Event Information
 
  - List information
- The beams
- Initialization
- The event type
- Hard process initiators
- Hard process parton densities and scales
- Hard process kinematics
- Soft Diffraction
- Hard Diffraction
- Photons from lepton beams
- Event activity
- Multiparton interactions
- Cross sections
- Loop counters
- Parton shower history
- Les Houches Event File information
- Heavy Ion Information
TheInfo class collects various one-of-a-kind 
information, some relevant for all events and others for the current 
event.  An object info is a public member of the 
Pythia 
class, so if you e.g. have declared Pythia pythia, the 
Info methods can be accessed by 
pythia.info.method(). Most of this is information that 
could also be obtained e.g. from the event record, but is here more 
directly available. It is primarily intended for processes generated 
internally in PYTHIA, but many of the methods would work also for 
events fed in via the Les Houches Accord. 
 
 
Note that further Info methods related to cross sections 
and event weights are collected on the 
Cross Sections and Weights page. 
 
 
List information
 
 
 void Info::list()   
a listing of most of the information set for the current event. 
   
 
 
The beams
 
 
 int Info::idA()   
   
 int Info::idB()   
the identities of the two beam particles. 
   
 
 double Info::pzA()   
   
 double Info::pzB()   
the longitudinal momenta of the two beam particles. 
   
 
 double Info::eA()   
   
 double Info::eB()   
the energies of the two beam particles. 
   
 
 double Info::mA()   
   
 double Info::mB()   
the masses of the two beam particles. 
   
 
 double Info::eCM()   
   
 double Info::s()   
the CM energy and its square for the two beams. 
   
 
 
Initialization
 
 
 bool Info::tooLowPTmin()   
normally false, but true if the proposed pTmin scale was too low 
in timelike or spacelike showers, or in multiparton interactions. In the 
former case the pTmin is raised to some minimal value, in the 
latter the initialization fails (it is impossible to obtain a minijet 
cross section bigger than the nondiffractive one by reducing 
pTmin). 
   
 
 
The event type
 
 
 string Info::name()   
   
 int Info::code()   
the name and code of the process that occurred. 
   
 
 int Info::nFinal()   
the number of final-state partons in the hard process. 
   
 
 bool Info::isResolved()   
are beam particles resolved, i.e. were PDF's used for the process? 
   
 
 bool Info::isDiffractiveA()   
   
 bool Info::isDiffractiveB()   
is either beam soft diffractively excited? 
   
 
 bool Info::isDiffractiveC()   
is there soft central diffraction (a.k.a. double Pomeron exchange)? 
   
 
 bool Info::isHardDiffractiveA()   
   
 bool Info::isHardDiffractiveB()   
is either beam hard diffractively excited? 
   
 
 bool Info::isNonDiffractive()   
is the process the SoftQCD:nonDiffractive one, 
i.e. corresponding to the full inelastic nondiffractive part of the 
total cross section. (Note that a hard process, say Z^0 
production, normally is nondiffractive, but this is not what we 
aim at here, and so the method would return false, 
unless the Z^0 had been generated as part of the MPI 
machinery for the SoftQCD:nonDiffractive component.) 
   
 
 bool Info::isMinBias()   
the same as above, retained for backwards compatibility, but to 
be removed in PYTHIA 8.2. 
   
 
 bool Info::isLHA()   
has the process been generated from external Les Houches Accord 
information? 
   
 
 bool Info::atEndOfFile()   
true if a linked Les Houches class refuses to return any further 
events, presumably because it has reached the end of the file from 
which events have been read in. 
   
 
 bool Info::hasSub()   
does the process have a subprocess classification? 
Currently only true for nondiffractive and Les Houches events, where 
it allows the hardest collision to be identified. 
   
 
 string Info::nameSub()   
   
 int Info::codeSub()   
   
 int Info::nFinalSub()   
the name, code and number of final-state partons in the subprocess 
that occurred when hasSub() is true. For a minimum-bias event 
the code would always be 101, while codeSub() 
would vary depending on the actual hardest interaction, e.g. 111 for 
g g → g g. For a Les Houches event the code would 
always be 9999, while codeSub() would be the external 
user-defined classification code. The methods below would also provide 
information for such particular subcollisions. 
   
