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## Switches for Event Type and Kinematics Selection

By default, if PYTHIA is run for a hadron collider, only QCD processes are generated, composed of hard interactions above PARP(81), with low- processes added on so as to give the full (parameterized) inelastic, non-diffractive cross section. In an collider, production is the default, and in an one it is Deeply Inelastic Scattering. With the help of the common block PYSUBS, it is possible to select the generation of another process, or combination of processes. It is also allowed to restrict the generation to specific incoming partons/particles at the hard interaction. This often automatically also restricts final-state flavours but, in processes such as resonance production or QCD/QED production of new flavours, switches in the PYTHIA program may be used to this end; see section .

The CKIN array may be used to impose specific kinematics cuts. You should here be warned that, if kinematical variables are too strongly restricted, the generation time per event may become very long. In extreme cases, where the cuts effectively close the full phase space, the event generation may run into an infinite loop. The generation of resonance production is performed in terms of the and variables, and so the ranges CKIN(1) - CKIN(2) and CKIN(7) - CKIN(8) may be arbitrarily restricted without a significant loss of speed. For processes, is added as a third generation variable, and so additionally the range CKIN(27) - CKIN(28) may be restricted without any loss of efficiency.

Effects from initial- and final-state radiation are not included, since they are not known at the time the kinematics at the hard interaction is selected. The sharp kinematical cut-offs that can be imposed on the generation process are therefore smeared, both by QCD radiation and by fragmentation. A few examples of such effects follow.

Initial-state radiation implies that each of the two incoming partons has a non-vanishing when they interact. The hard scattering subsystem thus receives a net transverse boost, and is rotated with respect to the beam directions. In a process, what typically happens is that one of the scattered partons receives an increased , while the of the other parton can be reduced or increased, depending on the detailed topology.
Since the initial-state radiation machinery assigns space-like virtualities to the incoming partons, the definitions of in terms of energy fractions and in terms of momentum fractions no longer coincide, and so the interacting subsystem may receive a net longitudinal boost compared with naïve expectations, as part of the parton-shower machinery.
Initial-state radiation gives rise to additional jets, which in extreme cases may be mistaken for either of the jets of the hard interaction.
Final-state radiation gives rise to additional jets, which smears the meaning of the basic scattering. The assignment of soft jets is not unique. The energy of a jet becomes dependent on the way it is identified, e.g. what jet cone size is used.
The beam-remnant description assigns primordial values, which also gives a net shift of the hard-interaction subsystem; except at low energies this effect is overshadowed by initial-state radiation, however. Beam remnants may also add further activity under the perturbative' event.
Fragmentation will further broaden jet profiles, and make jet assignments and energy determinations even more uncertain.
In a study of events within a given window of experimentally defined variables, it is up to you to leave such liberal margins that no events are missed. In other words, cuts have to be chosen such that a negligible fraction of events migrate from outside the simulated region to inside the interesting region. Often this may lead to low efficiency in terms of what fraction of the generated events are actually of interest to you. See also section .

In addition to the variables found in PYSUBS, also those in the PYPARS common block may be used to select exactly what one wants to have simulated. These possibilities will be described in the following section.

The notation used above and in the following is that ' denotes internal variables in the hard-scattering subsystem, while ' is for variables in the c.m. frame of the event as a whole.

Purpose:
to allow you to run the program with any desired subset of processes, or restrict flavours or kinematics. If the default values, denoted below by (D = ...), are not satisfactory, they must be changed before the PYINIT call.

