General Event Information

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 final-state 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 colour-flow topologies possible in QCD $2 \to 2$ processes, but easily generalizes e.g. if a quark is replaced by a squark. With $i =$MSTI(15), $j =$MSTI(16) and $k =$MSTI(2), the QCD possibilities are, in the classification scheme of [Ben84] (cf. section ):

ISUB = 11,

$i = j$, $\mathrm{q}_i \mathrm{q}_i \to \mathrm{q}_i \mathrm{q}_i$;
$k=1$ : colour configuration $A$.
$k=2$ : colour configuration $B$.

ISUB = 11,

$i \neq j$, $\mathrm{q}_i \mathrm{q}_j \to \mathrm{q}_i \mathrm{q}_j$;
$k=1$ : only possibility.

ISUB = 12,

$\mathrm{q}_i \overline{\mathrm{q}}_i \to \mathrm{q}_l \overline{\mathrm{q}}_l$;
$k=1$ : only possibility.

ISUB = 13,

$\mathrm{q}_i \overline{\mathrm{q}}_i \to \mathrm{g}\mathrm{g}$;
$k=1$ : colour configuration $A$.
$k=2$ : colour configuration $B$.

ISUB = 28,

$\mathrm{q}_i \mathrm{g}\to \mathrm{q}_i \mathrm{g}$;
$k=1$ : colour configuration $A$.
$k=2$ : colour configuration $B$.

ISUB = 53,

$\mathrm{g}\mathrm{g}\to \mathrm{q}_l \overline{\mathrm{q}}_l$;
$k=1$ : colour configuration $A$.
$k=2$ : colour configuration $B$.

ISUB = 68,

$\mathrm{g}\mathrm{g}\to \mathrm{g}\mathrm{g}$;
$k=1$ : colour configuration $A$.
$k=2$ : colour configuration $B$.
$k = 3$ : colour configuration $C$.

ISUB = 83,

$\mathrm{f}\mathrm{q}\to \mathrm{f}' \mathrm{Q}$ (by $t$-channel $\mathrm{W}$ exchange; does not distinguish colour flows but result of user selection);
$k=1$ : heavy flavour $\mathrm{Q}$ is produced on side 1.
$k=2$ : heavy flavour $\mathrm{Q}$ is produced on side 2.

MSTI(3) :

the number of partons produced in the hard interactions, i.e. the number $n$ of the $2 \to n$ matrix elements used; it is sometimes 3 or 4 when a basic $2 \to 1$ or $2 \to 2$ process has been folded with two $1 \to 2$ initial branchings (like $\mathrm{q}_i \mathrm{q}_j \to \mathrm{q}_k \mathrm{q}_l \mathrm{h}^0$).

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 variable-energy 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 $2 \to 2$ or $2 \to 1 \to 2$ processes (else = 0).

MSTI(9) :

event class used in current event for $\gamma\mathrm{p}$ or $\gamma\gamma$ events. The code depends on which process is being studied.

= 0 :

for other processes than the ones listed below.

For $\gamma\mathrm{p}$ or $\gamma^*\mathrm{p}$ events,

generated with the MSTP(14) = 10 or MSTP(14) = 30 options:

= 1 :

VMD.

= 2 :

direct.

= 3 :

anomalous.

= 4 :

DIS (only for $\gamma^*\mathrm{p}$, i.e. MSTP(14) = 30).

For real $\gamma\gamma$ events,

i.e. MSTP(14) = 10:

= 1 :

VMD$\times$VMD.

= 2 :

VMD$\times$direct.

= 3 :

VMD$\times$anomalous .

= 4 :

direct$\times$direct.

= 5 :

direct$\times$anomalous.

= 6 :

anomalous$\times$anomalous.

For virtual $\gamma^*\gamma^*$ events,

i.e. MSTP(14) = 30, where the two incoming photons are not equivalent and the order therefore matters:

= 1 :

direct$\times$direct.

= 2 :

direct$\times$VMD.

= 3 :

direct$\times$anomalous.

= 4 :

VMD$\times$direct.

= 5 :

VMD$\times$VMD.

= 6 :

VMD$\times$anomalous.

= 7 :

anomalous$\times$direct.

