main234
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// main234.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:
// Hadronization
// This is a simple test program.
// It illustrates how to feed in a single particle (including a resonance)
// or a toy parton-level configurations.
#include "Pythia8/Pythia.h"
using namespace Pythia8;
//==========================================================================
// Single-particle gun. The particle must be a colour singlet.
// Input: flavour, energy, direction (theta, phi).
// If theta < 0 then random choice over solid angle.
// Optional final argument to put particle at rest => E = m.
void fillParticle(int id, double ee, double thetaIn, double phiIn,
Event& event, ParticleData& pdt, Rndm& rndm, bool atRest = false,
bool hasLifetime = false) {
// Reset event record to allow for new event.
event.reset();
// Select particle mass; where relevant according to Breit-Wigner.
double mm = pdt.mSel(id);
double pp = sqrtpos(ee*ee - mm*mm);
// Special case when particle is supposed to be at rest.
if (atRest) {
ee = mm;
pp = 0.;
}
// Angles as input or uniform in solid angle.
double cThe, sThe, phi;
if (thetaIn >= 0.) {
cThe = cos(thetaIn);
sThe = sin(thetaIn);
phi = phiIn;
} else {
cThe = 2. * rndm.flat() - 1.;
sThe = sqrtpos(1. - cThe * cThe);
phi = 2. * M_PI * rndm.flat();
}
// Store the particle in the event record.
int iNew = event.append( id, 1, 0, 0, pp * sThe * cos(phi),
pp * sThe * sin(phi), pp * cThe, ee, mm);
// Generate lifetime, to give decay away from primary vertex.
if (hasLifetime) event[iNew].tau( event[iNew].tau0() * rndm.exp() );
}
//==========================================================================
// Simple method to do the filling of partons into the event record.
void fillPartons(int type, double ee, Event& event, ParticleData& pdt,
Rndm& rndm) {
// Reset event record to allow for new event.
event.reset();
// Information on a q qbar system, to be hadronized.
if (type == 1 || type == 12) {
int id = 2;
double mm = pdt.m0(id);
double pp = sqrtpos(ee*ee - mm*mm);
event.append( id, 23, 101, 0, 0., 0., pp, ee, mm);
event.append( -id, 23, 0, 101, 0., 0., -pp, ee, mm);
// Information on a g g system, to be hadronized.
} else if (type == 2 || type == 13) {
event.append( 21, 23, 101, 102, 0., 0., ee, ee);
event.append( 21, 23, 102, 101, 0., 0., -ee, ee);
// Information on a g g g system, to be hadronized.
} else if (type == 3) {
event.append( 21, 23, 101, 102, 0., 0., ee, ee);
event.append( 21, 23, 102, 103, 0.8 * ee, 0., -0.6 * ee, ee);
event.append( 21, 23, 103, 101, -0.8 * ee, 0., -0.6 * ee, ee);
// Information on a q q q junction system, to be hadronized.
} else if (type == 4 || type == 5) {
// Need a colour singlet mother parton to define junction origin.
event.append( 1000022, -21, 0, 0, 2, 4, 0, 0,
0., 0., 1.01 * ee, 1.01 * ee);
// The three endpoint q q q; the minimal system.
double rt75 = sqrt(0.75);
event.append( 2, 23, 1, 0, 0, 0, 101, 0,
0., 0., 1.01 * ee, 1.01 * ee);
event.append( 2, 23, 1, 0, 0, 0, 102, 0,
rt75 * ee, 0., -0.5 * ee, ee );
event.append( 1, 23, 1, 0, 0, 0, 103, 0,
-rt75 * ee, 0., -0.5 * ee, ee );
// Define the qqq configuration as starting point for adding gluons.
