Rivet analyses

title: BESIII_2021_I1966612

Cross section for $e^+e^-\to$ neutron and antineutron between 2. and 3.08 GeV

Experiment: BESIII (BEPC II)

Inspire ID: 1966612

Status: VALIDATED

Authors: - Peter Richardson

References: - Nature Phys. 17 (2021) 11, 1200-1204

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); (1.3, 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)GeV

Run details: - e+ e- to hadrons

Measurement of the cross section for $e^+e^-\to p\bar{p}$ for energies between 2. and 3.08 GeV.

Source code:BESIII_2021_I1966612.cc

```c++ // -- C++ --

include "Rivet/Analysis.hh"

include "Rivet/Projections/FinalState.hh"

namespace Rivet {

/// @brief e+ e- > n nbar class BESIII_2021_I1966612 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2021_I1966612);


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

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

  // Initialise and register projections
  declare(FinalState(), "FS");

  // Book histograms
  book(_nneutron, 1, 1, 1);

  for (const string& en : _nneutron.binning().edges<0>()) {
    const double eval = stod(en) * GeV;
    if (isCompatibleWithSqrtS(eval)) {
      _sqs = en;
      break;
    }
  }
  raiseBeamErrorIf(_sqs.empty());
}


/// Perform the per-event analysis
void analyze(const Event& event) {
  const FinalState& fs = apply<FinalState>(event, "FS");
  if (fs.particles().size() != 2) vetoEvent;
  for (const Particle& p : fs.particles()) {
    if (p.abspid() != PID::NEUTRON) vetoEvent;
  }
  _nneutron->fill(_sqs);
}


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

///@}


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

};

RIVET_DECLARE_PLUGIN(BESIII_2021_I1966612);

} ```