main77
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// main77.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.
// Authors:
// Boris Blok
// Paolo Gunnellini
// Keywords:
// MPI
// DPS
// The program calculates the 4 jet DPS cross section as a function of
// standard observables according to the model described in
// "Dynamical approach to MPI four-jet production in Pythia",
// B. Blok , P. Gunnellini
// Eur.Phys.J. C75 (2015) no.6, 282 [arXiv:1503.08246 [hep-ph]]
// The model is simulated by a dynamical reweighting of Pythia events.
// The notations and observables calculated are as in Fig. 4 of the article.
#include "Pythia8/Pythia.h"
using namespace Pythia8;
//==========================================================================
// Method to calculate DPS weight according to the model above,
// relative to the default one in Pythia.
double Reweighting(double x1, double x2, double x3, double x4,
double scale1, double scale2){
double newsigmaeff=2.*3.1415*(4.1+0.28*log(0.0012/x1)+4.1
+0.28*log(0.0012/x2)+4.1+0.28*log(0.0012/x3)+4.1+0.28*log(0.0012/x4));
newsigmaeff=newsigmaeff*0.389; //in mb
//rescaling function
double y=1.0;//0.5, 1 or 2
double c1=0.18527-0.2058534*log(2*y)+0.0736003*log(2*y)*log(2*y);
double c2=-0.016428+0.0254615*log(2*y)-0.0097517*log(2*y)*log(2*y);
double c3=0.001444-0.00073161*log(2*y)+0.000314591*log(2*y)*log(2*y);
double R=c1*log(scale2*scale2/y)+c2*log(scale2*scale2/y)*log(scale2*scale2/y)
+c3*log(scale2*scale2/y)*log(scale2*scale2/y)*log(scale2*scale2/y);
double ReweightFactor=R+(0.03-0.014427*log(y))*log((scale1*scale1)
/(scale2*scale2));
newsigmaeff=newsigmaeff/(1+ReweightFactor);
// The final weight is the sigma effective implemented in the tune
// reweighted with the value of the new sigma effective that you
// obtained after the x-dependence and the Q^2 dependence.
// Sigma effective value from our paper https://arxiv.org/pdf/1503.08246.pdf
double FinalReweight=29.719/newsigmaeff;//only valid for 7 TeV
return FinalReweight;
}
//==========================================================================
int main(){
// Number of events, generated and listed ones (for jets).
int nEvent = 1000;
// Select common parameters for SlowJet and FastJet analyses.
int power = -1; // -1 = anti-kT; 0 = C/A; 1 = kT.
double R = 0.5; // Jet size.
double pTMin = 15.0; // Min jet pT.
double etaMax = 5.0; // Pseudorapidity range of detector.
int select = 2; // Which particles are included?
int massSet = 2; // Which mass are they assumed to have?
// Generator.
Pythia pythia;
// Process selection.
pythia.readString("HardQCD:all = on");
pythia.readString("PhaseSpace:pTHatMin = 15.");
// pThat reweighting.
pythia.readString("PhaseSpace:bias2Selection = on");
pythia.readString("PhaseSpace:bias2SelectionPow = 4.");
pythia.readString("PhaseSpace:bias2SelectionRef = 15.");
// No event record printout.
pythia.readString("Next:numberShowInfo = 0");
pythia.readString("Next:numberShowProcess = 0");
pythia.readString("Next:numberShowEvent = 0");
// Tune from our paper https://arxiv.org/pdf/1503.08246.pdf
pythia.readString("Tune:pp=5");
pythia.readString("Tune:ee=3");
pythia.readString("MultipartonInteractions:bProfile=1");
pythia.readString("MultipartonInteractions:pT0Ref=2.659");
pythia.readString("ColourReconnection:range=3.540");
// LHC initialization.
pythia.readString("Beams:eCM = 7000.");
pythia.init();
//Histograms
Hist _hist_DeltaS_norm("DeltaS between the two jet systems", 14, 0., 3.1415);
Hist _hist_DeltaPtRelSoft_norm("Normalized pT balance between the two jet"
" systems", 10, 0., 1.);
// Set up SlowJet jet finder in native mode.