= 8 :

anomalous$\times$VMD.

= 9 :

anomalous$\times$anomalous.

= 10 :

DIS$\times$VMD.

= 11 :

DIS$\times$anomalous.

= 12 :

VMD$\times$DIS.

= 13 :

anomalous$\times$DIS.

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 initial-state 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 process-dependent, 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 $s$-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 semi-hard scatterings that occurred in the current event in the multiple-interaction scenario; is = 0 for a low-$p_{\perp}$ 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 pile-up events generated in the latest PYEVNT call (including the first, `hard' event).

MSTI(42) - MSTI(50) :

ISUB codes for the events 2-10 generated in the pile-up-events 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 pile-up 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 non-vanishing 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 by-product of the selection of hard-process 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 variable-energy 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 $-1$ for events with negative cross section, with IDWTUP = 4 it can be an arbitrary non-negative 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) $\geq 1$, 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) :

$E_{\mathrm{cm}}$, i.e. total c.m. energy (except when using the 'gamma/lepton' machinery, see PARI(101).

PARI(12) :

$s$, i.e. squared total c.m. energy (except when using the 'gamma/lepton' machinery, see PARI(102).

PARI(13) :

$\hat{m} = \sqrt{\hat{s}}$, i.e. mass of the hard-scattering subsystem.

PARI(14) :

$\hat{s}$ of the hard subprocess ($2 \to 2$ or $2 \to 1$).

PARI(15) :

$\hat{t}$ of the hard subprocess ($2 \to 2$ or $2 \to 1 \to 2$).

PARI(16) :

$\hat{u}$ of the hard subprocess ($2 \to 2$ or $2 \to 1 \to 2$).

PARI(17) :

$\hat{p}_{\perp}$ of the hard subprocess ($2 \to 2$ or $2 \to 1 \to 2$), evaluated in the rest frame of the hard interaction.

PARI(18) :

$\hat{p}_{\perp}^2$ of the hard subprocess; see PARI(17).

PARI(19) :

$\hat{m}'$, the mass of the complete three- or four-body final state in $2 \to 3$ or $2 \to 4$ processes (while $\hat{m}$, given in PARI(13), here corresponds to the one- or two-body central system). Kinematically $\hat{m} \leq \hat{m}' \leq E_{\mathrm{cm}}$.

PARI(20) :

$\hat{s}' = \hat{m}'^2$; see PARI(19).

PARI(21) :

$Q$ of the hard-scattering subprocess. The exact definition is process-dependent, see MSTP(32).

PARI(22) :

$Q^2$ of the hard-scattering subprocess; see PARI(21).

PARI(23) :

$Q$ of the outer hard-scattering subprocess. Agrees with PARI(21) for a $2 \to 1$ or $2 \to 2$ process. For a $2 \to 3$ or $2 \to 4$ $\mathrm{W}/ \mathrm{Z}$ fusion process, it is set by the $\mathrm{W}/ \mathrm{Z}$ mass scale, and for subprocesses 121 and 122 by the heavy-quark mass.

PARI(24) :

$Q^2$ of the outer hard-scattering subprocess; see PARI(23).

PARI(25) :

$Q$ scale used as maximum virtuality in parton showers. Is equal to PARI(23), except for Deeply Inelastic Scattering processes when MSTP(22) $\geq 1$.

PARI(26) :

$Q^2$ scale in parton showers; see PARI(25).

PARI(31), PARI(32) :

the momentum fractions $x$ of the initial-state parton-shower initiators on side 1 and 2, respectively.

PARI(33), PARI(34) :

the momentum fractions $x$ taken by the partons at the hard interaction, as used e.g. in the parton-distribution functions.

PARI(35) :

Feynman-$x$, $x_{\mathrm{F}} = x_1 - x_2 =$PARI(33)$-$PARI(34).

PARI(36) :

$\tau = \hat{s}/s = x_1 \, x_2 =$PARI(33)$\times$PARI(34).

PARI(37) :

$y = (1/2) \ln(x_1/x_2)$, i.e. rapidity of the hard-interaction subsystem in the c.m. frame of the event as a whole.

PARI(38) :

$\tau' = \hat{s}'/s =$PARI(20)$/$PARI(12).

