Rivet analyses

title: BESIII_2020_I1830421

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

Experiment: BESIII (BEPC)

Inspire ID: 1830421

Status: VALIDATED NOHEPDATA

Authors: - Peter Richardson

References: - Phys.Rev.D 103 (2021) 9, 092005

Beams: * *

Beam energies: ANY

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

Differential decay rates for $\eta^\prime\to \pi^+\pi^-e^+e^-$ decays. Due to the resolution and backgrounds only the pion pair mass is used. The data were read from the plots in the paper.

Source code:BESIII_2020_I1830421.cc

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/DecayedParticles.hh"

include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {

/// @brief eta' -> pi+pi- e+e- decay class BESIII_2020_I1830421 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2020_I1830421);


/// @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);
  declare(ETA, "ETA");
  // Book histograms
  //book(_h_mee  , 1, 1, 1);
  book(_h_mpipi, 1, 1, 2);
}

/// Perform the per-event analysis
void analyze(const Event& event) {
  static const map<PdgId, unsigned int>& mode = {{211, 1}, {-211, 1}, {11, 1}, {-11, 1}};
  DecayedParticles ETA = apply<DecayedParticles>(event, "ETA");
  // loop over particles
  for (unsigned int ix = 0; ix < ETA.decaying().size(); ++ix) {
    if (!ETA.modeMatches(ix, 4, mode)) continue;
    const Particle& pip = ETA.decayProducts()[ix].at(211)[0];
    const Particle& pim = ETA.decayProducts()[ix].at(-211)[0];
    //const Particle & ep  = ETA.decayProducts()[ix].at(  11)[0];
    //const Particle & em  = ETA.decayProducts()[ix].at(- 11)[0];
    //_h_mee  ->fill((em .momentum()+ep .momentum()).mass());
    _h_mpipi->fill((pim.momentum() + pip.momentum()).mass());
  }
}


/// Normalise histograms etc., after the run
void finalize() {
  //normalize(_h_mee);
  normalize(_h_mpipi);
}

/// @}


/// @name Histograms
/// @{
//Histo1DPtr _h_mee;
Histo1DPtr _h_mpipi;
/// @}

};

RIVET_DECLARE_PLUGIN(BESIII_2020_I1830421);

} ```