if (type == 5) {
int colNow[4] = {0, 101, 102, 103};
Vec4 pQ[4];
pQ[1] = Vec4(0., 0., 1., 0.);
pQ[2] = Vec4( rt75, 0., -0.5, 0.);
pQ[3] = Vec4(-rt75, 0., -0.5, 0.);
// Minimal cos(q-g opening angle), allows more or less nasty events.
double cosThetaMin =0.;
// Add a few gluons (almost) at random.
for (int nglu = 0; nglu < 5; ++nglu) {
int iq = 1 + int( 2.99999 * rndm.flat() );
double px, py, pz, e, prod;
do {
e = ee * rndm.flat();
double cThe = 2. * rndm.flat() - 1.;
double phi = 2. * M_PI * rndm.flat();
px = e * sqrt(1. - cThe*cThe) * cos(phi);
py = e * sqrt(1. - cThe*cThe) * sin(phi);
pz = e * cThe;
prod = ( px * pQ[iq].px() + py * pQ[iq].py() + pz * pQ[iq].pz() )
/ e;
} while (prod < cosThetaMin);
int colNew = 104 + nglu;
event.append( 21, 23, 1, 0, 0, 0, colNew, colNow[iq],
px, py, pz, e, 0.);
colNow[iq] = colNew;
}
// Update daughter range of mother.
event[1].daughters(2, event.size() - 1);
}
// Information on a q q qbar qbar dijunction system, to be hadronized.
} else if (type >= 6) {
// The two fictitious beam remnant particles; needed for junctions.
event.append( 2212, -12, 0, 0, 3, 5, 0, 0, 0., 0., ee, ee, 0.);
event.append(-2212, -12, 0, 0, 6, 8, 0, 0, 0., 0., ee, ee, 0.);
// Opening angle between "diquark" legs.
double theta = 0.2;
double cThe = cos(theta);
double sThe = sin(theta);
// Set one colour depending on whether more gluons or not.
int acol = (type == 6) ? 103 : 106;
// The four endpoint q q qbar qbar; the minimal system.
// Two additional fictitious partons to make up original beams.
event.append( 2, 23, 1, 0, 0, 0, 101, 0,
ee * sThe, 0., ee * cThe, ee, 0.);
event.append( 1, 23, 1, 0, 0, 0, 102, 0,
-ee * sThe, 0., ee * cThe, ee, 0.);
event.append( 2, -21, 1, 0, 0, 0, 103, 0,
0., 0., ee , ee, 0.);
event.append( -2, 23, 2, 0, 0, 0, 0, 104,
ee * sThe, 0., -ee * cThe, ee, 0.);
event.append( -1, 23, 2, 0, 0, 0, 0, 105,
-ee * sThe, 0., -ee * cThe, ee, 0.);
event.append( -2, -21, 2, 0, 0, 0, 0, acol,
0., 0., -ee , ee, 0.);
// Add extra gluons on string between junctions.
if (type == 7) {
event.append( 21, 23, 8, 5, 0, 0, 103, 106, 0., ee, 0., ee, 0.);
} else if (type == 8) {
event.append( 21, 23, 8, 5, 0, 0, 103, 108, 0., ee, 0., ee, 0.);
event.append( 21, 23, 8, 5, 0, 0, 108, 106, 0.,-ee, 0., ee, 0.);
} else if (type == 9) {
event.append( 21, 23, 8, 5, 0, 0, 103, 107, 0., ee, 0., ee, 0.);
event.append( 21, 23, 8, 5, 0, 0, 107, 108, ee, 0., 0., ee, 0.);
event.append( 21, 23, 8, 5, 0, 0, 108, 106, 0.,-ee, 0., ee, 0.);
} else if (type == 10) {
event.append( 21, 23, 8, 5, 0, 0, 103, 107, 0., ee, 0., ee, 0.);
event.append( 21, 23, 8, 5, 0, 0, 107, 108, ee, 0., 0., ee, 0.);
event.append( 21, 23, 8, 5, 0, 0, 108, 109, 0.,-ee, 0., ee, 0.);
event.append( 21, 23, 8, 5, 0, 0, 109, 106,-ee, 0., 0., ee, 0.);
}
// No more cases: done.