 
 
Hard process initiators
 
 
The methods in this sections refer to the two initial partons of the 
hard 2 → n process (diffraction excluded; see below). 
 
 int Info::id1()   
   
 int Info::id2()   
the identities of the two partons coming in to the hard process. 
   
 
 double Info::x1()   
   
 double Info::x2()   
x fractions of the two partons coming in to the hard process. 
   
 
 double Info::y()   
   
 double Info::tau()   
rapidity and scaled mass-squared of the hard-process subsystem, as 
defined by the above x values. 
   
 
 bool Info::isValence1()   
   
 bool Info::isValence2()   
true if the two hard incoming partons have been picked 
to belong to the valence piece of the parton-density distribution, 
else false. Should be interpreted with caution. 
Information is not set if you switch off parton-level processing. 
   
 
 
Hard process parton densities and scales
 
 
The methods in this section refer to the partons for which parton 
densities have been defined, in order to calculate the cross section 
of the hard process (diffraction excluded; see below). 
 
 
These partons would normally agree with the 
ones above, the initiators of the 2 → n process, but it 
does not have to be so. Currently the one counterexample is POWHEG 
events [Ali10]. Here the original hard process could be 
2 → (n-1). The NLO machinery at times would add an 
initial-state branching to give a 2 → n process with a 
changed initial state. In that case the values in this section 
refer to the original 2 → (n-1) state and the initiator 
ones above to the complete2 → n process. The 
Info::list() printout will contain a warning in such cases. 
 
 
For external events in the Les Houches format, the pdf information 
is obtained from the optional #pdf line. When this 
information is absent, the parton identities and x values agree 
with the initiator ones above, while the pdf values are unknown and 
therefore set to vanish. The alpha_s and alpha_em 
values are part of the compulsory information. The factorization and 
renormalization scales are both equated with the one compulsory scale 
value in the Les Houches standard, except when a #pdf 
line provides the factorization scale separately. If alpha_s, 
alpha_em or the compulsory scale value are negative at input 
then new values are defined as for internal processes. 
 
 int Info::id1pdf()   
   
 int Info::id2pdf()   
the identities of the two partons for which parton density values 
are defined. 
   
 
 double Info::x1pdf()   
   
 double Info::x2pdf()   
x fractions of the two partons for which parton density values 
are defined. 
   
 
 double Info::pdf1()   
   
 double Info::pdf2()   
parton densities x*f(x,Q^2) evaluated for the two incoming 
partons; could be used e.g. for reweighting purposes in conjunction 
with the idpdf, xpdf and QFac 
methods. Events obtained from external programs or files may not 
contain this information and, if so, 0 is returned. 
   
 
 double Info::QFac()   
   
 double Info::Q2Fac()   
the Q or Q^2 factorization scale at which the 
densities were evaluated. 
   
 
 double Info::alphaS()   
   
 double Info::alphaEM()   
the alpha_strong and alpha_electromagnetic values used 
for the hard process. 
   
 
 double Info::QRen()   
   
 double Info::Q2Ren()   
the Q or Q^2 renormalization scale at which 
alpha_strong and alpha_electromagnetic were evaluated. 
   
 
 double Info::scalup()   
returns the stored SCALUP value for Les Houches events, 
and else zero. It may agree with both the QFac() and 
QRen() values, as explained above. However, to repeat, 
should the input SCALUP scale be negative, separate positive 
factorization and renormalization scales are calculated and set as for 
internally generated events. Furthermore, when PDF info is supplied for 
the Les Houches event, the factorization scale is set by this PDF info 
(scalePDF), which can disagree with SCALUP. 
   
 
 
Hard process kinematics
 
 
The methods in this section provide info on the kinematics of the hard 
processes, with special emphasis on 2 → 2 (diffraction excluded; 
see below). 
 
 double Info::mHat()   
   
 double Info::sHat()   
the invariant mass and its square for the hard process. 
   
 
 double Info::tHat()   
   
 double Info::uHat()   
the remaining two Mandelstam variables; only defined for 2 → 2 
processes. 
   
 
 double Info::pTHat()   
   
 double Info::pT2Hat()   
transverse momentum and its square in the rest frame of a 2 → 2 
processes. 
   
 
 double Info::m3Hat()   
   
 double Info::m4Hat()   
the masses of the two outgoing particles in a 2 → 2 processes. 
   
 
 double Info::thetaHat()   
   
 double Info::phiHat()   
the polar and azimuthal scattering angles in the rest frame of 
a 2 → 2 process. 
   
