Rivet analyses

Photon Spectrum in J/ψ → 3γ decays

Experiment: BESIII (BEPC)

Inspire ID: 1126137

Status: VALIDATED NOHEPDATA

Authors: - Peter Richardson

References: - Phys.Rev.D 87 (2013) 3, 032003

Beams: * *

Beam energies: ANY

Run details: - Any process producing J/psi mesons, originally e+e- collisions

Measurement of the photon pectrum in J/ψ → 3γ decays by BES. The data was read from the figure in the paper but is corrected.

Source code:BESIII_2013_I1126137.cc

// -*- C++ -*-
#include "Rivet/Analysis.hh"
#include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {


  /// @brief J/psi -> 3 gamma
  class BESIII_2013_I1126137 : public Analysis {
  public:

    /// Constructor
    RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2013_I1126137);


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

    /// Book histograms and initialise projections before the run
    void init() {
      // projections
      declare(UnstableParticles(Cuts::pid == 443), "UFS");
      // histos
      book(_h, 1, 1, 1);
      book(_c, "TMP/nJPsi");
    }


    /// Perform the per-event analysis
    void analyze(const Event& event) {
      // Find the J/psi among the unstables
      for (const Particle& psi : apply<UnstableParticles>(event, "UFS").particles()) {
        _c->fill();
        unsigned int nhadron(0);
        Particles photons;
        for (const Particle& child : psi.children()) {
          if (PID::isHadron(child.pid()))
            ++nhadron;
          else if (child.pid() == PID::PHOTON)
            photons.push_back(child);
        }
        if (nhadron == 0 && photons.size() == 3) {
          LorentzTransform boost;
          if (psi.p3().mod() > 1 * MeV)
            boost = LorentzTransform::mkFrameTransformFromBeta(psi.momentum().betaVec());
          for (const Particle& gamma : photons)
            _h->fill(2. * boost.transform(gamma.momentum()).E() / psi.mass());
        }
      }
    }


    /// Normalise histograms etc., after the run
    void finalize() {
      scale(_h, 1e6 / 3. / *_c);
    }

    /// @}


    /// @name Histograms
    /// @{
    Histo1DPtr _h;
    CounterPtr _c;
    /// @}
  };


  RIVET_DECLARE_PLUGIN(BESIII_2013_I1126137);

}