}
}
//==========================================================================
int main() {
// Loop over kind of events to generate:
// 0 = single-particle gun.
// 1 = q qbar.
// 2 = g g.
// 3 = g g g.
// 4 = minimal q q q junction topology.
// 5 = q q q junction topology with gluons on the strings.
// 6 = q q qbar qbar dijunction topology, no gluons.
// 7 - 10 = ditto, but with 1 - 4 gluons on string between junctions.
// 11 = single-resonance gun.
// 12 = q qbar plus parton shower.
// 13 = g g plus parton shower.
// It is easy to edit the line below to only study one of them.
for (int type = 0; type < 14; ++type) {
// Set particle species and energy for single-particle gun.
int idGun = (type == 0) ? 15 : 25;
double eeGun = (type == 0) ? 20. : 125.;
bool atRest = (type == 0) ? false : true;
// The single-particle gun produces a particle at the origin, and
// by default decays it there. When hasLifetime = true instead a
// finite lifetime is selected and used to generate a displaced
// decay vertex.
bool hasLifetime = (type == 0) ? true : false;
// Set typical energy per parton.
double ee = 20.0;
// Set number of events to generate and to list.
int nEvent = 10000;
int nList = 1;
// Generator; shorthand for event and particleData.
Pythia pythia;
Event& event = pythia.event;
ParticleData& pdt = pythia.particleData;
// Key requirement: switch off ProcessLevel, and thereby also PartonLevel.
pythia.readString("ProcessLevel:all = off");
// Avoid the standard scrutiny of mother/daughter relations.
// But note that other event checks are done below.
pythia.readString("Check:event = off");
// Optionally switch off resonance decays.
//pythia.readString("ProcessLevel:resonanceDecays = off");
// For type = 12 and 13 showers are done by forceTimeShower,
// so should not be done a second time.
if (type == 12 || type == 13)
pythia.readString("PartonLevel:FSRinResonances = off");
// Optionally switch off ordinary decays.
//pythia.readString("HadronLevel:Decay = off");
// Switch off automatic event listing in favour of manual.
pythia.readString("Next:numberShowInfo = 0");
pythia.readString("Next:numberShowProcess = 0");
pythia.readString("Next:numberShowEvent = 0");
// Set true to also see space-time information in event listings.
bool showScaleAndVertex = (type == 0) ? true : false;
// Initialize.
cout << "\n Now begin type = " << type << endl;
// If Pythia fails to initialize, exit with error.
if (!pythia.init()) return 1;
// Book histograms.
Hist epCons("deviation from energy-momentum conservation",100,0.,1e-4);
Hist chgCons("deviation from charge conservation",57,-9.5,9.5);
Hist nFinal("final particle multiplicity",100,-0.5,99.5);
Hist dnparticledp("dn/dp for particles",100,0.,ee);
Hist status85("multiplicity status code 85",50,-0.5,49.5);
Hist status86("multiplicity status code 86",50,-0.5,49.5);
Hist status83("multiplicity status code 83",50,-0.5,49.5);
Hist status84("multiplicity status code 84",50,-0.5,49.5);
Hist dndtheta("particle flow in event plane",100,-M_PI,M_PI);
Hist dedtheta("energy flow in event plane",100,-M_PI,M_PI);
Hist dpartondtheta("parton flow in event plane",100,-M_PI,M_PI);
Hist dndyAnti("dn/dy primaries antijunction",100, -10., 10.);
Hist dndyJun("dn/dy primaries junction",100, -10., 10.);
Hist dndySum("dn/dy all primaries",100, -10., 10.);
// Begin of event loop.
for (int iEvent = 0; iEvent < nEvent; ++iEvent) {
// Set up single particle, with random direction in solid angle.
if (type == 0 || type == 11) fillParticle( idGun, eeGun, -1., 0.,
event, pdt, pythia.rndm, atRest, hasLifetime);
// Set up parton-level configuration.
else fillPartons( type, ee, event, pdt, pythia.rndm);
// To have partons shower they must be set maximum allowed scale.
