Rivet analyses

title: BESIII_2013_I1126137

Photon Spectrum in $J/\psi\to3\gamma$ 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 $J/\psi\to3\gamma$ decays by BES. The data was read from the figure in the paper but is corrected.

Source code:BESIII_2013_I1126137.cc

```c++ // -- 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);

} ```