SlowJet slowJet( power, R, pTMin, etaMax, select, massSet, 0, false);
//setup generation
double sumWeights = 0.;
// Generate events.
for(int iEvent=0;iEvent<nEvent;++iEvent){
if (!(pythia.next())){
continue;//skip event in case of problem.
}
// Initial values for current event.
unsigned int num_jets=0;
unsigned int num_Softjets=0;
double x1=pythia.info.x1();
double x2=pythia.info.x2();
double x3=-1.;
double x4=-1.;
bool hasMPI=false;
// Identify incoming state of second MPI, if present.
for (int i=7;i<pythia.event.size();++i){
if(pythia.event[i].statusAbs()==31){
x3=pythia.event[i].e()/pythia.event[1].e();
x4=pythia.event[i+1].e()/pythia.event[2].e();
hasMPI=true;
break;
}
}
// Find reweighting factor; check if the MPI scale is higher than 15 GeV.
double scale1=pythia.info.pTMPI(0);
double scale2=(hasMPI)? pythia.info.pTMPI(1):1.;
double ReweightFactor= (scale2>15 && x3>0 && x4>0)
? Reweighting(x1,x2,x3,x4,scale1,scale2) : 1.;
double weightFirst = pythia.info.weight();
double weight=weightFirst*ReweightFactor;
// Analyze jet content of event.
slowJet.analyze( pythia.event );
int nSlow = slowJet.sizeJet();
vector <Vec4> myJets;
for (int i = 0; i < nSlow; ++i) {
if(slowJet.pT(i)>=50 && abs(slowJet.y(i))<4.7) num_jets+=1;
if(slowJet.pT(i)>=20 && abs(slowJet.y(i))<4.7){
num_Softjets+=1;
myJets.push_back(slowJet.p(i));
}
}
// Events with at least two hard and exactly four soft jets.
if (num_jets >= 2 && num_Softjets == 4) {
sumWeights+=weight;
double SptSoft = (myJets.at(2)+myJets.at(3)).pT()
/ (myJets.at(2).pT()+myJets.at(3).pT());
_hist_DeltaPtRelSoft_norm.fill(SptSoft, weight);
double DeltaS = abs( (myJets.at(0)+myJets.at(1)).phi()
- (myJets.at(2)+myJets.at(3)).phi() );
if (DeltaS > M_PI) DeltaS = 2. * M_PI - DeltaS;
_hist_DeltaS_norm.fill(DeltaS,weight);
}
}
// Print run statistics and normalized histograms.
pythia.stat();
double binWidthDeltaPtRel = 0.1;
double binWidthDeltaS = M_PI/14;
_hist_DeltaPtRelSoft_norm /= (sumWeights*binWidthDeltaPtRel);
_hist_DeltaS_norm /= (sumWeights*binWidthDeltaS);
cout << _hist_DeltaPtRelSoft_norm << _hist_DeltaS_norm;
// Histogram in format to allow Python plotting.
HistPlot hpl("main77plot");
hpl.plotFrame("out77plot1", _hist_DeltaS_norm, "$\\Delta$ S (rad)",
"1/$\\sigma$ d$\\sigma$/d$\\Delta$S",
"1/$\\sigma$ d$\\sigma$/d$\\Delta$S",
"h", "proton-proton collisions, $\\sqrt{s}$ = 7 TeV", true);
hpl.plotFrame("out77plot2", _hist_DeltaPtRelSoft_norm, "$\\Delta$ p_T",
"1/$\\sigma$ d$\\sigma$/d$\\Delta$p_T",
"1/$\\sigma$ d$\\sigma$/d$\\Delta$p_T",
"h", "proton-proton collisions, $\\sqrt{s}$ = 7 TeV", true);
hpl.plot();
// Done.
return 0;
}