The status codes and parameters relevant for the
routines are
found in the common block `PYDAT1`. This common block also contains
more general status codes and parameters, described elsewhere.

**Purpose:**- to give access to a number of status codes and
parameters regulating the performance of the
event generation
routines.
`MSTJ(101) :`- (D = 5) gives the type of QCD
corrections used for continuum events.
`= 0 :`- only events are generated.
`= 1 :`- events are generated according to first-order QCD.
`= 2 :`- events are generated according to second-order QCD.
`= 3 :`- events are generated, but without second-order corrections to the 3-jet rate.
`= 5 :`- a parton shower is allowed to develop from an original
pair, see
`MSTJ(38) - MSTJ(50)`for details. `= -1 :`- only
events are generated (within same
matrix-element cuts as for
`= 1`). Since the change in flavour composition from mass cuts or radiative corrections is not taken into account, this option is not intended for quantitative studies. `= -2 :`- only
and
events are
generated (as for
`= 2`). The same warning as for`= -1`applies. `= -3 :`- only
events are generated (as for
`= 2`). The same warning as for`= -1`applies. `= -4 :`- only
events are generated
(as for
`= 2`). The same warning as for`= -1`applies. **Note 1:**`MSTJ(101)`is also used in`PYONIA`, with`4 :`- events are generated according to lowest-order matrix elements.
`5 :`- a parton shower is allowed to develop from the
original
or
configuration, see
`MSTJ(38) - MSTJ(50)`for details. **Note 2:**- the default values of fragmentation parameters have
been chosen to work well with the default parton-shower approach
above. If any of the other options are used, or if the parton
shower is used in non-default mode, it is normally necessary to
retune fragmentation parameters. As an example, we note that
the second-order matrix-element approach (
`MSTJ(101) = 2`) at PETRA/PEP energies gives a better description when the and parameters of the symmetric fragmentation function are set to`PARJ(41) = 1`,`PARJ(42) = 0.7`, and the width of the transverse momentum distribution to`PARJ(21) = 0.40`. In principle, one also ought to change the joining parameter to`PARJ(33) = PARJ(35) = 1.1`to preserve a flat rapidity plateau, but if this should be forgotten, it does not make too much difference. For applications at TRISTAN or LEP, one has to change the matrix-element approach parameters even more, to make up for additional soft gluon effects not covered in this approach.

`MSTJ(102) :`- (D = 2) inclusion of weak effects ( exchange)
for flavour production, angular orientation, cross sections and
initial-state photon radiation in continuum events.
`= 1 :`- QED, i.e. no weak effects are included.
`= 2 :`- QFD, i.e. including weak effects.
`= 3 :`- as
`= 2`, but at initialization in`PYXTEE`the width is calculated from , and and quark masses (including bottom and top threshold factors for`MSTJ(103)`odd), assuming three full generations, and the result is stored in`PARJ(124)`.

`MSTJ(103) :`- (D = 7) mass effects in continuum matrix elements,
in the form
`MSTJ(103)`, where if no mass effects and if mass effects should be included. Here;`:`- threshold factor for new flavour production according to QFD result;
`:`- gluon emission probability (only applies for
`|MSTJ(101)|`, otherwise no mass effects anyhow); `:`- angular orientation of event (only applies for
`|MSTJ(101)|`and`MSTJ(102) = 1`, otherwise no mass effects anyhow).

`MSTJ(104) :`- (D = 5) number of allowed flavours, i.e. flavours
that can be produced in a continuum event if the energy is enough.
A change to 6 makes top production allowed above the threshold, etc.
Note that in
events only the first five flavours
are allowed in the secondary pair, produced by a gluon breakup.
`MSTJ(105) :`- (D = 1) fragmentation and decay in
`PYEEVT`and`PYONIA`calls.`= 0 :`- no
`PYEXEC`calls, i.e. only matrix-element and/or parton-shower treatment, and collapse of small jet systems into one or two particles (in`PYPREP`). `= 1 :``PYEXEC`calls are made to generate fragmentation and decay chain.`= -1 :`- no
`PYEXEC`calls and no collapse of small jet systems into one or two particles (in`PYPREP`).

