Rivet analyses

e+e− → e+e− and μ+μ− cross sections for $\sqrt{s}=3.05$ to 3.12 GeV

Experiment: BESIII (BEPC)

Inspire ID: 2102287

Status: VALIDATED NOHEPDATA SINGLEWEIGHT

Authors: - Peter Richardson

References: - arXiv: 2206.13674

Beams: e+ e-

Beam energies: ANY

Run details: - e+ e- -> e+ e- and mu+mu-

Measurement of the observed e+e− and μ+μ− cross sections near the J/ψ by BESIII. As the analyses requires the beam energy smearing described in the paper then central CMS energy should be specified using the ECENT (in GeV) option.

Source code:BESIII_2022_I2102287.cc

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

namespace Rivet {


  /// @brief e+ e- -> e+e- / mu+mu-
  class BESIII_2022_I2102287 : public Analysis {
  public:

    /// Constructor
    RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2022_I2102287);


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

    /// Book histograms and initialise projections before the run
    void init() {
      // Initialise and register projections
      declare(Beam(), "Beams");
      declare(FinalState(), "FS");
      // histograms
      for (unsigned int ix = 0; ix < 2; ++ix) {
        book(_sigma[ix], 1 + ix, 1, 1);
      }
      // central beam energy
      _eCent = getOption<string>("ECENT", std::to_string(sqrtS() / GeV));
    }


    /// Perform the per-event analysis
    void analyze(const Event& event) {
      static const double cos34 = cos(34. / 180. * M_PI);
      // get the axis, direction of incoming positron
      const ParticlePair& beams = apply<Beam>(event, "Beams").beams();
      Vector3 axis1 = beams.first.momentum().p3().unit();
      Vector3 axis2 = beams.second.momentum().p3().unit();
      if (beams.first.pid() < 0) swap(axis1, axis2);
      // loop over FS particles
      Particles fs = apply<FinalState>(event, "FS").particles();
      Particles em, ep, mm, mp;
      for (const Particle& p : fs) {
        if (p.abspid() == PID::MUON) {
          if (p.pid() > 0)
            mm.push_back(p);
          else
            mp.push_back(p);
        }
        else if (p.abspid() == PID::ELECTRON) {
          if (abs(axis1.dot(p.p3().unit())) > cos34) continue;
          if (p.pid() > 0)
            em.push_back(p);
          else
            ep.push_back(p);
        }
        else if (p.pid() != PID::GAMMA) {
          vetoEvent;
        }
      }
      if (em.size() == 1 && ep.size() == 1 && mm.size() == 0 && mp.size() == 0) {
        _sigma[0]->fill(_eCent);
      }
      else if (mm.size() == 1 && mp.size() == 1 && em.size() == 0 && ep.size() == 0) {
        _sigma[1]->fill(_eCent);
      }
    }


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

    /// @}


    /// @name Histograms
    /// @{
    BinnedHistoPtr<string> _sigma[2];
    string _eCent;
    /// @}
  };


  RIVET_DECLARE_PLUGIN(BESIII_2022_I2102287);

}