Rivet analyses

Cross section for e+e− → ηϕ for centre-of-mass energies between 2 and 3.08 GeV

Experiment: BESIII (BEPC)

Inspire ID: 1857930

Status: VALIDATED

Authors: - Peter Richardson

References: - arXiv: 2104.05549

Beams: e+ e-

Beam energies: (1.0, 1.0); (1.0, 1.0); (1.1, 1.1); (1.1, 1.1); (1.1, 1.1); (1.1, 1.1); (1.1, 1.1); (1.1, 1.1); (1.2, 1.2); (1.2, 1.2); (1.2, 1.2); (1.2, 1.2); (1.3, 1.3); (1.3, 1.3); (1.4, 1.4); (1.5, 1.5); (1.5, 1.5); (1.5, 1.5); (1.5, 1.5); (1.5, 1.5); (1.5, 1.5); (1.5, 1.5)GeV

Run details: - e+ e- to hadrons

Measurement of the vross section for e+e− → ηϕ for centre-of-mass energies between 2 and 3.08 GeV by the BESIII collaboration.

Source code:BESIII_2021_I1857930.cc

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

namespace Rivet {


  /// @brief e+e- > phi eta
  class BESIII_2021_I1857930 : public Analysis {
  public:

    /// Constructor
    RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2021_I1857930);


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

    /// Book histograms and initialise projections before the run
    void init() {

      // Initialise and register projections
      declare(FinalState(), "FS");
      declare(UnstableParticles(), "UFS");
      book(_nPhiEta, 1, 1, 1);
      for (const string& en : _nPhiEta.binning().edges<0>()) {
        const double eval = stod(en)*GeV;
        if (isCompatibleWithSqrtS(eval)) {
          _sqs = en; break;
        }
      }
      raiseBeamErrorIf(_sqs.empty());
    }

    void findChildren(const Particle& p, map<long,int>& nRes, int& ncount) const {
      for (const Particle &child : p.children()) {
        if (child.children().empty()) {
          nRes[child.pid()]-=1;
          --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");

      // loop over eta mesons
      for (const Particle& p : ufs.particles(Cuts::pid==221)) {
        if (p.children().empty()) continue;
        map<long,int> nRes = nCount;
        int ncount = ntotal;
        findChildren(p,nRes,ncount);
        // loop over phi mesons
        for (const Particle& p2 : ufs.particles(Cuts::pid==333)) {
          if (p2.parents()[0].isSame(p)) continue;
          map<long,int> nResB = nRes;
          int ncountB = ncount;
          findChildren(p2,nResB,ncountB);
          if (ncountB!=0) continue;
          bool matched = true;
          for (const auto& val : nResB) {
            if (val.second!=0) {
              matched = false;
              break;
            }
          }
          if (matched) _nPhiEta->fill(_sqs);
        }
      }
    }


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

    ///@}


    /// @name Histograms
    ///@{
    BinnedHistoPtr<string> _nPhiEta;
    string _sqs = "";
    ///@}


  };


  RIVET_DECLARE_PLUGIN(BESIII_2021_I1857930);

}