MSEL :
(D = 1) a switch to select between full user control and some preprogrammed alternatives.
= 0 :
desired subprocesses have to be switched on in MSUB, i.e. full user control.
= 1 :
depending on incoming particles, different alternatives are used.
Lepton-lepton: or production (ISUB = 1 or 2).
Lepton-hadron: Deeply Inelastic Scattering (ISUB = 10; this option is now out of date for most applications, superseded by the 'gamma/lepton' machinery).
Hadron-hadron: QCD high- processes (ISUB = 11, 12, 13, 28, 53, 68); additionally low- production if CKIN(3) PARP(81) or PARP(82), depending on MSTP(82) (ISUB = 95). If low- is switched on, the other CKIN cuts are not used.
A resolved photon counts as hadron. When the photon is not resolved, the following cases are possible.
Photon-lepton: Compton scattering (ISUB = 34).
Photon-hadron: photon-parton scattering (ISUB = 33, 34, 54).
Photon-photon: fermion pair production (ISUB = 58).
When photons are given by the 'gamma/lepton' argument in the PYINIT call, the outcome depends on the MSTP(14) value. Default is a mixture of many kinds of processes, as described in section .
= 2 :
as MSEL = 1 for lepton-lepton, lepton-hadron and unresolved photons. For hadron-hadron (including resolved photons) all QCD processes, including low-, single and double diffractive and elastic scattering, are included (ISUB = 11, 12, 13, 28, 53, 68, 91, 92, 93, 94, 95). The CKIN cuts are here not used.
For photons given with the 'gamma/lepton' argument in the PYINIT call, the above processes are replaced by other ones that also include the photon virtuality in the cross sections. The principle remains to include both high- and low- processes, however.
= 4 :
charm ( ) production with massive matrix elements (ISUB = 81, 82, 84, 85).
= 5 :
bottom ( ) production with massive matrix elements (ISUB = 81, 82, 84, 85).
= 6 :
top ( ) production with massive matrix elements (ISUB = 81, 82, 84, 85).
= 7 :
fourth generation ( ) production with massive matrix elements (ISUB = 81, 82, 84, 85).
= 8 :
fourth generation ( ) production with massive matrix elements (ISUB = 81, 82, 84, 85).
= 10 :
prompt photons (ISUB = 14, 18, 29).
= 11 :
production (ISUB = 1).
= 12 :
production (ISUB = 2).
= 13 :
+ jet production (ISUB = 15, 30).
= 14 :
+ jet production (ISUB = 16, 31).
= 15 :
pair production of different combinations of , and (except ; see MSEL = 10) (ISUB = 19, 20, 22, 23, 25).
= 16 :
production (ISUB = 3, 102, 103, 123, 124).
= 17 :
or (ISUB = 24, 26).
= 18 :
production, combination relevant for annihilation (ISUB = 24, 103, 123, 124).
= 19 :
, and production, excepting pair production (ISUB = 24, 103, 123, 124, 153, 158, 171, 173, 174, 176, 178, 179).
= 21 :
production (ISUB = 141).
= 22 :
production (ISUB = 142).
= 23 :
production (ISUB = 143).
= 24 :
production (ISUB = 144).
= 25 :
(leptoquark) production (ISUB = 145, 162, 163, 164).
= 35 :
single bottom production by exchange (ISUB = 83).
= 36 :
single top production by exchange (ISUB = 83).
= 37 :
single production by exchange (ISUB = 83).
= 38 :
single production by exchange (ISUB = 83).
= 39 :
all MSSM processes except Higgs production.
= 40 :
squark and gluino production (ISUB = 243, 244, 258, 259, 271-280).
= 41 :
stop pair production (ISUB = 261-265).
= 42 :
slepton pair production (ISUB = 201-214).
= 43 :
squark or gluino with chargino or neutralino, (ISUB = 237-242, 246-256).
= 44 :
chargino-neutralino pair production (ISUB = 216-236).
= 45 :
sbottom production (ISUB = 281-296).
= 50 :
pair production of technipions and gauge bosons by exchange (ISUB = 361-377).
= 51 :
standard QCD processes 381-386, with possibility to introduce compositeness/technicolor modifications, see ITCM(5).
= 61 :
charmonimum production in the NRQCD framework, (ISUB = 421-439).
= 62 :
bottomonimum production in the NRQCD framework, (ISUB = 461-479).
= 63 :
both charmonimum and bottomonimum production in the NRQCD framework, (ISUB = 421-439, 461-479).

MSUB :
(D = 500*0) array to be set when MSEL = 0 (for MSEL relevant entries are set in PYINIT) to choose which subset of subprocesses to include in the generation. The ordering follows the ISUB code given in section (with comments as given there).
MSUB(ISUB) = 0 :
the subprocess is excluded.
MSUB(ISUB) = 1 :
the subprocess is included.
Note:
when MSEL = 0, the MSUB values set by you are never changed by PYTHIA. If you want to combine several different subruns', each with its own PYINIT call, into one single run, it is up to you to remember not only to switch on the new processes before each new PYINIT call, but also to switch off the old ones that are no longer desired.

KFIN(I,J) :
provides an option to selectively switch on and off contributions to the cross sections from the different incoming partons/particles at the hard interaction. In combination with the PYTHIA resonance decay switches, this also allows you to set restrictions on flavours appearing in the final state.
I :
is 1 for beam side of event and 2 for target side.
J :
enumerates flavours according to the KF code; see section .
KFIN(I,J) = 0 :
the parton/particle is forbidden.
KFIN(I,J) = 1 :
the parton/particle is allowed.
Note:
by default, the following are switched on: , , , , , , , , , , , , , , and their antiparticles. In particular, top is off, and has to be switched on explicitly if needed.

