Rivet analyses

η′ → γγπ0 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 γγ mass distribution in the η′ → γγπ0 decay by the BESIII collaboration.

Source code:BESIII_2016_I1504943.cc

// -*- 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);

}