) last year of change for PYTHIA.

MSTP(184) :
(R) last month of change for PYTHIA.

MSTP(185) :
(R) last day of change for PYTHIA.


PARP(1) :
(D = 0.25 GeV) nominal $\Lambda_{\mathrm{QCD}}$ used in running $\alpha_{\mathrm{s}}$ for hard scattering (see MSTP(3)).

PARP(2) :
(D = 10. GeV) lowest c.m. energy for the event as a whole that the program will accept to simulate.

PARP(13) :
(D = 1. GeV$^2$) $Q_{\mathrm{max}}^2$ scale, to be set by you for defining maximum scale allowed for photoproduction when using the option MSTP(13) = 2.

PARP(14) :
(D = 0.01) in the numerical integration of quark and gluon parton distributions inside an electron, the successive halvings of evaluation-point spacing is interrupted when two values agree in relative size, $\vert$new$-$old$\vert$/(new$+$old), to better than PARP(14). There are hardwired lower and upper limits of 2 and 8 halvings, respectively.

PARP(15) :
(D = 0.5 GeV) lower cut-off $p_0$ used to define minimum transverse momentum in branchings $\gamma \to \mathrm{q}\overline{\mathrm{q}}$ in the anomalous event class of $\gamma\mathrm{p}$ interactions, i.e. sets the dividing line between the VMD and GVMD event classes.

PARP(16) :
(D = 1.) the anomalous parton-distribution functions of the photon are taken to have the charm and bottom flavour thresholds at virtuality PARP(16) $\times m_{\mathrm{q}}^2$.

PARP(17) :
(D = 1.) rescaling factor used for the $Q$ argument of the anomalous parton distributions of the photon, see MSTP(15).

PARP(18) :
(D = 0.4 GeV) scale $k_{\rho}$, such that the cross sections of a GVMD state of scale $k_{\perp}$ is suppressed by a factor $k_{\rho}^2/k_{\perp}^2$ relative to those of a VMD state. Should be of order $m_{\rho}/2$, with some finetuning to fit data.

PARP(25) :
(D = 0.) parameter $\eta$ describing the admixture of CP-odd Higgs decays for MSTP(25) = 3.

PARP(31) :
(D = 1.5) common $K$ factor multiplying the differential cross section for hard parton-parton processes when MSTP(33) = 1 or 2, with the exception of colour annihilation graphs in the latter case.

PARP(32) :
(D = 2.0) special $K$ factor multiplying the differential cross section in hard colour annihilation graphs, including resonance production, when MSTP(33) = 2.

PARP(33) :
(D = 0.075) this factor is used to multiply the ordinary $Q^2$ scale in $\alpha_{\mathrm{s}}$ at the hard interaction for MSTP(33) = 3. With the default value, which is only to be taken as an example, the effective $K$ factor thus obtained for jet production is in accordance with the NLO results in [Ell86], modulo the danger of double-counting because of parton-shower corrections to jet rates.

PARP(34) :
(D = 1.) the $Q^2$ scale defined by MSTP(32) is multiplied by PARP(34) when it is used as argument for parton distributions and $\alpha_{\mathrm{s}}$ at the hard interaction. It does not affect $\alpha_{\mathrm{s}}$ when MSTP(33) = 3, nor does it change the $Q^2$ argument of parton showers.

PARP(35) :
(D = 0.20) fix $\alpha_{\mathrm{s}}$ value that is used in the heavy-flavour threshold factor when MSTP(35) = 1.

PARP(36) :
(D = 0. GeV) the width $\Gamma_{\mathrm{Q}}$ for the heavy flavour studied in processes ISUB = 81 or 82; to be used for the threshold factor when MSTP(35) = 2.

PARP(37) :
(D = 1.) for MSTP(37) = 1 this regulates the point at which the reference on-shell quark mass in Higgs and technicolor couplings is assumed defined in PYMRUN calls; specifically the running quark mass is assumed to coincide with the fix one at an energy scale PARP(37) times the fix quark mass, i.e. $m_{\mathrm{running}}($PARP(37) $\times m_{\mathrm{fix}}) =
m_{\mathrm{fix}}$. See discussion at eq. ([*]) on ambiguity of PARP(37) choice.

