[1] | 1 | // main28.cc is a part of the PYTHIA event generator. |
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| 2 | // Copyright (C) 2012 Peter Skands, Torbjorn Sjostrand. |
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| 3 | // PYTHIA is licenced under the GNU GPL version 2, see COPYING for details. |
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| 4 | // Please respect the MCnet Guidelines, see GUIDELINES for details. |
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| 5 | |
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| 6 | // Example of of R-hadron production. |
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| 7 | // Several of the possibilities shown here, like displaced vertices, |
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| 8 | // are extras that need not be used for the basic setup. |
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| 9 | |
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| 10 | #include "Pythia.h" |
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| 11 | |
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| 12 | using namespace Pythia8; |
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| 13 | |
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| 14 | int main() { |
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| 15 | |
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| 16 | // Key settings to be used in the main program. |
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| 17 | // nGluino = 0, 1, 2 give stop pair, single gluino or gluino pair. |
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| 18 | int nGluino = 2; |
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| 19 | int nEvent = 200; |
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| 20 | int nAbort = 3; |
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| 21 | int nList = 0; |
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| 22 | double eCM = 7000.; |
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| 23 | |
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| 24 | // Generator. Shorthand for the event. |
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| 25 | Pythia pythia; |
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| 26 | Event& event = pythia.event; |
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| 27 | |
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| 28 | // Set up beams: p p is default so only need set energy. |
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| 29 | pythia.settings.parm("Beams:eCM", eCM); |
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| 30 | |
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| 31 | // Squark pair: use stop-antistop as example. |
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| 32 | if (nGluino == 0) { |
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| 33 | pythia.readString("SUSY:gg2squarkantisquark = on"); |
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| 34 | pythia.readString("SUSY:idA = 1000006"); |
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| 35 | pythia.readString("SUSY:idB = 1000006"); |
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| 36 | // Squark-gluino pair: also supersymmetric u has been made long-lived. |
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| 37 | // Stop does not work since then one would need inoming top PDF. |
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| 38 | // Nevertheless R-hadrons are numbered/named as if containing a stop. |
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| 39 | } else if (nGluino == 1) { |
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| 40 | pythia.readString("SUSY:qg2squarkgluino = on"); |
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| 41 | pythia.readString("SUSY:idA = 1000002"); |
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| 42 | pythia.readString("RHadrons:idStop = 1000002"); |
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| 43 | pythia.readString("SUSY:idB = 1000021"); |
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| 44 | // Gluino pair. |
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| 45 | } else { |
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| 46 | pythia.readString("SUSY:gg2gluinogluino = on"); |
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| 47 | } |
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| 48 | |
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| 49 | // Use hacked sps1a file, with stop (+su) and gluino made long-lived. |
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| 50 | // This is based on the width being less than 0.2 GeV by default. |
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| 51 | pythia.readString("SLHA:file = sps1aNarrowStopGluino.spc"); |
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| 52 | // Further hacked file, to test R-parity violating gluino decay. |
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| 53 | //pythia.readString("SLHA:file = sps1aNarrowStopGluinoRPV.spc"); |
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| 54 | |
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| 55 | // Allow R-hadron formation. |
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| 56 | pythia.readString("Rhadrons:allow = on"); |
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| 57 | |
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| 58 | // If you want to do the decay separately later, |
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| 59 | // you need to switch off automatic decays. |
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| 60 | pythia.readString("RHadrons:allowDecay = off"); |
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| 61 | |
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| 62 | // Fraction of gluinoballs. |
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| 63 | pythia.readString("RHadrons:probGluinoball = 0.1"); |
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| 64 | |
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| 65 | // Switch off key components. |
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| 66 | //pythia.readString("PartonLevel:MPI = off"); |
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| 67 | //pythia.readString("PartonLevel:ISR = off"); |
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| 68 | //pythia.readString("PartonLevel:FSR = off"); |