`MSTJ(106) :`- (D = 1) angular orientation in
`PYEEVT`and`PYONIA`.`= 0 :`- standard orientation of events, i.e. along axis and along axis or in plane with for continuum events, and or in plane with or along the axis for onium events.
`= 1 :`- random orientation according to matrix elements.

`MSTJ(107) :`- (D = 0) radiative corrections to continuum events.
`= 0 :`- no radiative corrections.
`= 1 :`- initial-state radiative corrections (including weak
effects for
`MSTJ(102) =`2 or 3).

`MSTJ(108) :`- (D = 2) calculation of
for matrix-element
alternatives. The
`MSTU(111)`and`PARU(112)`values are automatically overwritten in`PYEEVT`or`PYONIA`calls accordingly.`= 0 :`- fixed
value as given in
`PARU(111)`. `= 1 :`- first-order formula is always used, with
given by
`PARJ(121)`. `= 2 :`- first- or second-order formula is used, depending on
value of
`MSTJ(101)`, with given by`PARJ(121)`or`PARJ(122)`.

`MSTJ(109) :`- (D = 0) gives a possibility to switch from QCD
matrix elements to some alternative toy models. Is not relevant for
shower evolution,
`MSTJ(101) = 5`, where one can use`MSTJ(49)`instead.`= 0 :`- standard QCD scenario.
`= 1 :`- a scalar gluon model. Since no second-order corrections
are available in this scenario, one can only use this with
`MSTJ(101) = 1`or`-1`. Also note that the event-as-a-whole angular distribution is for photon exchange only (i.e. no weak effects), and that no higher-order corrections to the total cross section are included. `= 2 :`- an Abelian vector gluon theory, with the colour factors
( in QCD), ( in QCD) and
( in QCD). If one selects
,
the 3-jet cross section will agree with
the QCD one, and differences are to be found only in 4-jets.
The
`MSTJ(109) = 2`option has to be run with`MSTJ(110) = 1`and`MSTJ(111) = 0`; if need be, the latter variables will be overwritten by the program.**Warning:**second-order corrections give a large negative contribution to the 3-jet cross section, so large that the whole scenario is of doubtful use. In order to make the second-order options work at all, the 3-jet cross section is here by hand set exactly equal to zero for`MSTJ(101) = 2`. It is here probably better to use the option`MSTJ(101) = 3`, although this is not a consistent procedure either.

`MSTJ(110) :`- (D = 2) choice of second-order contributions
to the 3-jet rate.
`= 1 :`- the GKS second-order matrix elements.
`= 2 :`- the Zhu parameterization of the ERT matrix elements, based on the program of Kunszt and Ali, i.e. in historical sequence ERT/Kunszt/Ali/Zhu. The parameterization is available for 0.01, 0.02, 0.03, 0.04 and 0.05. Values outside this range are put at the nearest border, while those inside it are given by a linear interpolation between the two nearest points. Since this procedure is rather primitive, one should try to work at one of the values given above. Note that no Abelian QCD parameterization is available for this option.

`MSTJ(111) :`- (D = 0) use of optimized perturbation theory for
second-order matrix elements (it can also be used for first-order
matrix elements, but here it only corresponds to a trivial
rescaling of the
argument).
`= 0 :`- no optimization procedure; i.e. .
`= 1 :`- an optimized scale is chosen as
, where
`PARJ(128)`for the total cross section factor, while`PARJ(129)`for the 3- and 4-jet rates. This value enters via the , and also via a term proportional to . Some constraints are imposed; thus the optimized `3-jet' contribution to is assumed to be positive (for`PARJ(128)`), the total 3-jet rate is not allowed to be negative (for`PARJ(129)`), etc. However, there is no guarantee that the differential 3-jet cross section is not negative (and truncated to 0) somewhere (this can also happen with , but is then less frequent). The actually obtained values are stored in`PARJ(168)`and`PARJ(169)`, respectively. If an optimized scale is used, then the (and ) should also be changed. With the value , it has been shown [Bet89] that a GeV gives a reasonable agreement; the parameter to be changed is`PARJ(122)`for a second-order running . Note that, since the optimized scale is sometimes below the charm threshold, the effective number of flavours used in may well be 4 only. If one feels that it is still appropriate to use 5 flavours (one choice might be as good as the other), it is necessary to put`MSTU(113) = 5`.