CKIN :
kinematics cuts that can be set by you before the PYINIT call, and that affect the region of phase space within which events are generated. Some cuts are hardwired' while most are softwired'. The hardwired ones are directly related to the kinematical variables used in the event selection procedure, and therefore have negligible effects on program efficiency. The most important of these are CKIN(1) - CKIN(8), CKIN(27) - CKIN(28), and CKIN(31) - CKIN(32). The softwired ones are most of the remaining ones, that cannot be fully taken into account in the kinematical variable selection, so that generation in constrained regions of phase space may be slow. In extreme cases the phase space may be so small that the maximization procedure fails to find any allowed points at all (although some small region might still exist somewhere), and therefore switches off some subprocesses, or aborts altogether.

CKIN(1), CKIN(2) :
(D = 2., GeV) range of allowed values. If CKIN(2) , the upper limit is inactive.

CKIN(3), CKIN(4) :
(D = 0., GeV) range of allowed values for hard processes, with transverse momentum defined in the rest frame of the hard interaction. If CKIN(4) , the upper limit is inactive. For processes that are singular in the limit (see CKIN(6)), CKIN(5) provides an additional constraint. The CKIN(3) and CKIN(4) limits can also be used in processes. Here, however, the product masses are not known and hence are assumed to be vanishing in the event selection. The actual range for massive products is thus shifted downwards with respect to the nominal one.
Note 1:
for processes that are singular in the limit , a careful choice of CKIN(3) value is not only a matter of technical convenience, but a requirement for obtaining sensible results. One example is the hadroproduction of a or gauge boson together with a jet, discussed in section . Here the point is that this is a first-order process (in ), correcting the zeroth-order process of a or without any jet. A full first-order description would also have to include virtual corrections in the low- region.
Generalizing also to other processes, the simple-minded higher-order description breaks down when CKIN(3) is selected so small that the higher-order process cross section corresponds to a non-negligible fraction of the lower-order one. This number will vary depending on the process considered and the c.m. energy used, but could easily be tens of GeV rather than the default 1 GeV provided as technical cut-off in CKIN(5). Processes singular in should therefore only be used to describe the high- behaviour, while the lowest-order process complemented with parton showers should give the inclusive distribution and in particular the one at small values.
Technically the case of QCD production of two jets is slightly more complicated, and involves eikonalization to multiple parton-parton scattering, section , but again the conclusion is that the processes have to be handled with care at small values.
Note 2:
there are a few situations in which CKIN(3) may be overwritten; especially when different subprocess classes are mixed in or collisions, see section .

CKIN(5) :
(D = 1. GeV) lower cut-off on values, in addition to the CKIN(3) cut above, for processes that are singular in the limit (see CKIN(6)).

CKIN(6) :
(D = 1. GeV) hard processes, which do not proceed only via an intermediate resonance (i.e. are processes), are classified as singular in the limit if either or both of the two final-state products has a mass CKIN(6).

CKIN(7), CKIN(8) :
(D = , 10.) range of allowed scattering subsystem rapidities in the c.m. frame of the event, where . (Following the notation of this section, the variable should be given as , but because of its frequent use, it was called in section .)

CKIN(9), CKIN(10) :
(D = , 40.) range of allowed (true) rapidities for the product with largest rapidity in a or a process, defined in the c.m. frame of the event, i.e. . Note that rapidities are counted with sign, i.e. if and then .

CKIN(11), CKIN(12) :
(D = , 40.) range of allowed (true) rapidities for the product with smallest rapidity in a or a process, defined in the c.m. frame of the event, i.e. . Consistency thus requires CKIN(11) CKIN(9) and CKIN(12) CKIN(10).

CKIN(13), CKIN(14) :
(D = , 40.) range of allowed pseudorapidities for the product with largest pseudorapidity in a or a process, defined in the c.m. frame of the event, i.e. . Note that pseudorapidities are counted with sign, i.e. if and then .

CKIN(15), CKIN(16) :
(D = , 40.) range of allowed pseudorapidities for the product with smallest pseudorapidity in a or a process, defined in the c.m. frame of the event, i.e. . Consistency thus requires CKIN(15) CKIN(13) and CKIN(16) CKIN(14).

