Rivet analyses

Cross sections for e⁺e⁻ → μ⁺μ⁻, π⁺π⁻η and 2π⁺2π⁻π⁰ near the J/ψ

Experiment: BESIII (BEPC)

Inspire ID: 1685351

Status: VALIDATED NOHEPDATA SINGLEWEIGHT

Authors: - Peter Richardson

References: - Phys.Lett.B 791 (2019) 375-384

Beams: e+ e-

Beam energies: ANY

Run details: - e+ e- > mu+ mu- or hadrons, pi0 set stable

Measurement of the cross sections for e⁺e⁻ → μ⁺μ⁻, π⁺π⁻η and 2π⁺2π⁻π⁰ near the J/ψ, not corrected for photon ISR which should be included in the simulation. Useful for looking at the simulation of QED ISR at low energies. As the analyses requires the beam energy smearing described in the paper then central CMS energy should be specified using the ECENT (in MeV) option.

Source code:`BESIII_2019_I1685351.cc`

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

namespace Rivet {


  /// @brief e+ e- > mu+ mu-, pi+ pi- eta and 5pi near J/psi
  class BESIII_2019_I1685351 : public Analysis {
  public:

    /// Constructor
    RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2019_I1685351);


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

    /// Book histograms and initialise projections before the run
    void init() {
      // Projections
      declare(FinalState(), "FS");
      declare(UnstableParticles(Cuts::pid==PID::ETA), "UFS");
      // histograms
      for (unsigned int ix=0; ix<3; ++ix) {
        book(_sigma[ix], 1, 1, 1+ix);
      }
      // central beam energy
      string ecms = std::to_string(0.01*double(round(sqrtS()/MeV*100)));
      const size_t idx = ecms.find(".");
      ecms = ecms.substr(0,idx+3);
      if(ecms[ecms.length()-1]=='0') ecms.pop_back();
      _eCent = getOption<string>("ECENT", ecms);
    }

    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;
      }
      // mu+mu- + photons
      if (nCount[-13]==1 && nCount[13]==1 && ntotal==2+nCount[22]) {
        _sigma[0]->fill(_eCent);
      }
      else if (nCount[111]==1 && nCount[211]==2 && nCount[-211]==2  && ntotal==5+nCount[22]) {
        _sigma[2]->fill(_eCent);
      }

      const FinalState& ufs = apply<FinalState>(event, "UFS");
      // loop over eta mesons
      for (const Particle& p : ufs.particles()) {
        map<long,int> nRes = nCount;
        int ncount = ntotal;
        findChildren(p,nRes,ncount);
        bool matched = true;
        for (const auto& val : nRes) {
          if (abs(val.first)==211) {
            if (val.second !=1) {
              matched = false;
              break;
            }
          }
          else if (val.first!=PID::PHOTON && val.second!=0) {
            matched = false;
            break;
          }
        }
        if (!matched) continue;
        _sigma[1]->fill(_eCent);
        break;
      }
    }


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

    /// @}


    /// @name Histograms
    /// @{
    BinnedHistoPtr<string> _sigma[3];
    string _eCent;
    /// @}


  };


  RIVET_DECLARE_PLUGIN(BESIII_2019_I1685351);

}

Rivet analyses

Cross sections for e+e− → μ+μ−, π+π−η and 2π+2π−π0 near the J/ψ

Source code:BESIII_2019_I1685351.cc

Cross sections for e⁺e⁻ → μ⁺μ⁻, π⁺π⁻η and 2π⁺2π⁻π⁰ near the J/ψ

Source code:`BESIII_2019_I1685351.cc`