`MSTJ(115) :`- (D = 1) documentation of continuum or onium
events, in increasing order of completeness.
`= 0 :`- only the parton shower, the fragmenting partons and the
generated hadronic system are stored in the
`PYJETS`common block. `= 1 :`- also a radiative photon is stored (for continuum events).
`= 2 :`- also the original
are stored (with
`K(I,1) = 21`). `= 3 :`- also the or
exchanged for continuum
events, the onium state for resonance events is stored (with
`K(I,1) = 21`).

`MSTJ(116) :`- (D = 1) initialization of total cross section and
radiative photon spectrum in
`PYEEVT`calls.`= 0 :`- never; cannot be used together with radiative corrections.
`= 1 :`- calculated at first call and then whenever
`KFL`or`MSTJ(102)`is changed or`ECM`is changed by more than`PARJ(139)`. `= 2 :`- calculated at each call.
`= 3 :`- everything is re-initialized in the next call, but
`MSTJ(116)`is afterwards automatically put`= 1`for use in subsequent calls.

`MSTJ(119) :`- (I) check on need to re-initialize
`PYXTEE`. `MSTJ(120) :`- (R) type of continuum event generated with the
matrix-element option (with the shower one, the result is always
`= 1`).`= 1 :`- .
`= 2 :`- .
`= 3 :`- from Abelian (QED-like) graphs in matrix element.
`= 4 :`- from non-Abelian (i.e. containing triple-gluon coupling) graphs in matrix element.
`= 5 :`- .

`MSTJ(121) :`- (R) flag set if a negative differential
cross section was encountered in the latest
`PYX3JT`call. Events are still generated, but maybe not quite according to the distribution one would like (the rate is set to zero in the regions of negative cross section, and the differential rate in the regions of positive cross section is rescaled to give the `correct' total 3-jet rate).