 
 
Soft Diffraction
 
 
Information on the primary elastic or 
diffractive process 
(A B → A B, X1 B, A X2, X1 X2, A X B) can be obtained with 
the methods in the "Hard process kinematics" section above. The 
variables here obviously are s, t, u, ... rather than 
sHat, tHat, uHat, ..., but the method names remain to avoid 
unnecessary duplication. Most other methods are irrelevant for a 
primary elastic/diffractive process. 
 
Central diffraction A B → A X B is a 2 → 3 
process, and therefore most of the 2 → 2 variables are 
no longer relevant. The tHat() and uHat() 
methods instead return the two t values at the A → A 
and B → B vertices, and pTHat() the average 
transverse momentum of the three outgoing "particles", while 
thetaHat() and phiHat() are undefined. 
 
 
While the primary interaction does not contain a hard process, 
the diffractive subsystems can contain them, but need not. 
Specifically, double diffraction can contain two separate hard 
subprocesses, which breaks the methods above. Most of them have been 
expanded with an optional argument to address properties of diffractive 
subsystems. This argument can take four values: 
 
- 0 : default argument, used for normal nondiffractive events or 
the primary elastic/diffractive process (see above); 
- 1 : the X1 system in single diffraction 
A B → X1 B or double diffraction A B → X1 X2;
- 2 : the X2 system in single diffraction 
A B → A X2 or double diffraction A B → X1 X2;
- 3 : the X system in central diffraction 
A B → A X B. 
The argument is defined for all of the methods in the three sections above, 
"Hard process initiators", "Hard process parton densities and scales" and 
"Hard process kinematics", with the exception of theisValence 
methods. Also the four final methods of "The event type" section, the 
...Sub() methods, take this argument. But recall that they 
will only provide meaningful answers, firstly if there is a system of the 
requested type, and secondly if there is a hard subprocess in this system. 
A simple check for this is that id1() has to be nonvanishing. 
The methods below this section do not currently provide information 
specific to diffractive subsystems, e.g. the MPI information is not 
bookkept in such cases. 
 
 
Hard Diffraction
 
 
Information on the momentum fraction taken from the beam 
and the momentum transfer in the hard diffractive process. 
Note that when side A is diffractively exited, then the Pomeron 
has been taken from side B and vice versa. 
 
 double Info::xPomeronA()   
   
 double Info::xPomeronB()   
x fractions of momenta carried by the Pomeron in the hard 
diffractive process. 
   
 double Info::tPomeronA()   
   
 double Info::tPomeronB()   
The momentum transfer t in the hard diffractive process. 
   
 
 
Photons from lepton beams
 
 
Information about the kinematics of photon-photon collisions from lepton 
beams. 
 double Info::eCMsub()   
Collision energy of the gamma-gamma sub-system. 
   
 double Info::xGammaA()   
   
 double Info::xGammaB()   
x fractions of lepton momenta carried by the photons. 
   
 double Info::Q2GammaA()   
   
 double Info::Q2GammaB()   
Virtualities of the photons emitted by the leptons. 
   
 double Info::thetaScatLepA()   
   
 double Info::thetaScatLepB()   
Scattering angles of the leptons wrt. the beam direction. 
   
 int Info::photonMode()   
Type of photon process, see 
Photoproduction for details. 
   
 
 
Event activity
 
 
 int Info::nISR()   
   
 int Info::nFSRinProc()   
   
 int Info::nFSRinRes()   
the number of emissions in the initial-state showering, in the final-state 
showering excluding resonance decays, and in the final-state showering 
inside resonance decays, respectively. 
   
 
 double Info::pTmaxMPI()   
   
 double Info::pTmaxISR()   
   
 double Info::pTmaxFSR()   
Maximum pT scales set for MPI, ISR and FSR, given the 
process type and scale choice for the hard interactions. The actual 
evolution will run down from these scales. 
   
 
 double Info::pTnow()   
The current pT scale in the combined MPI, ISR and FSR evolution. 
Useful for classification in user hooks, 
but not once the event has been evolved. 
   