PARP(38) :
(D = 0.70 GeV$^3$) the squared wave function at the origin, $\vert R(0)\vert^2$, of the $\mathrm{J}/\psi $ wave function. Used for processes 86 and 106-108. See ref. [Glo88].

PARP(39) :
(D = 0.006 GeV$^3$) the squared derivative of the wave function at the origin, $\vert R'(0)\vert^2/m^2$, of the $\chi_{\c }$ wave functions. Used for processes 87-89 and 104-105. See ref. [Glo88].

PARP(41) :
(D = 0.020 GeV) in the process of generating mass for resonances, and optionally to force that mass to be in a given range, only resonances with a total width in excess of PARP(41) are generated according to a Breit-Wigner shape (if allowed by MSTP(42)), while narrower resonances are put on the mass shell.

PARP(42) :
(D = 2. GeV) minimum mass of resonances assumed to be allowed when evaluating total width of $\mathrm{h}^0$ to $\mathrm{Z}^0 \mathrm{Z}^0$ or $\mathrm{W}^+ \mathrm{W}^-$ for cases when the $\mathrm{h}^0$ is so light that (at least) one $\mathrm{Z}/\mathrm{W}$ is forced to be off the mass shell. Also generally used as safety check on minimum mass of resonance. Note that some CKIN values may provide additional constraints.

PARP(43) :
(D = 0.10) precision parameter used in numerical integration of width for a channel with at least one daughter off the mass shell.

PARP(44) :
(D = 1000.) the $\nu$ parameter of the strongly interacting $\mathrm{Z}/\mathrm{W}$ model of Dobado, Herrero and Terron [Dob91]; see MSTP(46) = 3.

PARP(45) :
(D = 2054. GeV) the effective techni-$\rho$ mass parameter of the strongly interacting model of Dobado, Herrero and Terron [Dob91]; see MSTP(46) = 5. On physical grounds it should not be chosen smaller than about 1 TeV or larger than about the default value.

PARP(46) :
(D = 123. GeV) the $F_{\pi}$ decay constant that appears inversely quadratically in all techni-$\eta$ partial decay widths [Eic84,App92].

PARP(47) :
(D = 246. GeV) vacuum expectation value $v$ used in the DHT scenario [Dob91] to define the width of the techni-$\rho$; this width is inversely proportional $v^2$.

PARP(48) :
(D = 50.) the Breit-Wigner factor in the cross section is set to vanish for masses that deviate from the nominal one by more than PARP(48) times the nominal resonance width (i.e. the width evaluated at the nominal mass). Is used in most processes with a single $s$-channel resonance, but there are some exceptions, notably $\gamma^* / \mathrm{Z}^0$ and $\mathrm{W}^{\pm}$. The reason for this option is that the conventional Breit-Wigner description is at times not really valid far away from the resonance position, e.g. because of interference with other graphs that should then be included. The wings of the Breit-Wigner can therefore be removed.

PARP(50) :
(D = 0.054) dimensionless coupling, which enters quadratically in all partial widths of the excited graviton $\mathrm{G}^*$ resonance, is $\kappa m_{\mathrm{G}^*} = \sqrt{2} x_1 k /\overline{M}_{\mathrm{Pl}}$, where $x_1 \approx 3.83$ is the first zero of the $J_1$ Bessel function and $\overline{M}_{\mathrm{Pl}}$ is the modified Planck mass scale [Ran99,Bij01].

PARP(61) - PARP(65) :
(C) parameters for initial-state radiation, see section [*].

PARP(71) - PARP(72) :
(C) parameter for final-state radiation, see section [*].

PARP(78) - PARP(90) :
parameters for multiple interactions, see section [*].

PARP(91) - PARP(100) :
parameters for beam-remnant treatment, see section [*].

PARP(101) :
(D = 0.50) fraction of diffractive systems in which a quark is assumed kicked out by the pomeron rather than a gluon; applicable for option MSTP(101) = 3.

PARP(102) :
(D = 0.28 GeV) the mass spectrum of diffractive states (in single and double diffractive scattering) is assumed to start PARP(102) above the mass of the particle that is diffractively excited. In this connection, an incoming $\gamma$ is taken to have the selected VMD meson mass, i.e. $m_{\rho}$, $m_{\omega}$, $m_{\phi}$ or $m_{\mathrm{J}/\psi }$.

