main28
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// main28.cc is a part of the PYTHIA event generator.
// Copyright (C) 2024 Torbjorn Sjostrand.
// PYTHIA is licenced under the GNU GPL v2 or later, see COPYING for details.
// Please respect the MCnet Guidelines, see GUIDELINES for details.
// Keywords:
// BSM
// R‑hadron
// Displaced vertex
// Example of of R-hadron production.
// Several of the possibilities shown here, like displaced vertices,
// are extras that need not be used for the basic setup.
#include "Pythia8/Pythia.h"
using namespace Pythia8;
int main() {
// Key settings to be used in the main program.
// nGluino = 0, 1, 2 give stop pair, single gluino or gluino pair.
int nGluino = 2;
int nEvent = 200;
int nAbort = 3;
int nList = 0;
double eCM = 7000.;
// Generator. Shorthand for the event.
Pythia pythia;
Event& event = pythia.event;
// Set up beams: p p is default so only need set energy.
pythia.settings.parm("Beams:eCM", eCM);
// Squark pair: use stop-antistop as example.
if (nGluino == 0) {
pythia.readString("SUSY:gg2squarkantisquark = on");
pythia.readString("SUSY:idA = 1000006");
pythia.readString("SUSY:idB = 1000006");
// Squark-gluino pair: also supersymmetric u has been made long-lived.
// Stop does not work since then one would need inoming top PDF.
// Nevertheless R-hadrons are numbered/named as if containing a stop.
} else if (nGluino == 1) {
pythia.readString("SUSY:qg2squarkgluino = on");
pythia.readString("SUSY:idA = 1000002");
pythia.readString("RHadrons:idStop = 1000002");
pythia.readString("SUSY:idB = 1000021");
// Gluino pair.
} else {
pythia.readString("SUSY:gg2gluinogluino = on");
}
// Use hacked sps1a file, with stop (+su) and gluino made long-lived.
// This is based on the width being less than 0.2 GeV by default.
pythia.readString("SLHA:file = sps1aNarrowStopGluino.spc");
// Further hacked file, to test R-parity violating gluino decay.
//pythia.readString("SLHA:file = sps1aNarrowStopGluinoRPV.spc");
// Allow R-hadron formation.
pythia.readString("Rhadrons:allow = on");
// If you want to do the decay separately later,
// you need to switch off automatic decays.
pythia.readString("RHadrons:allowDecay = off");
// Fraction of gluinoballs.
pythia.readString("RHadrons:probGluinoball = 0.1");
// Switch off key components.
//pythia.readString("PartonLevel:MPI = off");
//pythia.readString("PartonLevel:ISR = off");
//pythia.readString("PartonLevel:FSR = off");
//pythia.readString("HadronLevel:Hadronize = off");
// Allow the R-hadrons to have secondary vertices: set c*tau in mm.
// Note that width and lifetime can be set independently.
// (Nonzero small widths are needed e.g. to select branching ratios.)
pythia.readString("1000002:tau0 = 200.");
pythia.readString("1000006:tau0 = 250.");
pythia.readString("1000021:tau0 = 300.");
// Checks. Optionally relax E-p-conservation.
pythia.readString("Check:nErrList = 2");
//pythia.readString("Check:epTolErr = 2e-3");
// Possibility to switch off particle data and event listings.
// Also to shop location of displaced vertices.
pythia.readString("Init:showChangedSettings = on");
pythia.readString("Init:showChangedParticleData = off");
pythia.readString("Next:numberShowInfo = 1");
pythia.readString("Next:numberShowProcess = 1");
pythia.readString("Next:numberShowEvent = 0");
pythia.readString("Next:showScaleAndVertex = on");
// Initialize.
pythia.init();
// Histograms.