 
 
Multiparton interactions
 
 
As already noted, these methods do not make sense for diffractive 
topologies, and should not be used there. Partly this is physics, 
but mainly it is for technical reasons, e.g. that double diffraction 
involves two separate systems that would have to be bookkept as such. 
 
 double Info::a0MPI()   
The value of a0 when an x-dependent matter profile is used, 
MultipartonInteractions:bProfile = 4. 
   
 
 double Info::bMPI()   
The impact parameter b assumed for the current collision when 
multiparton interactions are simulated. Is not expressed in any physical 
size (like fm), but only rescaled so that the average should be unity 
for minimum-bias events (meaning less than that for events with hard 
processes). 
   
 
 double Info::enhanceMPI()   
The choice of impact parameter implies an enhancement or depletion of 
the rate of subsequent interactions, as given by this number. Again 
the average is normalized to be unity for minimum-bias events (meaning 
more than that for events with hard processes). 
   
 
 double Info::enhanceMPIavg()   
The average enhancement factor expected for hard processes, in those 
cases where it can be calculated already at initialization, i.e. excluding 
the x-dependent b profile. The normalization is here 
chosen to apply to cases with two hard interactions A and 
B preselected in the process level, and there multiplies 
sigma_A * sigma_B / sigma_{nondiff} to give the joint cross 
section. (Additional corrections from joint PDF weights somewhat reduce 
the final number.) The normalization is slightly different (typically 
around 5%) from the average of the enhanceMPI() method above, 
which instead is normalized to average value unity for nondiffractive events. 
As used internally the two are consistent. 
   
 
 int Info::nMPI()   
The number of hard interactions in the current event. Is 0 for elastic 
and diffractive events, and else at least 1, with more possible from 
multiparton interactions. 
   
 
 int Info::codeMPI(int i)   
   
 double Info::pTMPI(int i)   
the process code and transverse momentum of the i'th 
subprocess, with i in the range from 0 to 
nMPI() - 1. The values for subprocess 0 is redundant with 
information already provided above. 
   
 
 int Info::iAMPI(int i)   
   
 int Info::iBMPI(int i)   
are normally zero. However, if the i'th subprocess is 
a rescattering, i.e. either or both incoming partons come from the 
outgoing state of previous scatterings, they give the position in the 
event record of the outgoing-state parton that rescatters. 
iAMPI and iBMPI then denote partons coming from 
the first or second beam, respectively. 
   
 
 double Info::eMPI(int i)   
The enhancement or depletion of the rate of the i'th 
subprocess. Is primarily of interest for the 
MultipartonInteractions:bProfile = 4 option, where the 
size of the proton depends on the x values of the colliding 
partons. Note that eMPI(0) = enhanceMPI(). 
   
 
 double Info::bMPIold()   
   
 double Info::enhanceMPIold()   
   
 double Info::enhanceMPIoldavg()   
These methods are only relevant for hard diffraction with the requirement 
of no MPI in the hadron-hadron collision. Then an impact parameter 
and associated enhancement factor is picked for this collision, but 
afterwards overwritten when the Pomeron-hadron subcollision is considered. 
In such cases the old hadron-hadron values can be found here, while 
bMPI, enhanceMPI and enhanceMPIavg 
provide the new Pomeron-hadron ones. 
   
 
 
Cross sections
 
 
Dedicated documentation describes the details of 
cross sections and weights. 
 
 
Loop counters
 
 
Mainly for internal/debug purposes, a number of loop counters from 
various parts of the program are stored in the Info class, 
so that one can keep track of how the event generation is progressing. 
This may be especially useful in the context of the 
User Hooks facility. 
 
 int Info::getCounter(int i)   
the method that gives you access to the value of the various loop 
counters. 
argument i   :  the counter number you want to access: 
argumentoption  0 - 9 :  counters that refer to the run as a whole, 
i.e. are set 0 at the beginning of the run and then only can increase. 
   
argumentoption  0 :  the number of successful constructor calls for the 
Pythia class (can only be 0 or 1). 
   
argumentoption  1 :  the number of times a Pythia::init() 
call has been begun. 
   
argumentoption  2 :  the number of times a Pythia::init() 
call has been completed successfully. 
   
argumentoption  3 :  the number of times a Pythia::next() 
call has been begun. 
   
argumentoption  4 :  the number of times a Pythia::next() 
call has been completed successfully. 
   
argumentoption  10 - 19 :  counters that refer to each individual event, 
and are reset and updated in the top-level Pythia::next() 
method. 
   
argumentoption  10 :  the number of times the selection of a new hard 
process has been begun. Normally this should only happen once, unless a 
user veto is set to abort the current process and try a new one. 
   
argumentoption  11 :  the number of times the selection of a new hard 
process has been completed successfully. 
   
argumentoption  12 :  as 11, but additionally the process should 
survive any user veto and go on to the parton- and hadron-level stages. 
   
