Next: How to Generate Weighted
Up: The Process Generation Program
Previous: Supersymmetry CommonBlocks and Routines
Contents
When an event is generated with PYEVNT, some information on
it is stored in the MSTI and PARI arrays of the
PYPARS common block (often copied directly from the internal
MINT and VINT variables). Further information is stored
in the complete event record; see section .
Part of the information is only relevant for some subprocesses; by
default everything irrelevant is set to 0. Kindly note that, like the
CKIN constraints described in section ,
kinematical variables normally (i.e. where it is not explicitly stated
otherwise) refer to the naïve hard scattering, before initial and
finalstate radiation effects have been included.
 Purpose:
 to provide information on latest event generated or,
in a few cases, on statistics accumulated during the run.
 MSTI(1) :
 specifies the general type of
subprocess that has occurred, according to the ISUB code given
in section .
 MSTI(2) :
 whenever MSTI(1) (together with MSTI(15)
and MSTI(16)) are not enough to specify the type of process
uniquely, MSTI(2) provides an ordering of the different
possibilities. This is particularly relevant for the different
colourflow topologies possible in QCD processes, but
easily generalizes e.g. if a quark is replaced by a squark. With
MSTI(15), MSTI(16) and MSTI(2),
the QCD possibilities are, in the classification scheme of
[Ben84] (cf. section ):
 ISUB = 11,
 ,
;
: colour configuration .
: colour configuration .
 ISUB = 11,
 ,
;
: only possibility.
 ISUB = 12,

;
: only possibility.
 ISUB = 13,

;
: colour configuration .
: colour configuration .
 ISUB = 28,

;
: colour configuration .
: colour configuration .
 ISUB = 53,

;
: colour configuration .
: colour configuration .
 ISUB = 68,

;
: colour configuration .
: colour configuration .
: colour configuration .
 ISUB = 83,

