rivet is hosted by Hepforge, IPPP Durham

Rivet analyses reference

STAR_2020_I1792394

Measurement of Central Exclusive Production of charged hadron pairs h+h- (h=pi,K,p) at sqrt(s)=200 GeV with forward proton tagging in Roman Pots
Experiment: STAR (RHIC)
Inspire ID: 1792394
Status: VALIDATED
Authors:
  • Rafal Sikora
References: Beams: p+ p+
Beam energies: (100.0, 100.0) GeV
    No run details listed

The differential fiducial cross sections are measured for the process of CEP of h+h- pairs (h=pi,K,p). For the details of analysis and definition of the fiducial region see the arXiv.

Source code: STAR_2020_I1792394.cc
  1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
// -*- 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
    DEFAULT_RIVET_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
    //@}


  };


  DECLARE_RIVET_PLUGIN(STAR_2020_I1792394);

}