`PARJ(121) :`- (D = 1.0 GeV) value
used in first-order
calculation of
in the matrix-element alternative.
`PARJ(122) :`- (D = 0.25 GeV) values used in second-order
calculation of
in the matrix-element alternative.
`PARJ(123) :`- (D = 91.187 GeV) mass of as used in
propagators for the QFD case.
`PARJ(124) :`- (D = 2.489 GeV) width of as used in
propagators for the QFD case. Overwritten at initialization if
`MSTJ(102) = 3`. `PARJ(125) :`- (D = 0.01)
, minimum squared scaled
invariant mass of any two partons in 3- or 4-jet events; the main
user-controlled matrix-element cut.
`PARJ(126)`provides an additional constraint. For each new event, it is additionally checked that the total 3- plus 4-jet fraction does not exceed unity; if so the effective cut will be dynamically increased. The actual -cut value is stored in`PARJ(150)`, event by event. `PARJ(126) :`- (D = 2. GeV) minimum invariant mass of any two
partons in 3- or 4-jet events; a cut in addition to the one above,
mainly for the case of a radiative photon lowering the hadronic
c.m. energy significantly.
`PARJ(127) :`- (D = 1. GeV) is used as a safety margin for small
colour-singlet jet systems, cf.
`PARJ(32)`, specifically masses in 4-jet events and mass in onium events. `PARJ(128) :`- (D = 0.25) optimized scale for the QCD
(total rate) factor for the
`MSTJ(111) = 1`option is given by , where`PARJ(128)`. For various reasons the actually used value may be increased compared with the nominal one; while`PARJ(128)`gives the nominal value,`PARJ(168)`gives the actual one for the current event. `PARJ(129) :`- (D = 0.002) optimized scale for the 3-
and 4-jet rate for the
`MSTJ(111) = 1`option is given by , where`PARJ(129)`. For various reasons the actually used value may be increased compared with the nominal one; while`PARJ(129)`gives the nominal value,`PARJ(169)`gives the actual one for the current event. The default value is in agreement with the studies of Bethke [Bet89]. `PARJ(131), PARJ(132) :`- (D = 2*0.) longitudinal polarizations
and
of incoming and .
`PARJ(133) :`- (D = 0.) transverse polarization
, with
and
transverse
polarizations of incoming and .
`PARJ(134) :`- (D = 0.) mean of transverse polarization
directions of incoming and ,
, with
the azimuthal angle of polarization, leading to a shift in the
distribution of jets by
.
`PARJ(135) :`- (D = 0.01) minimum photon energy fraction
(of beam energy) in initial-state radiation; should normally
never be changed (if lowered too much, the fraction of events
containing a radiative photon will exceed unity, leading to
problems).
`PARJ(136) :`- (D = 0.99) maximum photon energy fraction
(of beam energy) in initial-state radiation; may be changed
to reflect actual trigger conditions of a detector (but must
always be larger than
`PARJ(135)`). `PARJ(139) :`- (D = 0.2 GeV) maximum deviation of
from the corresponding value at last
`PYXTEE`call, above which a new call is made if`MSTJ(116) = 1`. `PARJ(141) :`- (R) value of , the ratio of continuum
cross section to the lowest-order muon pair production cross section,
as given in massless QED (i.e. three times the sum of active
quark squared charges, possibly modified for polarization).
`PARJ(142) :`- (R) value of including quark-mass effects
(for
`MSTJ(102) = 1`) and/or weak propagator effects (for`MSTJ(102) = 2`). `PARJ(143) :`- (R) value of as
`PARJ(142)`, but including QCD corrections as given by`MSTJ(101)`. `PARJ(144) :`- (R) value of as
`PARJ(143)`, but additionally including corrections from initial-state photon radiation (if`MSTJ(107) = 1`). Since the effects of heavy flavour thresholds are not simply integrable, the initial value of`PARJ(144)`is updated during the course of the run to improve accuracy. `PARJ(145) - PARJ(148) :`- (R) absolute cross sections in nb
as for the cases
`PARJ(141) - PARJ(144)`above. `PARJ(150) :`- (R) current effective matrix element cut-off
, as given by
`PARJ(125), PARJ(126)`and the requirements of having non-negative cross sections for 2-, 3- and 4-jet events. Not used in parton showers. `PARJ(151) :`- (R) value of c.m. energy
`ECM`at last`PYXTEE`call. `PARJ(152) :`- (R) current first-order contribution to the
3-jet fraction; modified by mass effects. Not used in parton
showers.
`PARJ(153) :`- (R) current second-order contribution to the
3-jet fraction; modified by mass effects. Not used in parton
showers.
`PARJ(154) :`- (R) current second-order contribution to the
4-jet fraction; modified by mass effects. Not used in parton
showers.
`PARJ(155) :`- (R) current fraction of 4-jet rate
attributable to
events rather than
ones; modified by mass effects. Not used in parton showers.
`PARJ(156) :`- (R) has two functions when using second-order
QCD. For a 3-jet event, it gives the ratio of the second-order
to the total 3-jet cross section in the given kinematical
point. For a 4-jet event, it gives the ratio of the
modified 4-jet cross section, obtained when neglecting interference
terms whose colour flow is not well defined, to the full
unmodified one, all evaluated in the given kinematical point.
Not used in parton showers.
`PARJ(157) - PARJ(159) :`- (I) used for cross-section
calculations to include mass threshold effects to radiative
photon cross section. What is stored is basic cross section,
number of events generated and number that passed cuts.
`PARJ(160) :`- (R) nominal fraction of events that should contain a radiative photon.
`PARJ(161) - PARJ(164) :`- (I) give shape of radiative photon
spectrum including weak effects.
`PARJ(168) :`- (R) actual value of current event in
optimized perturbation theory for ; see
`MSTJ(111)`and`PARJ(128)`. `PARJ(169) :`- (R) actual value of current event in
optimized perturbation theory for 3- and 4-jet rate;
see
`MSTJ(111)`and`PARJ(129)`. `PARJ(171) :`- (R) fraction of cross section corresponding
to the axial coupling of quark pair to the intermediate
state; needed for the Abelian gluon model 3-jet matrix
element.