argumentoption  13 :  as 11, but additionally the process should 
survive the parton- and hadron-level stage and any user cuts. 
   
argumentoption  14 :  the number of times the loop over parton- and 
hadron-level processing has begun for a hard process. Is reset each 
time counter 12 above is reached. 
   
argumentoption  15 :  the number of times the above loop has successfully 
completed the parton-level step. 
   
argumentoption  16 :  the number of times the above loop has successfully 
completed the checks and user vetoes after the parton-level step. 
   
argumentoption  17 :  the number of times the above loop has successfully 
completed the hadron-level step. 
   
argumentoption  18 :  the number of times the above loop has successfully 
completed the checks and user vetoes after the hadron-level step. 
   
argumentoption  20 - 39 :  counters that refer to a local part of the 
individual event, and are reset at the beginning of this part. 
   
argumentoption  20 :  the current system being processed in 
PartonLevel::next(). Is almost always 1, but for double 
diffraction the two diffractive systems are 1 and 2, respectively. 
   
argumentoption  21 :  the number of times the processing of the 
current system (see above) has begun. 
   
argumentoption  22 :  the number of times a step has begun in the 
combined MPI/ISR/FSR evolution downwards in pT 
for the current system. 
   
argumentoption  23 :  the number of times MPI has been selected for the 
downwards step above. 
   
argumentoption  24 :  the number of times ISR has been selected for the 
downwards step above. 
   
argumentoption  25 :  the number of times FSR has been selected for the 
downwards step above. 
   
argumentoption  26 :   the number of times MPI has been accepted as the 
downwards step above, after the vetoes. 
   
argumentoption  27 :   the number of times ISR has been accepted as the 
downwards step above, after the vetoes. 
   
argumentoption  28 :   the number of times FSR has been accepted as the 
downwards step above, after the vetoes. 
   
argumentoption  29 :  the number of times a step has begun in the 
separate (optional) FSR evolution downwards in pT 
for the current system. 
   
argumentoption  30 :  the number of times FSR has been selected for the 
downwards step above. 
   
argumentoption  31 :   the number of times FSR has been accepted as the 
downwards step above, after the vetoes. 
   
argumentoption  40 :  keeps track of vetoed emission for shower 
reweighting. 
   
argumentoption  41 - 49 :  counters that are unused (currently), and 
that therefore are free to use, with the help of the two methods below. 
   
   
   
 
 void Info::setCounter(int i, int value = 0)   
set the above counters to a given value. Only to be used by you 
for the unassigned counters 40 - 49. 
argument i   :  the counter number, see above. 
   
argument value  (default = 0) :  set the counter to this number; 
normally the default value is what you want. 
   
   
 
 void Info::addCounter(int i, int value = 0)   
increase the above counters by a given amount. Only to be used by you 
for the unassigned counters 40 - 49. 
argument i   :  the counter number, see above. 
   
argument value  (default = 1) :  increase the counter by this amount; 
normally the default value is what you want. 
   
   
 
 
Parton shower history
 
 
The following methods are mainly intended for internal use, 
e.g. for matrix-element matching. 
 
 void Info::hasHistory(bool hasHistoryIn)   
   
 bool Info::hasHistory()   
set/get knowledge whether the likely shower history of an event 
has been traced. 
   
 
 void Info::zNowISR(bool zNowIn)   
   
 double Info::zNowISR()   
set/get value of z in latest ISR branching. 
   
 
 void Info::pT2NowISR(bool pT2NowIn)   
   
 double Info::pT2NowISR()   
set/get value of pT^2 in latest ISR branching. 
   
 
 
Les Houches Event File information
 
 
Since the Info class is one of the main interfaces between 
the PYTHIA generation and the user, it also handles the retrieval of 
information that is passed to PYTHIA through input Les Houches Event 
files. 
The Info class further provides the interface to 
the information stored after reading Les Houches Event files in the 
updated format [But14]. An example main program using LHEF 3.0 
information is main127.cc. 
 
For the documentation of the necessary Info class 
retrieval functions, please consult the 
Les Houches Event Files  section. 
 
 
Heavy Ion Information
 
 
When generating collisions involving heavy 
ions, the Info object will contain a non-null 
pointer Info::hiInfo to a special HIInfo 
object. The information stored there is typically related to the 
Glauber modelling of the nucleons in a nuclei and is detailed on the 
Heavy Ion Collisions page.