Rivet analyses

Kinematic distributions in the decay η → π0γγ

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 η → π0γγ.

Source code:A2_2014_I1297221.cc

// -*- 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);

}