(by channel exchange;
does not distinguish colour flows but result of user selection);
: heavy flavour is produced on side 1.
: heavy flavour is produced on side 2.
 MSTI(3) :
 the number of partons produced in the hard
interactions, i.e. the number of the matrix elements
used; it is sometimes 3 or 4 when a basic or
process has been folded with two initial branchings (like
).
 MSTI(4) :
 number of documentation lines at the beginning of
the common block PYJETS that are given with K(I,1) = 21;
0 for MSTP(125) = 0.
 MSTI(5) :
 number of events generated to date in current
run. In runs with the variableenergy option, MSTP(171) = 1
and MSTP(172) = 2, only those events that survive (i.e. that
do not have MSTI(61) = 1) are counted in this number. That
is, MSTI(5) may be less than the total number of PYEVNT
calls.
 MSTI(6) :
 current frame of event, cf. MSTP(124).
 MSTI(7), MSTI(8) :
 line number for documentation of
outgoing partons/particles from hard scattering for or
processes (else = 0).
 MSTI(9) :
 event class used in current event for
or
events. The code depends on which process is being studied.
 = 0 :
 for other processes than the ones listed below.
 For
or
events,
 generated with the
MSTP(14) = 10 or MSTP(14) = 30 options:
 = 1 :
 VMD.
 = 2 :
 direct.
 = 3 :
 anomalous.
 = 4 :
 DIS (only for
, i.e. MSTP(14) = 30).
 For real events,
 i.e. MSTP(14) = 10:
 = 1 :
 VMDVMD.
 = 2 :
 VMDdirect.
 = 3 :
 VMDanomalous .
 = 4 :
 directdirect.
 = 5 :
 directanomalous.
 = 6 :
 anomalousanomalous.
 For virtual
events,
 i.e. MSTP(14) = 30,
where the two incoming photons are not equivalent and the order
therefore matters:
 = 1 :
 directdirect.
 = 2 :
 directVMD.
 = 3 :
 directanomalous.
 = 4 :
 VMDdirect.
 = 5 :
 VMDVMD.
 = 6 :
 VMDanomalous.
 = 7 :
 anomalousdirect.
 = 8 :
 anomalousVMD.
 = 9 :
 anomalousanomalous.
 = 10 :
 DISVMD.
 = 11 :
 DISanomalous.
 = 12 :
 VMDDIS.
 = 13 :
 anomalousDIS.
 MSTI(10) :
 is 1 if cross section maximum was violated
in current event, and 0 if not.
 MSTI(11) :
 KF flavour code for beam (side 1) particle.
 MSTI(12) :
 KF flavour code for target (side 2) particle.
 MSTI(13), MSTI(14) :
 KF flavour codes for side 1 and side 2
initialstate shower initiators.
 MSTI(15), MSTI(16) :
 KF flavour codes for side 1 and side 2
incoming partons to the hard interaction.
 MSTI(17), MSTI(18) :
 flag to signal if particle on side
1 or side 2 has been scattered diffractively; 0 if no, 1 if yes.
 MSTI(21)  MSTI(24) :
 KF flavour codes for outgoing partons
from the hard interaction. The number of positions actually used is
processdependent, see MSTI(3); trailing positions not used
are set = 0. For events with many outgoing partons, e.g. in external
processes, also MSTI(25) and MSTI(26) could be used.
 MSTI(25), MSTI(26) :
 KF flavour codes of the products in the
decay of a single channel resonance formed in the hard interaction.
Are thus only used when MSTI(3) = 1 and the resonance is allowed
to decay.
 MSTI(31) :
 number of hard or semihard scatterings that occurred
in the current event in the multipleinteraction scenario; is = 0 for a
low event.
 MSTI(32) :
 information on whether a reconnection occurred in the
current event; is 0 normally but 1 in case of reconnection.
 MSTI(41) :
 the number of pileup events generated in the latest
PYEVNT call (including the first, `hard' event).
 MSTI(42)  MSTI(50) :
 ISUB codes for the events 210
generated in the pileupevents scenario. The first event ISUB code is
stored in MSTI(1). If MSTI(41) is less than 10, only as
many positions are filled as there are pileup events. If MSTI(41) is
above 10, some ISUB codes will not appear anywhere.
 MSTI(51) :
 normally 0 but set to 1 if a UPEVNT call
did not return an event, such that PYEVNT could not generate
an event. For further details, see section .
 MSTI(52) :
 counter for the number of times the current event
configuration failed in the generation machinery. For accepted events
this is always 0, but the counter can be used inside UPEVNT
to check on anomalous occurrences. For further
details, see section .
 MSTI(53) :
 normally 0, but 1 if no processes with nonvanishing
cross sections were found in a PYINIT call, for the case that
MSTP(127) = 1.
 MSTI(61) :
 status flag set when events are generated. It is
only of interest for runs with variable energies, MSTP(171) = 1,
with the option MSTP(172) = 2.
 = 0 :
 an event has been generated.
 = 1 :
 no event was generated, either because the c.m. energy
was too low or because the Monte Carlo phase space point selection
machinery rejected the trial point. A new energy is to be picked by
you.
 MSTI(71), MSTI(72) :
 KF code for incoming lepton beam or target
particles, when a flux of virtual photons are generated internally for
'gamma/lepton' beams, while MSTI(11) and MSTI(12) is
then the photon code.
 PARI(1) :
 total integrated cross section for
the processes
under study, in mb. This number is obtained as a byproduct of the
selection of hardprocess kinematics, and is thus known with better
accuracy when more events have been generated. The value stored here
is based on all events until the latest one generated.
 PARI(2) :
 for unweighted events, MSTP(142) = 0 or = 2,
it is the ratio PARI(1)/MSTI(5), i.e. the ratio of total
integrated cross section and number of events generated. Histograms
should then be filled with unit event weight and, at the end of the run,
multiplied by PARI(2) and divided by the bin width to convert
results to mb/(dimension of the horizontal axis).
For weighted events, MSTP(142) = 1, MSTI(5) is replaced by the
sum of PARI(10) values. Histograms should then be filled with
event weight PARI(10) and, as before, be multiplied by PARI(2)
and divided by the bin width at the end of the run.
In runs with the variableenergy option, MSTP(171) = 1
and MSTP(172) = 2, only those events that survive (i.e. that
do not have MSTI(61) = 1) are counted.
 PARI(7) :
 an event weight, normally 1 and thus uninteresting,
but for external processes with IDWTUP = 1, 2 or 3 it can
be for events with negative cross section, with IDWTUP = 4 it
can be an arbitrary nonnegative weight of dimension mb, and with
IDWTUP = 4 it can be an arbitrary weight of dimension mb.
(The difference being that in most cases a rejection step is involved
to bring the accepted events to a common weight normalization, up to
a sign, while no rejection need be involved in the last two cases.)
 PARI(9) :
 is weight WTXS returned from PYEVWT
call when MSTP(142) , otherwise is 1.
 PARI(10) :
 is compensating weight 1./WTXS that should
be associated to events when MSTP(142) = 1, else is 1.
 PARI(11) :