PARP(103) :
(D = 1.0 GeV) if the mass of a diffractive state is less than PARP(103) above the mass of the particle that is diffractively excited, the state is forced to decay isotropically into a two-body channel. In this connection, an incoming $\gamma$ is taken to have the selected VMD meson mass, i.e. $m_{\rho}$, $m_{\omega}$, $m_{\phi}$ or $m_{\mathrm{J}/\psi }$. If the mass is higher than this threshold, the standard string fragmentation machinery is used. The forced two-body decay is always carried out, also when MSTP(111) = 0.

PARP(104) :
(D = 0.8 GeV) minimum energy above threshold for which hadron-hadron total, elastic and diffractive cross sections are defined. Below this energy, an alternative description in terms of specific few-body channels would have been required, and this is not modelled in PYTHIA.

PARP(110) :
(D = 1.) a rescaling factor for resonance widths, applied when MSTP(110) is switched on.

PARP(111) :
(D = 2. GeV) used to define the minimum invariant mass of the remnant hadronic system (i.e. when interacting partons have been taken away), together with original hadron masses and extra parton masses. For a hadron or resolved photon beam, this also implies a further constraint that the $x$ of an interacting parton be below $1 - 2 \times \mbox{\texttt{PARP(111)}}/E_{\mathrm{cm}}$.

PARP(115) :
(D = 1.5 fm) (C) the average fragmentation time of a string, giving the exponential suppression that a reconnection cannot occur if strings decayed before crossing. Is implicitly fixed by the string constant and the fragmentation function parameters, and so a significant change is not recommended.

PARP(116) :
(D = 0.5 fm) (C) width of the type I string in reconnection calculations, giving the radius of the Gaussian distribution in $x$ and $y$ separately.

PARP(117) :
(D = 0.6) (C) $k_{\mathrm{I}}$, the main free parameter in the reconnection probability for scenario I; the probability is given by PARP(117) times the overlap volume, up to saturation effects.

PARP(118), PARP(119) :
(D = 2.5, 2.0) (C) $f_r$ and $f_t$, respectively, used in the Monte Carlo sampling of the phase space volume in scenario I. There is no real reason to change these numbers.

PARP(120) :
(D = 1.0) (D) (C) fraction of events in the GH, intermediate and instantaneous scenarios where a reconnection is allowed to occur. For the GH one a further suppression of the reconnection rate occurs from the requirement of reduced string length in a reconnection.

PARP(121) :
(D = 1.) the maxima obtained at initial maximization are multiplied by this factor if MSTP(121) = 1; typically PARP(121) would be given as the product of the violation factors observed (i.e. the ratio of final maximum value to initial maximum value) for the given process(es).

PARP(122) :
(D = 0.4) fraction of total probability that is shared democratically between the COEF coefficients open for the given variable, with the remaining fraction distributed according to the optimization results of PYMAXI.

PARP(131) :
parameter for pile-up events, see section [*].

PARP(141) - PARP(150) :
(D = 10*1.) matrix elements for charmonium and bottomonium production in the non-relativistic QCD framework (NRQCD). Current values are dummy only, and will be updated soon. These values are used in processes 421-439 and 461-479.
PARP(141) :
$\langle \mathcal{O}^{\mathrm{J}/\psi }[^3S_1^{(1)}] \rangle$.
PARP(142) :
$\langle \mathcal{O}^{\mathrm{J}/\psi }[^3S_1^{(8)}] \rangle$.
PARP(143) :
$\langle \mathcal{O}^{\mathrm{J}/\psi }[^1S_0^{(8)}] \rangle$.
PARP(144) :
$\langle \mathcal{O}^{\mathrm{J}/\psi }[^3P_0^{(8)}] %
\rangle / m_{\c }^2$.
PARP(145) :
$\langle \mathcal{O}^{\chi_{\c0}}[^3P_0^{(1)}] %
\rangle / m_{\c }^2$.
PARP(146) :
$\langle \mathcal{O}^{\Upsilon}[^3S_1^{(1)}] \rangle$.
PARP(147) :
$\langle \mathcal{O}^{\Upsilon}[^3S_1^{(8)}] \rangle$.
PARP(148) :
$\langle \mathcal{O}^{\Upsilon}[^1S_0^{(8)}] \rangle$.
PARP(149) :
$\langle \mathcal{O}^{\Upsilon}[^3P_0^{(8)}] %
\rangle / m_{\b }^2$.
PARP(150) :
$\langle \mathcal{O}^{\chi_{\b0}}[^3P_0^{(1)}] %
\rangle / m_{\b }^2$.