PARI(39), PARI(40) :

the primordial $k_{\perp}$ values selected in the two beam remnants.

PARI(41) :

$\cos\hat{\theta}$, where $\hat{\theta}$ is the scattering angle of a $2 \to 2$ (or $2 \to 1 \to 2$) interaction, defined in the rest frame of the hard-scattering subsystem.

PARI(42) :

$x_{\perp}$, i.e. scaled transverse momentum of the hard-scattering subprocess, $x_{\perp} = 2 \hat{p}_{\perp}/E_{\mathrm{cm}} = 2$PARI(17)$/$PARI(11).

PARI(43), PARI(44) :

$x_{L3}$ and $x_{L4}$, i.e. longitudinal momentum fractions of the two scattered partons, in the range $-1 < x_{\mathrm{L}} < 1$, in the c.m. frame of the event as a whole.

PARI(45), PARI(46) :

$x_3$ and $x_4$, 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) :

$y^*_3$ and $y^*_4$, i.e. rapidities of the two scattered partons in the c.m. frame of the event as a whole.

PARI(49), PARI(50) :

$\eta^*_3$ and $\eta^*_4$, i.e. pseudorapidities of the two scattered partons in the c.m. frame of the event as a whole.

PARI(51), PARI(52) :

$\cos\theta^*_3$ and $\cos\theta^*_4$, 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) :

$\theta^*_3$ and $\theta^*_4$, i.e. polar angles of the two scattered partons, defined in the range $0 < \theta^* < \pi$, in the c.m. frame of the event as a whole.

PARI(55), PARI(56) :

azimuthal angles $\phi^*_3$ and $\phi^*_4$ of the two scattered partons, defined in the range $-\pi < \phi^* < \pi$, 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 final-state radiation, i.e. $2 \times$PARI(17). Only intended for $2 \to 2$ or $2 \to 1 \to 2$ processes, i.e. not implemented for $2 \to 3$ ones.

PARI(66) :

sum of the transverse momenta of all partons generated at the hardest interaction, including initial- and final-state radiation, resonance decay products, and primordial $k_{\perp}$.

PARI(67) :

scalar sum of transverse momenta of partons generated at hard interactions, excluding the hardest one, along with its initial- and final-state radiation (see PARI(66)). Is non-vanishing only in the multiple-interaction scenarios. In the new scenario the initial- and final-state 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 beam-remnant partons.

PARI(71), PARI(72) :

sum of the momentum fractions $x$ taken by initial-state parton-shower initiators on side 1 and and side 2, excluding those of the hardest interaction. Is non-vanishing only in the multiple-interaction scenario.

PARI(73), PARI(74) :

sum of the momentum fractions $x$ taken by the partons at the hard interaction on side 1 and side 2, excluding those of the hardest interaction. Is non-vanishing only in the multiple-interaction scenario.

PARI(75), PARI(76) :

the $x$ value of a photon that branches into quarks or gluons, i.e. $x$ at interface between initial-state QED and QCD cascades, for the old photoproduction machinery.

PARI(77), PARI(78) :

the $\chi$ 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 heavy-flavour production; see MSTP(35).

PARI(91) :

average multiplicity $\overline{n}$ of pile-up events, see MSTP(133). Only relevant for MSTP(133) = 1 or 2.

PARI(92) :

average multiplicity $\langle n \rangle$ of pile-up events as actually simulated, i.e. with multiplicity = 0 events removed and the high-end 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 pile-up event at all, i.e. that there will be at least one hadron-hadron interaction; for MSTP(133) = 2 the probability that a beam crossing will produce a pile-up event with one hadron-hadron interaction of the desired rare type. See section .

PARI(101) :

c.m. energy for the full collision, while PARI(11) gives the $\gamma$-hadron or $\gamma\gamma$ 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) :

$x$ 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) :

$Q^2$ or $P^2$, 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) :

$y$ values, i.e. respective photon light-cone energy fraction of the incoming lepton; used for virtual photons generated internally with the 'gamma/lepton' option.

PARI(109), PARI(110) :

$\theta$, 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) :

$\phi$, 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 $R$ factor defined at MSTP(17), giving a cross section enhancement from the contribution of resolved longitudinal photons.