, i.e. total c.m. energy
(except when using the 'gamma/lepton' machinery, see PARI(101).
 PARI(12) :
 , i.e. squared total c.m. energy
(except when using the 'gamma/lepton' machinery, see PARI(102).
 PARI(13) :

, i.e. mass of the
hardscattering subsystem.
 PARI(14) :
 of the hard subprocess
( or ).
 PARI(15) :
 of the hard subprocess
( or ).
 PARI(16) :
 of the hard subprocess
( or ).
 PARI(17) :

of the hard subprocess
( or ),
evaluated in the rest frame of the hard interaction.
 PARI(18) :

of the hard subprocess;
see PARI(17).
 PARI(19) :
 , the mass of the complete three or
fourbody final state in or processes (while
, given in PARI(13), here corresponds to the one
or twobody central system).
Kinematically
.
 PARI(20) :

; see PARI(19).
 PARI(21) :
 of the hardscattering subprocess. The exact
definition is processdependent, see MSTP(32).
 PARI(22) :
 of the hardscattering subprocess; see
PARI(21).
 PARI(23) :
 of the outer hardscattering subprocess.
Agrees with PARI(21) for a or process.
For a or
fusion process, it is set by
the
mass scale, and for subprocesses 121 and 122 by the
heavyquark mass.
 PARI(24) :
 of the outer hardscattering subprocess;
see PARI(23).
 PARI(25) :
 scale used as maximum virtuality in parton
showers. Is equal to PARI(23), except for
Deeply Inelastic Scattering processes when MSTP(22) .
 PARI(26) :
 scale in parton showers; see PARI(25).
 PARI(31), PARI(32) :
 the momentum fractions of the
initialstate partonshower initiators on side 1 and 2, respectively.
 PARI(33), PARI(34) :
 the momentum fractions taken by the
partons at the hard interaction, as used e.g. in the
partondistribution functions.
 PARI(35) :
 Feynman,
PARI(33)PARI(34).
 PARI(36) :

PARI(33)PARI(34).
 PARI(37) :

, i.e. rapidity of the
hardinteraction subsystem in the c.m. frame of the event as a whole.
 PARI(38) :

PARI(20)PARI(12).
 PARI(39), PARI(40) :
 the primordial values
selected in the two beam remnants.
 PARI(41) :

, where is the
scattering angle of a (or ) interaction,
defined in the rest frame of the hardscattering subsystem.
 PARI(42) :
 , i.e. scaled transverse momentum of the
hardscattering subprocess,
PARI(17)PARI(11).
 PARI(43), PARI(44) :
 and , i.e. longitudinal
momentum fractions of the two scattered partons, in the range
, in the c.m. frame of the event as a whole.
 PARI(45), PARI(46) :
 and , i.e. scaled energy
fractions of the two scattered partons, in the c.m. frame of the
event as a whole.
 PARI(47), PARI(48) :
 and , i.e. rapidities
of the two scattered partons in the c.m. frame of the
event as a whole.
 PARI(49), PARI(50) :
 and , i.e. pseudorapidities of the two scattered partons in the c.m. frame
of the event as a whole.
 PARI(51), PARI(52) :

