Rivet analyses

Cross section for e+e− → ηJ/ψ at $\sqrt{s}=3.773$ GeV

Experiment: BESIII (BEPC)

Inspire ID: 2617922

Status: VALIDATED NOHEPDATA

Authors: - Peter Richardson

References: - Phys.Rev.D 102 (2020) 3, 031101

Beams: e+ e-

Beam energies: (1.9, 1.9)GeV

Run details: - e+ e- to hadrons

Cross section for e+e− → ηJ/ψ for $\sqrt{s}=3.773$ GeV measured by the BESIII collaboration.

Source code:BESIII_2023_I2617922.cc

// -*- C++ -*-
#include "Rivet/Analysis.hh"
#include "Rivet/Projections/FinalState.hh"
#include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {


  /// @brief e+e- -> eta J/psi
  class BESIII_2023_I2617922 : public Analysis {
  public:

    /// Constructor
    RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2023_I2617922);


    /// @name Analysis methods
    /// @{

    /// Book histograms and initialise projections before the run
    void init() {
      declare(FinalState(), "FS");
      declare(UnstableParticles(), "UFS");
      book(_nJPsi,1,1,1);
    }

    void findChildren(const Particle & p,map<long,int> & nRes, int &ncount) {
      for (const Particle &child : p.children()) {
        if (child.children().empty()) {
          --nRes[child.pid()];
          --ncount;
        }
        else {
          findChildren(child,nRes,ncount);
        }
      }
    }


    /// Perform the per-event analysis
    void analyze(const Event& event) {
      const FinalState& fs = apply<FinalState>(event, "FS");
      map<long,int> nCount;
      int ntotal(0);
      for (const Particle& p : fs.particles()) {
        nCount[p.pid()] += 1;
        ++ntotal;
      }
      const FinalState& ufs = apply<FinalState>(event, "UFS");
      for (const Particle& p : ufs.particles(Cuts::pid==PID::JPSI)) {
        if (p.children().empty()) continue;
        // find the J/psi
        map<long,int> nRes = nCount;
        int ncount = ntotal;
        findChildren(p,nRes,ncount);
        bool matched=false;
        for (const Particle& p2 : ufs.particles(Cuts::pid==PID::ETA)) {
          if (p2.children().empty()) continue;
          bool JpsiParent=false;
          Particle parent=p2;
          while (!parent.parents().empty()) {
            parent=parent.parents()[0];
            if (parent.pid()==PID::JPSI) {
              JpsiParent=true;
              break;
            }
          }
          if (JpsiParent) continue;
          map<long,int> nRes2 = nRes;
          int ncount2 = ncount;
          findChildren(p2,nRes2,ncount2);
          if(ncount2!=0) continue;
          matched = true;
          for (const auto& val : nRes2) {
            if (val.second!=0) {
              matched = false;
              break;
            }
          }
          if (matched) {
            _nJPsi->fill("3.773"s);
            break;
          }
        }
        if (matched) break;
      }
    }


    /// Normalise histograms etc., after the run
    void finalize() {
      scale(_nJPsi,crossSection()/ sumOfWeights() /picobarn);
    }

    /// @}


    /// @name Histograms
    /// @{
    BinnedHistoPtr<string> _nJPsi;
    /// @}


  };


  RIVET_DECLARE_PLUGIN(BESIII_2023_I2617922);

}