Rivet analyses

title: BESIII_2016_I1504943

$\eta^\prime \to\gamma\gamma\pi^0$ decays

Experiment: BESIII (BEPC)

Inspire ID: 1504943

Status: VALIDATED NOHEPDATA

Authors: - Peter Richardson

References: - Phys.Rev.D 96 (2017) 1, 012005

Beams: * *

Beam energies: ANY

Run details: - Any process producing eta', original e+e-

Measurement of the $\gamma\gamma$ mass distribution in the $\eta^\prime \to\gamma\gamma\pi^0$ decay by the BESIII collaboration.

Source code:BESIII_2016_I1504943.cc

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/DecayedParticles.hh"

include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {

/// @brief eta' -> gamma gamma pi0 class BESIII_2016_I1504943 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2016_I1504943);


/// @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);
  ETA.addStable(PID::K0S);
  declare(ETA, "ETA");
  // Book histograms
  for (unsigned int ix = 0; ix < 3; ++ix) book(_h_br[ix], 1, 1, 1 + ix);
  book(_h_m, 2, 1, 1);
  book(_netap, "TMP/netap");
}

/// Perform the per-event analysis
void analyze(const Event& event) {
  static const map<PdgId, unsigned int>& mode = {{111, 1}, {22, 2}};
  DecayedParticles ETA = apply<DecayedParticles>(event, "ETA");
  // loop over particles
  for (unsigned int ix = 0; ix < ETA.decaying().size(); ++ix) {
    _netap->fill();
    // select right decay mode
    if (!ETA.modeMatches(ix, 3, mode)) continue;
    const Particles& gam = ETA.decayProducts()[ix].at(22);
    double mass2 = (gam[0].momentum() + gam[1].momentum()).mass2();
    _h_m->fill(mass2);
    _h_br[0]->fill();
    if (any(ETA.decaying()[ix].children(), hasAbsPID(PID::OMEGA))) _h_br[1]->fill();
    if (ETA.decaying()[ix].children().size() == 3) _h_br[2]->fill();
  }
}


/// Normalise histograms etc., after the run
void finalize() {
  // eta' width in kev
  double gammaEtap = 0.188e3;
  scale(_h_m, gammaEtap / *_netap);
  for (unsigned int ix = 0; ix < 3; ++ix) scale(_h_br[ix], 1e4 / *_netap);
}

/// @}


/// @name Histograms
/// @{
Histo1DPtr _h_m;
CounterPtr _netap, _h_br[3];
/// @}

};

RIVET_DECLARE_PLUGIN(BESIII_2016_I1504943);

} ```