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| // -*- C++ -*-
#include "Rivet/Analysis.hh"
#include "Rivet/Projections/Beam.hh"
#include "Rivet/Projections/FinalState.hh"
namespace Rivet {
/// @brief CEP of h+h- (h=pi,K,p) at sqrt(s)=200 GeV with forward proton tagging
class STAR_2020_I1792394 : public Analysis {
public:
/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(STAR_2020_I1792394);
/// @name Analysis methods
//@{
enum CENTRAL_PARTICLES_PID {_PION, _KAON, _PROTON, _nAllowedPids};
enum PARTICLE_CHARGE { _PLUS, _MINUS, _nSigns };
enum PARTICLE_DIRECTION { _E, _W, _nBeamDirections }; // E = negative p_z, W = positive p_Z
const double minPt[_nAllowedPids] = { 0.2*GeV, 0.3*GeV, 0.4*GeV };
const double maxMinPt[_nAllowedPids] = { 9e9*GeV, 0.7*GeV, 1.1*GeV };
/// Book histograms and initialise projections before the run
void init() {
// all final-state particles
const FinalState fs( Cuts::NOCUT );
declare(fs, "FS_all");
// all final-state particles within STAR acceptance for this
// measurement (reconstructed in the TPC and TOF)
Cut centralCuts = Cuts::abscharge > 0
&& Cuts::abseta < 0.7
&& Cuts::pT > 0.2*GeV
&& ( Cuts::abspid == PID::PIPLUS
|| Cuts::abspid == PID::KPLUS
|| Cuts::abspid == PID::PROTON );
const FinalState fs_central( centralCuts );
declare(fs_central, "FS_central");
// forward-scattered beam particles detectable in Roman Pots
// Checking the ID is not needed
Cut forwardCuts = Cuts::abscharge > 0
&& Cuts::abseta > 5.0; // inclusive cut to select forward particles
const FinalState fs_forward(forwardCuts);
declare(fs_forward, "FS_forward");
// Book histograms with binning taken from HEPdata
book(_h["m_pipi"], "d01-x01-y01"); _scaleFactor["m_pipi"] = 1.0;
book(_h["m_kk"], "d02-x01-y01"); _scaleFactor["m_kk"] = 1.0;
book(_h["m_ppbar"], "d03-x01-y01"); _scaleFactor["m_ppbar"] = 1.0e3;
book(_h["y_pipi"], "d04-x01-y01"); _scaleFactor["y_pipi"] = 1.0;
book(_h["y_kk"], "d05-x01-y01"); _scaleFactor["y_kk"] = 1.0;
book(_h["y_ppbar"], "d06-x01-y01"); _scaleFactor["y_ppbar"] = 1.0e3;
book(_h["deltaPhi_pipi"], "d07-x01-y01"); _scaleFactor["deltaPhi_pipi"] = 1.0;
book(_h["deltaPhi_kk"], "d08-x01-y01"); _scaleFactor["deltaPhi_kk"] = 1.0e3;
book(_h["deltaPhi_ppbar"], "d09-x01-y01"); _scaleFactor["deltaPhi_ppbar"] = 1.0e3;
book(_h["tSum_pipi"], "d10-x01-y01"); _scaleFactor["tSum_pipi"] = 1.0;
book(_h["tSum_kk"], "d11-x01-y01"); _scaleFactor["tSum_kk"] = 1.0;
book(_h["tSum_ppbar"], "d12-x01-y01"); _scaleFactor["tSum_ppbar"] = 1.0e3;
}
/// Perform the per-event analysis
void analyze(const Event& event) {
// Retrieve all final-state particles
const FinalState & fs = apply<FinalState>(event, "FS_all");
// Veto event if number of particles in the final state is different from 4
if(fs.size() != 4)
return;
// Retrieve accepted centrally produced particles
const FinalState & fs_central = apply<FinalState>(event, "FS_central");
// Veto event if number of centrally produced particles is different from 2
if(fs_central.size() != 2)
return;
// Retrieve forward-scattered particles
const FinalState & fs_forward = apply<FinalState>(event, "FS_forward");
// Veto event if number of forward particles is different from 2
if(fs_forward.size() != 2)
return;
// Continue checking forward particles (intact beam particles)
// Storing forward particles in an array with cell ID indicating the direction (p_z)
bool forwardParticlesInFiducialRegion[_nBeamDirections] = {false};
Particle forwardParticles[_nBeamDirections];
Vector3 forwardParticle2Vec[_nBeamDirections];
for(const Particle & p : fs_forward.particles()){
const int dir = p.pz() > 0 ? _W : _E;
forwardParticle2Vec[dir] = Vector3(p.px(), p.py(), 0.0);
forwardParticles[dir] = p;
forwardParticlesInFiducialRegion[dir] = p.px() > -0.2
&& fabs(p.py()) > 0.2
&& fabs(p.py()) < 0.4
&& (pow( p.px() + 0.3, 2) + pow( p.py(), 2 )) < 0.25;
}
if( !forwardParticlesInFiducialRegion[_E] || !forwardParticlesInFiducialRegion[_W] )
return;