CKIN(17), CKIN(18) :
(D = , 1.) range of allowed values for the product with largest value in a or a process, defined in the c.m. frame of the event, i.e. .

CKIN(19), CKIN(20) :
(D = , 1.) range of allowed values for the product with smallest value in a or a process, defined in the c.m. frame of the event, i.e. . Consistency thus requires CKIN(19) CKIN(17) and CKIN(20) CKIN(18).

CKIN(21), CKIN(22) :
(D = 0., 1.) range of allowed values for the parton on side 1 that enters the hard interaction.

CKIN(23), CKIN(24) :
(D = 0., 1.) range of allowed values for the parton on side 2 that enters the hard interaction.

CKIN(25), CKIN(26) :
(D = , 1.) range of allowed Feynman- values, where .

CKIN(27), CKIN(28) :
(D = , 1.) range of allowed values in a hard scattering, where is the scattering angle in the rest frame of the hard interaction.

CKIN(31), CKIN(32) :
(D = 2., GeV) range of allowed values, where is the mass of the complete three- or four-body final state in or processes (while , constrained in CKIN(1) and CKIN(2), here corresponds to the one- or two-body central system). If CKIN(32) , the upper limit is inactive.

CKIN(35), CKIN(36) :
(D = 0., GeV) range of allowed values in processes. Note that for Deeply Inelastic Scattering this is nothing but the scale, in the limit that initial- and final-state radiation is neglected. If CKIN(36) , the upper limit is inactive.

CKIN(37), CKIN(38) :
(D = 0., GeV) range of allowed values in processes. If CKIN(38) , the upper limit is inactive.

CKIN(39), CKIN(40) :
(D = 4., GeV) the range allowed in DIS processes, i.e. subprocess number 10. If CKIN(40) , the upper limit is inactive. Here is defined in terms of . This formula is not quite correct, in that (i) it neglects the target mass (for a proton), and (ii) it neglects initial-state photon radiation off the incoming electron. It should be good enough for loose cuts, however. These cuts are not checked if process 10 is called for two lepton beams.

CKIN(41) - CKIN(44) :
(D = 12., , 12., GeV) range of allowed mass values of the two (or one) resonances produced in a true' process, i.e. one not (only) proceeding through a single -channel resonance (). (These are the ones listed as in the tables in section .) Only particles with a width above PARP(41) are considered as bona fide resonances and tested against the CKIN limits; particles with a smaller width are put on the mass shell without applying any cuts. The exact interpretation of the CKIN variables depends on the flavours of the two produced resonances.
For two resonances like (produced from ), which are not identical and which are not each other's antiparticles, one has
CKIN(41) CKIN(42), and
CKIN(43) CKIN(44),
where and are the actually generated masses of the two resonances, and 1 and 2 are defined by the order in which they are given in the production process specification.
For two resonances like , which are identical, or , which are each other's antiparticles, one instead has
CKIN(41) CKIN(42), and
CKIN(43) CKIN(44).
In addition, whatever limits are set on CKIN(1) and, in particular, on CKIN(2) obviously affect the masses actually selected.
Note 1:
if MSTP(42) = 0, so that no mass smearing is allowed, the CKIN values have no effect (the same as for particles with too narrow a width).
Note 2:
if CKIN(42) CKIN(41) it means that the CKIN(42) limit is inactive; correspondingly, if CKIN(44) CKIN(43) then CKIN(44) is inactive.
Note 3:
if limits are active and the resonances are identical, it is up to you to ensure that CKIN(41) CKIN(43) and CKIN(42) CKIN(44).
Note 4:
for identical resonances, it is not possible to preselect which of the resonances is the lighter one; if, for instance, one is to decay to leptons and the other to quarks, there is no mechanism to guarantee that the lepton pair has a mass smaller than the quark one.
Note 5:
the CKIN values are applied to all relevant processes equally, which may not be what one desires if several processes are generated simultaneously. Some caution is therefore urged in the use of the CKIN(41) - CKIN(44) values. Also in other respects, you are recommended to take proper care: if a is only allowed to decay into , for example, setting its mass range to be 2-8 GeV is obviously not a good idea.