PARP(151) - PARP(154) :
(D = 4*0.) (C) regulate the assumed beam-spot size. For MSTP(151) = 1 the $x$, $y$, $z$ and $t$ coordinates of the primary vertex of each event are selected according to four independent Gaussians. The widths of these Gaussians are given by the four parameters, where the first three are in units of mm and the fourth in mm/$c$.

PARP(161) - PARP(164) :
(D = 2.20, 23.6, 18.4, 11.5) couplings $f_V^2/4\pi$ of the photon to the $\rho^0$, $\omega$, $\phi$ and $\mathrm{J}/\psi $ vector mesons.

PARP(165) :
(D = 0.5) a simple multiplicative factor applied to the cross section for the transverse resolved photons to take into account the effects of longitudinal resolved photons, see MSTP(17). No preferred value, but typically one could use PARP(165) = 1 as main contrast to the no-effect = 0, with the default arbitrarily chosen in the middle.

PARP(167), PARP(168) :
(D = 2*0) the longitudinal energy fraction $y$ of an incoming photon, side 1 or 2, used in the $R$ expression given for MSTP(17) to evaluate $f_L(y,Q^2)/f_T(y,Q^2)$. Need not be supplied when a photon spectrum is generated inside a lepton beam, but only when a photon is directly given as argument in the PYINIT call.

PARP(171) :
to be set, event-by-event, when variable energies are allowed, i.e. when MSTP(171) = 1. If PYINIT is called with FRAME = 'CMS' (= 'FIXT'), PARP(171) multiplies the c.m. energy (beam energy) used at initialization. For the options '3MOM', '4MOM' and '5MOM', PARP(171) is dummy, since there the momenta are set in the P array. It is also dummy for the 'USER' option, where the choice of variable energies is beyond the control of PYTHIA.

PARP(173) :
event weight to be given by you when MSTP(173) = 1.

PARP(174) :
(D = 1.) maximum event weight that will be encountered in PARP(173) during the course of a run with MSTP(173) = 1; to be used to optimize the efficiency of the event generation. It is always allowed to use a larger bound than the true one, but with a corresponding loss in efficiency.

PARP(181) - PARP(189) :
(D = 0.1, 0.01, 0.01, 0.01, 0.1, 0.01, 0.01, 0.01, 0.3) Yukawa couplings of leptons to $\H ^{++}$, assumed same for $\H _L^{++}$ and $\H _R^{++}$. Is a symmetric $3 \times 3$ array, where PARP(177+3*i+j) gives the coupling to a lepton pair with generation indices $i$ and $j$. Thus the default matrix is dominated by the diagonal elements and especially by the $\tau\tau$ one.

PARP(190) :
(D = 0.64) $g_L = e/\sin\theta_W$.

PARP(191) :
(D = 0.64) $g_R$, assumed same as $g_L$.

PARP(192) :
(D = 5 GeV) $v_L$ vacuum expectation value of the left-triplet. The corresponding $v_R$ is assumed given by $v_R = \sqrt{2} M_{\mathrm{W}_R} / g_R$ and is not stored explicitly.

PARP(193) :
(D = 1D4 GeV$^2$) factorization scale $Q^2$ for parton densities, to be set by user when MSTP(32) = 12 for $2 \to 2$ processes or MSTP(39) = 8 for $2 \to 3$ ones.

PARP(194) :
(D = 1D4 GeV$^2$) renormalization scale $Q^2$, to be set by user when MSTP(32) = 12 for $2 \to 2$ processes or MSTP(39) = 8 for $2 \to 3$ ones. For process 161 it also sets the scale of running quark masses.


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Next: Further Couplings Up: The Process Generation Program Previous: Switches for Event Type   Contents
Stephen Mrenna 2007-10-30