and
,
i.e. cosines of the polar angles of the two scattered partons in
the c.m. frame of the event as a whole.
 PARI(53), PARI(54) :
 and , i.e. polar angles of the two scattered partons, defined in the range
, in the c.m. frame of the event as a whole.
 PARI(55), PARI(56) :
 azimuthal angles and
of the two scattered partons, defined in the range
, in the c.m. frame of the event as a whole.
 PARI(61) :
 multiple interaction enhancement factor for
current event. A large value corresponds to a central collision
and a small value to a peripheral one.
 PARI(65) :
 sum of the transverse momenta of partons
generated at the hardest interaction of the event, excluding
initial and finalstate radiation, i.e. PARI(17).
Only intended for or processes,
i.e. not implemented for ones.
 PARI(66) :
 sum of the transverse momenta of all partons
generated at the hardest interaction, including initial and
finalstate radiation, resonance decay products, and primordial
.
 PARI(67) :
 scalar sum of transverse momenta of partons generated
at hard interactions, excluding the hardest one, along with its
initial and finalstate radiation (see PARI(66)). Is
nonvanishing only in the multipleinteraction scenarios. In the
new scenario the initial and finalstate radiation associated with
further interactions is included.
 PARI(68) :
 currently equal to PARI(67).
 PARI(69) :
 sum of transverse momenta of all partons generated
in hard interactions (PARI(66) + PARI(68)) and, additionally,
of all beamremnant partons.
 PARI(71), PARI(72) :
 sum of the momentum fractions taken
by initialstate partonshower initiators on side 1 and and side 2,
excluding those of the hardest interaction. Is nonvanishing only in
the multipleinteraction scenario.
 PARI(73), PARI(74) :
 sum of the momentum fractions taken
by the partons at the hard interaction on side 1 and side 2, excluding
those of the hardest interaction. Is nonvanishing only in the
multipleinteraction scenario.
 PARI(75), PARI(76) :
 the value of a photon that branches
into quarks or gluons, i.e. at interface between initialstate QED
and QCD cascades, for the old photoproduction machinery.
 PARI(77), PARI(78) :
 the values selected for beam
remnants that are split into two objects, describing how the energy
is shared (see MSTP(92) and MSTP(94)); is vanishing if
no splitting is needed.
 PARI(81) :
 size of the threshold factor (enhancement or
suppression) in the latest event with heavyflavour production;
see MSTP(35).
 PARI(91) :
 average multiplicity of pileup events,
see MSTP(133). Only relevant for MSTP(133) = 1 or 2.
 PARI(92) :
 average multiplicity
of pileup
events as actually simulated, i.e. with multiplicity = 0 events
removed and the highend tail truncated. Only relevant for
MSTP(133) = 1 or 2.
 PARI(93) :
 for MSTP(133) = 1 it is the probability that
a beam crossing will produce a pileup event at all, i.e. that there
will be at least one hadronhadron interaction; for
MSTP(133) = 2 the probability that a beam crossing will produce
a pileup event with one hadronhadron interaction of the desired rare
type. See section .
 PARI(101) :
 c.m. energy for the full collision, while PARI(11)
gives the hadron or subsystem energy; used for
virtual photons generated internally with the 'gamma/lepton' option.
 PARI(102) :
 full squared c.m. energy, while PARI(12) gives
the subsystem squared energy; used for virtual photons generated
internally with the 'gamma/lepton' option.
 PARI(103), PARI(104) :
 values, i.e. respective photon energy
fractions of the incoming lepton in the c.m. frame of the event; used for
virtual photons generated internally with the 'gamma/lepton' option.
 PARI(105), PARI(106) :
 or , virtuality of the
respective photon (thus the square of VINT(3), VINT(4)); used for
virtual photons generated internally with the 'gamma/lepton' option.
 PARI(107), PARI(108) :
 values, i.e. respective photon lightcone
energy fraction of the incoming lepton; used for virtual photons generated
internally with the 'gamma/lepton' option.
 PARI(109), PARI(110) :
 , scattering angle of the respective
lepton in the c.m. frame of the event; used for virtual photons generated
internally with the 'gamma/lepton' option.
 PARI(111), PARI(112) :
 , azimuthal angle of the respective
scattered lepton in the c.m. frame of the event; used for virtual photons
generated internally with the 'gamma/lepton' option.
 PARI(113), PARI(114):
 the factor defined at MSTP(17),
giving a cross section enhancement from the contribution of resolved
longitudinal photons.
Next: How to Generate Weighted
Up: The Process Generation Program
Previous: Supersymmetry CommonBlocks and Routines
Contents
Stephen Mrenna
20071030