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| 69 | //pythia.readString("HadronLevel:Hadronize = off"); |
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| 70 | |
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| 71 | // Allow the R-hadrons to have secondary vertices: set c*tau in mm. |
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| 72 | // Note that width and lifetime can be set independently. |
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| 73 | // (Nonzero small widths are needed e.g. to select branching ratios.) |
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| 74 | pythia.readString("1000002:tau0 = 200."); |
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| 75 | pythia.readString("1000006:tau0 = 250."); |
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| 76 | pythia.readString("1000021:tau0 = 300."); |
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| 77 | |
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| 78 | // Checks. Optionally relax E-p-conservation. |
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| 79 | pythia.readString("Check:nErrList = 2"); |
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| 80 | //pythia.readString("Check:epTolErr = 2e-3"); |
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| 81 | |
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| 82 | // Possibility to switch off particle data and event listings. |
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| 83 | // Also to shop location of displaced vertices. |
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| 84 | pythia.readString("Init:showChangedSettings = on"); |
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| 85 | pythia.readString("Init:showChangedParticleData = off"); |
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| 86 | pythia.readString("Next:numberShowInfo = 1"); |
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| 87 | pythia.readString("Next:numberShowProcess = 1"); |
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| 88 | pythia.readString("Next:numberShowEvent = 0"); |
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| 89 | pythia.readString("Next:showScaleAndVertex = on"); |
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| 90 | |
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| 91 | // Initialize. |
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| 92 | pythia.init(); |
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| 93 | |
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| 94 | // Histograms. |
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| 95 | Hist nChargedH("charged multiplicity", 100, -0.5, 799.5); |
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| 96 | Hist dndyChargedH("dn/dy charged", 100, -10., 10.); |
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| 97 | Hist dndyRH("dn/dy R-hadrons", 100, -5., 5.); |
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| 98 | Hist pTRH("pT R-hadrons", 100, 0., 1000.); |
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| 99 | Hist xRH("p_RHadron / p_sparticle", 100, 0.9, 1.1); |
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| 100 | Hist mDiff("m(Rhadron) - m(sparticle)", 100, 0., 5.); |
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| 101 | Hist decVtx("R-hadron decay vertex (mm from origin)", 100, 0., 1000.); |
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| 102 | |
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| 103 | // R-hadron flavour composition. |
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| 104 | map<int, int> flavours; |
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| 105 | |
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| 106 | // Begin event loop. |
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| 107 | int iAbort = 0; |
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| 108 | for (int iEvent = 0; iEvent < nEvent; ++iEvent) { |
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| 109 | |
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| 110 | // Generate events. Quit if failure. |
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| 111 | if (!pythia.next()) { |
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| 112 | if (++iAbort < nAbort) continue; |
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| 113 | cout << " Event generation aborted prematurely, owing to error!\n"; |
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| 114 | break; |
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| 115 | } |
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| 116 | |
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| 117 | // Loop over final charged particles in the event. |
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| 118 | // The R-hadrons may not yet have decayed here. |
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| 119 | int nCharged = 0; |
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| 120 | Vec4 pSum; |
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| 121 | for (int i = 0; i < event.size(); ++i) { |
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| 122 | if (event[i].isFinal()) { |
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| 123 | pSum += event[i].p(); |
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| 124 | if (event[i].isCharged()) { |
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| 125 | ++nCharged; |
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| 126 | dndyChargedH.fill( event[i].y() ); |
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| 127 | } |
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| 128 | } |
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| 129 | } |
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| 130 | nChargedH.fill( nCharged ); |
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| 131 | |
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| 132 | // Loop over final R-hadrons in the event: kinematic distribution |
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| 133 | for (int i = 0; i < event.size(); ++i) { |
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| 134 | int idAbs = event[i].idAbs(); |
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| 135 | if (idAbs > 1000100 && idAbs < 2000000 && idAbs != 1009002) { |
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| 136 | ++flavours[ event[i].id() ]; |
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| 137 | dndyRH.fill( event[i].y() ); |
