Rivet analyses

title: BESIII_2022_I2102287

$e^+e^-\to e^+e^-$ and $\mu^+\mu^-$ 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 $\mu^+\mu^-$ cross sections near the $J/\psi$ 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++ // -- 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);

} ```