Rivet analyses

title: BESIII_2015_I1364494

$\eta^\prime\to e^+e^-\gamma$ decays

Experiment: BESIII (BEPC)

Inspire ID: 1364494

Status: VALIDATED

Authors: - Peter Richardson

References: - Phys.Rev. D92 (2015) 012001, 2015

Beams: * *

Beam energies: ANY

Run details: - Events with eta prime decays, either particle guns or collisions.

Differential decay rates for $\eta^\prime\to e^+e^-\gamma$ decays

Source code:BESIII_2015_I1364494.cc

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/DecayedParticles.hh"

include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {

/// @brief eta' -> gamma e+e- class BESIII_2015_I1364494 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2015_I1364494);


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

/// Book histograms and initialise projections before the run
void init() {
  // Initialise and register projections
  UnstableParticles ufs = UnstableParticles(Cuts::pid == 331);
  declare(ufs, "UFS");
  DecayedParticles ETA(ufs);
  ETA.addStable(PID::PI0);
  declare(ETA, "ETA");
  // Book histograms
  book(_h_m, 1, 1, 3);
  book(_netap, "TMP/netap");
}

/// Perform the per-event analysis
void analyze(const Event& event) {
  static const map<PdgId, unsigned int>& mode0 = {{22, 2}};
  static const map<PdgId, unsigned int>& mode1 = {{11, 1}, {-11, 1}, {22, 1}};
  // Loop over eta' mesons
  DecayedParticles ETA = apply<DecayedParticles>(event, "ETA");
  // loop over particles
  for (unsigned int ix = 0; ix < ETA.decaying().size(); ++ix) {
    // reference mode for denominator
    if (ETA.modeMatches(ix, 2, mode0)) _netap->fill();
    // select right decay mode
    else if (ETA.modeMatches(ix, 3, mode1)) {
      const Particle& ep = ETA.decayProducts()[ix].at(11)[0];
      const Particle& em = ETA.decayProducts()[ix].at(-11)[0];
      _h_m->fill((em.momentum() + ep.momentum()).mass());
    }
  }
}


/// Normalise histograms etc., after the run
void finalize() {

  // divide by no so BR and mult by bin width
  // and 100 as in %
  scale(_h_m, 0.1 * 100. / *_netap);
}

/// @}


/// @name Histograms
/// @{
Histo1DPtr _h_m;
CounterPtr _netap;
/// @}

};

RIVET_DECLARE_PLUGIN(BESIII_2015_I1364494);

} ```