Hist nChargedH("charged multiplicity", 100, -0.5, 799.5);
Hist dndyChargedH("dn/dy charged", 100, -10., 10.);
Hist dndyRH("dn/dy R-hadrons", 100, -5., 5.);
Hist pTRH("pT R-hadrons", 100, 0., 1000.);
Hist xRH("p_RHadron / p_sparticle", 100, 0.9, 1.1);
Hist mDiff("m(Rhadron) - m(sparticle)", 100, 0., 5.);
Hist decVtx("R-hadron decay vertex (mm from origin)", 100, 0., 1000.);
// R-hadron flavour composition.
map<int, int> flavours;
// Begin event loop.
int iAbort = 0;
for (int iEvent = 0; iEvent < nEvent; ++iEvent) {
// Generate events. Quit if failure.
if (!pythia.next()) {
if (++iAbort < nAbort) continue;
cout << " Event generation aborted prematurely, owing to error!\n";
break;
}
// Loop over final charged particles in the event.
// The R-hadrons may not yet have decayed here.
int nCharged = 0;
Vec4 pSum;
for (int i = 0; i < event.size(); ++i) {
if (event[i].isFinal()) {
pSum += event[i].p();
if (event[i].isCharged()) {
++nCharged;
dndyChargedH.fill( event[i].y() );
}
}
}
nChargedH.fill( nCharged );
// Loop over final R-hadrons in the event: kinematic distribution
for (int i = 0; i < event.size(); ++i) {
int idAbs = event[i].idAbs();
if (idAbs > 1000100 && idAbs < 2000000 && idAbs != 1009002) {
++flavours[ event[i].id() ];
dndyRH.fill( event[i].y() );
pTRH.fill( event[i].pT() );
// Trace back to mother; compare momenta and masses.
int iMother = i;
while( event[iMother].statusAbs() > 100)
iMother = event[iMother].mother1();
double xFrac = event[i].pAbs() / event[iMother].pAbs();
xRH.fill( xFrac);
double mShift = event[i].m() - event[iMother].m();
mDiff.fill( mShift );
// Separation of R-hadron decay vertex from origin.
// Don't be fooled by pAbs(); it gives the three-vector length
// of any Vec4, also one representing spatial coordinates.
double dist = event[i].vDec().pAbs();
decVtx.fill( dist);
// This is a place where you could allow a R-hadron shift of
// identity, momentum and decay vertex to allow for detector effects.
// Identity not illustrated here; requires a change of mass as well.
// Toy model: assume an exponential energy loss, < > = 1 GeV,
// but at most half of kinetic energy. Unchanged direction.
// Note that event will no longer conserve energy and momentum.
double eLossAvg = 1.;
double eLoss = 0.;
do { eLoss = eLossAvg * pythia.rndm.exp(); }
while (eLoss > 0.5 * (event[i].e() - event[i].m()));
double eNew = event[i].e() - eLoss;
Vec4 pNew = event[i].p() * sqrt( pow2(eNew) - pow2(event[i].m()) )
/ event[i].pAbs();
pNew.e( eNew);
event[i].p( pNew);
// The decay vertex will be calculated based on the production vertex,
// the proper lifetime tau and the NEW four-momentum, rather than
// e.g. some average momentum, if you do not set it by hand.
// This commented-out piece illustrates brute-force setting,
// but you should provide real numbers from some tracking program.
// With tau = 0 the decay is right at the chosen point.
//event[i].tau( 0.);
//event[i].vProd( 132., 155., 233., 177.);
// End of loop over final R-hadrons.
}
}
// If you have set R-hadrons stable above,
// you can still force them to decay at this stage.
pythia.forceRHadronDecays();
if (iEvent < nList) pythia.event.list(true);
// End of event loop.
}
// Final statistics, flavour composition and histogram output.
pythia.stat();
cout << "\n Composition of produced R-hadrons \n code "
<< "name times " << endl;
for (map<int, int>::iterator flavNow = flavours.begin();
flavNow != flavours.end(); ++flavNow) cout << setw(8)
<< flavNow->first << setw(16) << pythia.particleData.name(flavNow->first)
<< setw(8) << flavNow->second << endl;
cout << nChargedH << dndyChargedH << dndyRH << pTRH << xRH << mDiff
<< decVtx;
// Done.
return 0;
}