Rivet analyses

title: A2_2014_I1297221

Kinematic distributions in the decay $\eta\to\pi^0\gamma\gamma$

Experiment: A2 (MAMI)

Inspire ID: 1297221

Status: VALIDATED

Authors: - Peter Richardson

References: - Phys.Rev.C 90 (2014) 025206, 2014.

Beams: * *

Beam energies: ANY

Run details: - Any process producing eta mesons

Measurement of kinematic distributions in the decay $\eta\to\pi^0\gamma\gamma$.

Source code:A2_2014_I1297221.cc

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/DecayedParticles.hh"

include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {

/// @brief eta -> pi0 gamma gamma class A2_2014_I1297221 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(A2_2014_I1297221);


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

/// Book histograms and initialise projections before the run
void init() {
  // Initialise and register projections
  UnstableParticles ufs = UnstableParticles(Cuts::pid == PID::ETA);
  declare(ufs, "UFS");
  DecayedParticles ETA(ufs);
  ETA.addStable(PID::PI0);
  ETA.addStable(PID::K0S);
  declare(ETA, "ETA");
  // histos
  for (unsigned int ix = 0; ix < 3; ++ix) book(_h[ix], 1, 1, 1 + ix);
  book(_c, "TMP/nEta");
}


/// 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) {
    _c->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();
    for (unsigned int ix = 0; ix < 3; ++ix) _h[ix]->fill(mass2);
  }
}


/// Normalise histograms etc., after the run
void finalize() {
  // eta width in eV from PDG2022
  double gam = 1.31e3;
  for (unsigned int ix = 0; ix < 3; ++ix) {
    scale(_h[ix], gam / *_c);
  }
}

/// @}


/// @name Histograms
/// @{
Histo1DPtr _h[3];
CounterPtr _c;
/// @}

};

RIVET_DECLARE_PLUGIN(A2_2014_I1297221);

} ```