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BABAR_2018_I1691222

$e^+e^-\to e^+e^-\eta^\prime$ via intermediate photons at 10.58 GeV
Experiment: BABAR (PEP-II)
Inspire ID: 1691222
Status: VALIDATED
Authors:
  • Peter Richardson
References:
  • Phys.Rev.D 98 (2018) 11, 112002
Beams: e+ e-
Beam energies: (5.3, 5.3) GeV
Run details:
  • e+ e- > e+e- meson via photon photon -> meson

Measurement of the cross section for the production of $\eta^\prime$ in photon-photon collisions, i.e. $e^+e^-\to \gamma\gamma e^+e^-$ followed by $\gamma\gamma\to\eta^\prime$, by the Babar experiment at 10.58 GeV. This measurement is doubly differential in the virtuality of the two photons.

Source code: BABAR_2018_I1691222.cc
  1// -*- C++ -*-
  2#include "Rivet/Analysis.hh"
  3#include "Rivet/Projections/FinalState.hh"
  4#include "Rivet/Projections/UnstableParticles.hh"
  5#include "Rivet/Projections/Beam.hh"
  6
  7namespace Rivet {
  8
  9
 10  /// @brief e+e- > e+e- eta'
 11  class BABAR_2018_I1691222 : public Analysis {
 12  public:
 13
 14    /// Constructor
 15    RIVET_DEFAULT_ANALYSIS_CTOR(BABAR_2018_I1691222);
 16
 17
 18    /// @name Analysis methods
 19    ///@{
 20
 21    /// Book histograms and initialise projections before the run
 22    void init() {
 23      // Initialise and register projections
 24      declare(Beam(), "Beams");
 25      declare(FinalState(),"FS");
 26      declare(UnstableParticles(), "UFS");
 27      // book the histograms
 28      book(_h_etap,1,1,1);
 29    }
 30
 31    void findChildren(const Particle & p,map<long,int> & nRes, int &ncount) {
 32      for (const Particle &child : p.children()) {
 33        if (child.children().empty()) {
 34          --nRes[child.pid()];
 35          --ncount;
 36        } else {
 37          findChildren(child,nRes,ncount);
 38        }
 39      }
 40    }
 41
 42    bool findScattered(Particle beam, double& q2) {
 43      bool found = false;
 44      Particle scat = beam;
 45      while (!scat.children().empty()) {
 46        found = false;
 47        for (const Particle & p : scat.children()) {
 48          if (p.pid()==scat.pid()) {
 49            scat=p;
 50            found=true;
 51            break;
 52          }
 53        }
 54        if (!found) break;
 55      }
 56      if (!found) return false;
 57      q2 = -(beam.momentum() - scat.momentum()).mass2();
 58      return true;
 59    }
 60
 61    /// Perform the per-event analysis
 62    void analyze(const Event& event) {
 63      // find scattered leptons and calc Q2
 64      const Beam& beams = apply<Beam>(event, "Beams");
 65      double q12 = -1, q22 = -1;
 66      if (!findScattered(beams.beams().first,  q12)) vetoEvent;
 67      if (!findScattered(beams.beams().second, q22)) vetoEvent;
 68      if(q22>q12) swap(q12,q22);
 69      // check the final state
 70      const FinalState & fs = apply<FinalState>(event, "FS");
 71      map<long,int> nCount;
 72      int ntotal(0);
 73      for (const Particle& p : fs.particles()) {
 74        nCount[p.pid()] += 1;
 75        ++ntotal;
 76      }
 77      // find the meson
 78      const FinalState& ufs = apply<FinalState>(event, "UFS");
 79      for (const Particle& p : ufs.particles(Cuts::pid==331)) {
 80        if(p.children().empty()) continue;
 81        map<long,int> nRes = nCount;
 82        int ncount = ntotal;
 83        findChildren(p,nRes,ncount);
 84        bool matched = true;
 85        for(auto const & val : nRes) {
 86          if(abs(val.first)==11) {
 87            if(val.second!=1) {
 88              matched = false;
 89              break;
 90            }
 91          }
 92          else if(val.second!=0) {
 93            matched = false;
 94            break;
 95          }
 96        }
 97        if (matched) {
 98	  // 2<Q2<10 for both photons bin
 99	  if(q12>2.&&q12<10.&&q22>2.&&q22<10.) {
100	    _h_etap->fill(0,1./sqr(8.));
101	  }
102	  // 10<Q2<30 for both photons bin
103	  else if(q12>10&&q12<30.&&q22>10.&&q22<30.)
104	    _h_etap->fill(1,1./sqr(20.));
105	  // 10<Q12<30 2<Q22<10
106	  else if(q22>2.&&q22<10.&&q12>10.&&q12<30.)
107	    _h_etap->fill(2,1./8./20./2.);
108	  // 2<Q22<30 30<Q12<60
109	  else if(q22>2.&&q22<30.&&q12>30.&&q12<60.)
110	    _h_etap->fill(3,1./28./30./2.);
111	  // 30<Q2<60 for both photons
112	  else if(q22>30.&&q22<60.&&q12>30.&&q12<60.)
113	    _h_etap->fill(4,1./sqr(30.));
114        }
115      }
116    }
117
118    /// Normalise histograms etc., after the run
119    void finalize() {
120      scale(_h_etap, 1e4*crossSection()/femtobarn/sumW());
121    }
122
123    ///@}
124
125
126    /// @name Histograms
127    ///@{
128    Histo1DPtr _h_etap;
129    unsigned int _ncount=0;
130    ///@}
131
132
133  };
134
135
136  RIVET_DECLARE_PLUGIN(BABAR_2018_I1691222);
137
138}