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| 138 | pTRH.fill( event[i].pT() ); |
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| 139 | // Trace back to mother; compare momenta and masses. |
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| 140 | int iMother = i; |
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| 141 | while( event[iMother].statusAbs() > 100) |
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| 142 | iMother = event[iMother].mother1(); |
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| 143 | double xFrac = event[i].pAbs() / event[iMother].pAbs(); |
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| 144 | xRH.fill( xFrac); |
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| 145 | double mShift = event[i].m() - event[iMother].m(); |
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| 146 | mDiff.fill( mShift ); |
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| 147 | // Separation of R-hadron decay vertex from origin. |
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| 148 | // Don't be fooled by pAbs(); it gives the three-vector length |
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| 149 | // of any Vec4, also one representing spatial coordinates. |
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| 150 | double dist = event[i].vDec().pAbs(); |
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| 151 | decVtx.fill( dist); |
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| 152 | |
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| 153 | // This is a place where you could allow a R-hadron shift of |
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| 154 | // identity, momentum and decay vertex to allow for detector effects. |
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| 155 | // Identity not illustrated here; requires a change of mass as well. |
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| 156 | // Toy model: assume an exponential energy loss, < > = 1 GeV, |
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| 157 | // but at most half of kinetic energy. Unchanged direction. |
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| 158 | // Note that event will no longer conserve energy and momentum. |
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| 159 | double eLossAvg = 1.; |
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| 160 | double eLoss = 0.; |
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| 161 | do { eLoss = eLossAvg * pythia.rndm.exp(); } |
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| 162 | while (eLoss > 0.5 * (event[i].e() - event[i].m())); |
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| 163 | double eNew = event[i].e() - eLoss; |
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| 164 | Vec4 pNew = event[i].p() * sqrt( pow2(eNew) - pow2(event[i].m()) ) |
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| 165 | / event[i].pAbs(); |
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| 166 | pNew.e( eNew); |
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| 167 | event[i].p( pNew); |
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| 168 | // The decay vertex will be calculated based on the production vertex, |
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| 169 | // the proper lifetime tau and the NEW four-momentum, rather than |
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| 170 | // e.g. some average momentum, if you do not set it by hand. |
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| 171 | // This commented-out piece illustrates brute-force setting, |
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| 172 | // but you should provide real numbers from some tracking program. |
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| 173 | // With tau = 0 the decay is right at the chosen point. |
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| 174 | //event[i].tau( 0.); |
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| 175 | //event[i].vProd( 132., 155., 233., 177.); |
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| 176 | |
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| 177 | // End of loop over final R-hadrons. |
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| 178 | } |
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| 179 | } |
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| 180 | |
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| 181 | // If you have set R-hadrons stable above, |
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| 182 | // you can still force them to decay at this stage. |
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| 183 | pythia.forceRHadronDecays(); |
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| 184 | if (iEvent < nList) pythia.event.list(true); |
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| 185 | |
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| 186 | // End of event loop. |
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| 187 | } |
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| 188 | |
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| 189 | // Final statistics, flavour composition and histogram output. |
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| 190 | pythia.stat(); |
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| 191 | cout << "\n Composition of produced R-hadrons \n code " |
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| 192 | << "name times " << endl; |
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| 193 | for (map<int, int>::iterator flavNow = flavours.begin(); |
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| 194 | flavNow != flavours.end(); ++flavNow) cout << setw(8) |
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| 195 | << flavNow->first << setw(16) << pythia.particleData.name(flavNow->first) |
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| 196 | << setw(8) << flavNow->second << endl; |
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| 197 | cout << nChargedH << dndyChargedH << dndyRH << pTRH << xRH << mDiff |
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| 198 | << decVtx; |
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| 199 | |
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| 200 | // Done. |
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| 201 | return 0; |
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| 202 | } |
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