// Storing central particles in an array with cell ID indicating the charge
Particle csParticles[_nSigns];
int totalCharge = 0;
for(const Particle & p : fs_central.particles()){
csParticles[ p.charge()>0 ? _PLUS : _MINUS] = p;
totalCharge += p.charge();
}
// Checking the charge conservation, just in case
if( totalCharge != 0 )
return;
// Determine PID of the central pair
const int pid = ( csParticles[_PLUS].pid()==PID::PIPLUS && csParticles[_MINUS].pid()==PID::PIMINUS ) ? _PION :
(( csParticles[_PLUS].pid()==PID::KPLUS && csParticles[_MINUS].pid()==PID::KMINUS) ? _KAON :
(( csParticles[_PLUS].pid()==PID::PROTON && csParticles[_MINUS].pid()==PID::ANTIPROTON ) ? _PROTON : _nAllowedPids) );
// skip event if particles in a pair are of different ID (should not happen)
if(pid == _nAllowedPids)
return;
// Checking if central particles pass selection (in principle important for KK and ppbar)
bool centralParticlesWithinFiducialRegion = true;
for(int i=0; i<_nSigns; ++i)
if( csParticles[i].pT() < minPt[pid] || min(csParticles[i].pT(), csParticles[1-i].pT()) > maxMinPt[pid] ){
centralParticlesWithinFiducialRegion = false;
break;
}
if( !centralParticlesWithinFiducialRegion )
return;
//-- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
// At this point event satisfies the definition of the fiducial region for events accepted
// in the CEP measurement at STAR at 200 GeV
const FourMomentum centralState4Mom = csParticles[_PLUS].momentum() + csParticles[_MINUS].momentum();
const double invMass = centralState4Mom.mass()/GeV;
const double rapidity = centralState4Mom.rapidity();
const double deltaPhi = fabs( forwardParticle2Vec[_W].angle( forwardParticle2Vec[_E] ) )/degree;
// We need beam particles to get momentum transfers
/* // Fragment below did not work for Pythia, unfortunately (four momenta were [0,0,0,0]); using a workaround
Particle beamParticles[_nBeamDirections];
const ParticlePair & beams = Rivet::Beam().beams();
beamParticles[_W] = beams.first;
beamParticles[_E] = beams.second;
*/
// workaround - at this point we know that process is exclusive (2 forward protons + 2 central state particles)
// assume that both beams are of the same type and collision in symmetric (lab frame = c.m.s. frame)
const double sqrt_s = (centralState4Mom + forwardParticles[_E].momentum() + forwardParticles[_W].momentum()).mass();
const double beamParticleMass = forwardParticles[_W].momentum().mass();
const double fabsPz = sqrt( sqrt_s*sqrt_s/4. - beamParticleMass*beamParticleMass );
FourMomentum beamParticles4Mom[_nBeamDirections];
beamParticles4Mom[_W] = FourMomentum( sqrt(beamParticleMass*beamParticleMass + fabsPz*fabsPz), 0., 0., fabsPz );
beamParticles4Mom[_E] = FourMomentum( sqrt(beamParticleMass*beamParticleMass + fabsPz*fabsPz), 0., 0., -fabsPz );
// end of workaround
double t[_nBeamDirections];
for(int dir=0; dir<_nBeamDirections; ++dir)
t[dir] = (beamParticles4Mom[dir] - forwardParticles[dir].momentum()).mass2()/(GeV*GeV);
const double tSum = fabs(t[_E] + t[_W]);
if( pid==_PION ){
_h["m_pipi"]->fill( invMass );
_h["y_pipi"]->fill( rapidity );
_h["deltaPhi_pipi"]->fill( deltaPhi );
_h["tSum_pipi"]->fill( tSum );
} else if( pid==_KAON ){
_h["m_kk"]->fill( invMass );
_h["y_kk"]->fill( rapidity );
_h["deltaPhi_kk"]->fill( deltaPhi );
_h["tSum_kk"]->fill( tSum );
} else{
_h["m_ppbar"]->fill( invMass );
_h["y_ppbar"]->fill( rapidity );
_h["deltaPhi_ppbar"]->fill( deltaPhi );
_h["tSum_ppbar"]->fill( tSum );
}
}
/// Normalise histograms etc., after the run
void finalize() {
const double scalingFactor = crossSection()/nanobarn/sumOfWeights();
// scale to cross section
for(auto &hist : _h)
scale(hist.second, scalingFactor*_scaleFactor[hist.first]);
}
//@}
/// @name Histograms
//@{
map<string, Histo1DPtr> _h;
map<string, double> _scaleFactor; // map with scale factors to ensure cross section units in agreement with HEPdata
//@}
};
RIVET_DECLARE_PLUGIN(STAR_2020_I1792394);
}
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