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// main15.cc is a part of the PYTHIA event generator.
// Copyright (C) 2012 Torbjorn Sjostrand.
// PYTHIA is licenced under the GNU GPL version 2, see COPYING for details.
// Please respect the MCnet Guidelines, see GUIDELINES for details.
// This is a simple test program.
// It illustrates how either
// (a) B decays (sections marked "Repeated decays:), or
// (b) all hadronization (sections marked "Repeated hadronization:")
// could be repeated a number of times for each event,
// to improve statistics when this could be a problem.
// Option (a) is faster than (b), but less generic.
#include "Pythia.h"
using namespace Pythia8;
int main() {
// Main switches: redo B decays only or redo all hadronization, but not both.
bool redoBDecays = false;
bool redoHadrons = true;
if (redoHadrons) redoBDecays = false;
// Number of events. Number to list redone events.
int nEvent = 100;
int nListRedo = 1;
// Number of times decays/hadronization should be redone for each event.
int nRepeat = 10;
if (!redoBDecays && !redoHadrons) nRepeat = 1;
// Generator. Shorthand for event.
Pythia pythia;
Event& event = pythia.event;
// Simulate b production above given pTmin scale.
// Warning: these processes do not catch all possible production modes.
// You would need to use HardQCD:all or even SoftQCD:minBias for that.
pythia.readString("HardQCD:gg2bbbar = on");
pythia.readString("HardQCD:qqbar2bbbar = on");
pythia.readString("PhaseSpace:pTHatMin = 50.");
// Repeated decays: list of weakly decaying B hadrons.
// Note: this list is overkill; some will never be produced.
int bCodes[28] = {511, 521, 531, 541, 5122, 5132, 5142, 5232, 5242,
5332, 5342, 5412, 5414, 5422, 5424, 5432, 5434, 5442, 5444, 5512,
5514, 5522, 5524, 5532, 5534, 5542, 5544, 5544 };
int nCodes = 28;
// Repeated decays: location of B handrons.
vector<int> iBHad;
int nBHad = 0;
// Repeated hadronization: spare copy of event.
Event savedEvent;
// Repeated hadronization: switch off normal HadronLevel call.
if (redoHadrons) pythia.readString("HadronLevel:all = off");
// Initialize for LHC energies; default 14 TeV
pythia.init();
// Histogram invariant mass of muon pairs.
Hist nBperEvent("number of b quarks in an event", 10, -0.5, 9.5);
Hist nSameEvent("number of times same event is used", 10, -0.5, 9.5);
Hist oppSignMass("mass of opposite-sign muon pair", 100, 0.0, 100.0);
Hist sameSignMass("mass of same-sign muon pair", 100, 0.0, 100.0);
// Begin event loop.
for (int iEvent = 0; iEvent < nEvent; ++iEvent) {
// Repeated decays: switch off decays of weakly decaying B hadrons.
// (More compact solution than repeated readString(..).)
if (redoBDecays) for (int iC = 0; iC < nCodes; ++iC)
pythia.particleData.mayDecay( bCodes[iC], false);
// Generate event. Skip it if error.
if (!pythia.next()) continue;
// Find and histogram number of b quarks.
int nBquark = 0;
int stat;
for (int i = 0; i < event.size(); ++i) {
stat = event[i].statusAbs();
if (event[i].idAbs() == 5 && (stat == 62 || stat == 63)) ++nBquark;
}
nBperEvent.fill( nBquark );
// Repeated decays: find all locations where B hadrons are stored.
if (redoBDecays) {
iBHad.resize(0);
for (int i = 0; i < event.size(); ++i) {
int idAbs = event[i].idAbs();
for (int iC = 0; iC < 28; ++iC)
if (idAbs == bCodes[iC]) {
iBHad.push_back(i);
break;
}
}
// Repeated decays: check that #b = #B.
nBHad = iBHad.size();
if (nBquark != nBHad) cout << " Warning: " << nBquark
<< " b quarks but " << nBHad << " B hadrons" << endl;
// Repeated decays: store size of current event.
event.saveSize();
// Repeated decays: switch back on weakly decaying B hadrons.
for (int iC = 0; iC < nCodes; ++iC)
pythia.particleData.mayDecay( bCodes[iC], true);
// Repeated hadronization: copy event into spare position.
} else if (redoHadrons) {
savedEvent = event;
}
// Begin loop over rounds of decays / hadronization for same event.
int nWithPair = 0;
for (int iRepeat = 0; iRepeat < nRepeat; ++iRepeat) {
// Repeated decays: remove B decay products from previous round.
if (redoBDecays) {
if (iRepeat > 0) {
event.restoreSize();
// Repeated decays: mark decayed B hadrons as undecayed.
for (int iB = 0; iB < nBHad; ++iB) event[ iBHad[iB] ].statusPos();
}
// Repeated decays: do decays of B hadrons, sequentially for products.
// Note: modeDecays does not work for bottomonium (or heavier) states,
// since there decays like Upsilon -> g g g also need hadronization.
// Also, there is no provision for Bose-Einstein effects.
if (!pythia.moreDecays()) continue;
// Repeated hadronization: restore saved event record.
} else if (redoHadrons) {
if (iRepeat > 0) event = savedEvent;
// Repeated hadronization: do HadronLevel (repeatedly).
// Note: argument false needed owing to bug in junction search??
if (!pythia.forceHadronLevel(false)) continue;
}
// List last repetition of first few events.
if ( (redoBDecays || redoHadrons) && iEvent < nListRedo
&& iRepeat == nRepeat - 1) event.list();
// Look for muons among decay products (also from charm/tau/...).
vector<int> iMuNeg, iMuPos;
for (int i = 0; i < event.size(); ++i) {
int id = event[i].id();
if (id == 13) iMuNeg.push_back(i);
if (id == -13) iMuPos.push_back(i);
}
// Check whether pair(s) present.
int nMuNeg = iMuNeg.size();
int nMuPos = iMuPos.size();
if (nMuNeg + nMuPos > 1) {
++nWithPair;
// Fill masses of opposite-sign pairs.
for (int iN = 0; iN < nMuNeg; ++iN)
for (int iP = 0; iP < nMuPos; ++iP)
oppSignMass.fill(
(event[iMuNeg[iN]].p() + event[iMuPos[iP]].p()).mCalc() );
// Fill masses of same-sign pairs.
for (int i1 = 0; i1 < nMuNeg - 1; ++i1)
for (int i2 = i1 + 1; i2 < nMuNeg; ++i2)
sameSignMass.fill(
(event[iMuNeg[i1]].p() + event[iMuNeg[i2]].p()).mCalc() );
for (int i1 = 0; i1 < nMuPos - 1; ++i1)
for (int i2 = i1 + 1; i2 < nMuPos; ++i2)
sameSignMass.fill(
(event[iMuPos[i1]].p() + event[iMuPos[i2]].p()).mCalc() );
// Finished analysis of current round.
}
// End of loop over many rounds. fill number of rounds with pairs.
}
nSameEvent.fill( nWithPair );
// End of event loop.
}
// Statistics. Histograms.
pythia.stat();
cout << nBperEvent << nSameEvent << oppSignMass << sameSignMass << endl;
// Done.
return 0;
}
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