CKIN(45) - CKIN(48) :
(D = 12., , 12., GeV) range of allowed mass values of the two (or one) secondary resonances produced in a process (like ) or even a (or 3) process (like ). Note that these CKIN values only affect the secondary resonances; the primary ones are constrained by CKIN(1), CKIN(2) and CKIN(41) - CKIN(44) (indirectly, of course, the choice of primary resonance masses affects the allowed mass range for the secondary ones). What is considered to be a resonance is defined by PARP(41); particles with a width smaller than this are automatically put on the mass shell. The description closely parallels the one given for CKIN(41) - CKIN(44). Thus, for two resonances that are not identical or each other's antiparticles, one has
CKIN(45) CKIN(46), and
CKIN(47) CKIN(48),
where and are the actually generated masses of the two resonances, and 1 and 2 are defined by the order in which they are given in the decay channel specification in the program (see e.g. output from PYSTAT(2) or PYLIST(12)). For two resonances that are identical or each other's antiparticles, one instead has
CKIN(45) CKIN(46), and
CKIN(47) CKIN(48).
Notes 1 - 5:
as for CKIN(41) - CKIN(44), with trivial modifications.
Note 6:
setting limits on secondary resonance masses is possible in any of the channels of the allowed types (see above). However, so far only and have been fully implemented, such that an arbitrary mass range below the naïve mass threshold may be picked. For other possible resonances, any restrictions made on the allowed mass range are not reflected in the cross section; and further it is not recommendable to pick mass windows that make a decay on the mass shell impossible.

CKIN(49) - CKIN(50) :
allow minimum mass limits to be passed from PYRESD to PYOFSH. They are used for tertiary and higher resonances, i.e. those not controlled by CKIN(41) - CKIN(48). They should not be touched by the user.

CKIN(51) - CKIN(56) :
(D = 0., , 0., , 0., GeV) range of allowed transverse momenta in a true process. This means subprocesses such as 121-124 for production, and their , and equivalents. CKIN(51) - CKIN(54) corresponds to ranges for scattered partons, in order of appearance, i.e. CKIN(51) - CKIN(52) for the parton scattered off the beam and CKIN(53) - CKIN(54) for the one scattered off the target. CKIN(55) and CKIN(56) here sets limits for the third product, the , i.e. the CKIN(3) and CKIN(4) values have no effect for this process. Since the of the Higgs is not one of the primary variables selected, any constraints here may mean reduced Monte Carlo efficiency, while for these processes CKIN(51) - CKIN(54) are hardwired' and therefore do not cost anything. As usual, a negative value implies that the upper limit is inactive.

CKIN(61) - CKIN(78) :
allows to restrict the range of kinematics for the photons generated off the lepton beams with the 'gamma/lepton' option of PYINIT. In each quartet of numbers, the first two corresponds to the range allowed on incoming side 1 (beam) and the last two to side 2 (target). The cuts are only applicable for a lepton beam. Note that the and () variables are the basis for the generation, and so can be restricted with no loss of efficiency. For leptoproduction (i.e. lepton on hadron) the is uniquely given by the one value of the problem, so here also cuts are fully efficient. The other cuts may imply a slowdown of the program, but not as much as if equivalent cuts only are introduced after events are fully generated. See [Fri00] for details.

CKIN(61) - CKIN(64) :
(D = 0.0001, 0.99, 0.0001, 0.99) allowed range for the energy fractions that the photon take of the respective incoming lepton energy. These fractions are defined in the c.m. frame of the collision, and differ from energy fractions as defined in another frame. (Watch out at HERA!) In order to avoid some technical problems, absolute lower and upper limits are set internally at 0.0001 and 0.9999.

CKIN(65) - CKIN(68) :
(D = 0., , 0., GeV) allowed range for the space-like virtuality of the photon, conventionally called either or , depending on process. A negative number means that the upper limit is inactive, i.e. purely given by kinematics. A nonzero lower limit is implicitly given by kinematics constraints.

CKIN(69) - CKIN(72) :
(D = 0., , 0., ) allowed range of the scattering angle of the lepton, defined in the c.m. frame of the event. (Watch out at HERA!) A negative number means that the upper limit is inactive, i.e. equal to .

CKIN(73) - CKIN(76) :
(D = 0.0001, 0.99, 0.0001, 0.99) allowed range for the light-cone fraction that the photon take of the respective incoming lepton energy. The light-cone is defined by the four-momentum of the lepton or hadron on the other side of the event (and thus deviates from true light-cone fraction by mass effects that normally are negligible). The value is related to the and () values by if mass terms are neglected.

CKIN(77), CKIN(78) :
(D = 2., GeV) allowed range for , i.e. either the photon-hadron or photon-photon invariant mass. A negative number means that the upper limit is inactive.

Next: The General Switches and Up: The Process Generation Program Previous: The Main Subroutines   Contents
Stephen Mrenna 2007-10-30