// (Can be set individually to restrict radiation differently.)
if (type == 12 || type == 13) {
double scale = ee;
event[1].scale( scale);
event[2].scale( scale);
// Now actually do the shower, for range of partons, and max scale.
// (Most restrictive of global and individual applied to each parton.)
pythia.forceTimeShower( 1, 2, scale);
}
// Generate events. Quit if failure.
if (!pythia.next()) {
cout << " Event generation aborted prematurely, owing to error!\n";
break;
}
// List first few events.
if (iEvent < nList) {
event.list(showScaleAndVertex);
// Also list junctions.
event.listJunctions();
}
// Initialize statistics.
Vec4 pSum = - event[0].p();
double chargeSum = 0.;
if (type == 0) chargeSum = -event[1].charge();
if (type == 4 || type == 5) chargeSum = -1;
int nFin = 0;
int n85 = 0;
int n86 = 0;
int n83 = 0;
int n84 = 0;
// Loop over all particles.
for (int i = 0; i < event.size(); ++i) {
int status = event[i].statusAbs();
// Find any unrecognized particle codes.
int id = event[i].id();
if (id == 0 || !pdt.isParticle(id))
cout << " Error! Unknown code id = " << id << "\n";
// Find particles with E-p mismatch.
double eCalc = event[i].eCalc();
if (abs(eCalc/event[i].e() - 1.) > 1e-6) cout << " e mismatch, i = "
<< i << " e_nominal = " << event[i].e() << " e-from-p = "
<< eCalc << " m-from-e " << event[i].mCalc() << "\n";
// Parton flow in event plane.
if (status == 71 || status == 72) {
double thetaXZ = event[i].thetaXZ();
dpartondtheta.fill(thetaXZ);
}
// Origin of primary hadrons.
if (status == 85) ++n85;
if (status == 86) ++n86;
if (status == 83) ++n83;
if (status == 84) ++n84;
// Flow of primary hadrons in the event plane.
if (status > 80 && status < 90) {
double eAbs = event[i].e();
if (eAbs < 0.) {cout << " e < 0 line " << i; event.list();}
double thetaXZ = event[i].thetaXZ();
dndtheta.fill(thetaXZ);
dedtheta.fill(thetaXZ, eAbs);
// Rapidity distribution of primary hadrons.
double y = event[i].y();
dndySum.fill(y);
if (type >= 6) {
int motherId = event[event[i].mother1()].id();
if (motherId > 0 ) dndyJun.fill(event[i].y());
else dndyAnti.fill(event[i].y());
}
}
// Study final-state particles.
if (event[i].isFinal()) {
pSum += event[i].p();
chargeSum += event[i].charge();
nFin++;
double pAbs = event[i].pAbs();
dnparticledp.fill(pAbs);
}
}
// Fill histograms once for each event.
double epDev = abs(pSum.e()) + abs(pSum.px()) + abs(pSum.py())
+ abs(pSum.pz());
epCons.fill(epDev);
chgCons.fill(chargeSum);
nFinal.fill(nFin);
status85.fill(n85);
status86.fill(n86);
status83.fill(n83);
status84.fill(n84);
if (epDev > 1e-3 || abs(chargeSum) > 0.1) event.list();
// End of event loop.
}
// Print statistics and histograms.
pythia.stat();
cout << epCons << chgCons << nFinal << dnparticledp
<< dndtheta << dedtheta << dpartondtheta << dndySum;
if (type >= 4) cout << status85 << status86 << status83
<< status84;
if (type >= 6) cout << dndyJun << dndyAnti;
// End of type loop and done.
}
return 0;
}