Rivet analyses

title: BESIII_2015_I1373915

Radiative $J/\psi$ decays to $\pi^0\pi^0$

Experiment: BESIII (BEPC)

Inspire ID: 1373915

Status: VALIDATED NOHEPDATA

Authors: - Peter Richardson

References: - Phys.Rev.D 92 (2015) 5, 052003

Beams: * *

Beam energies: ANY

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

Measurement of the $\pi^0\pi^0$ mass distribution in the decay $J/\psi\to\gamma\pi^0\pi^0$. Plots were read from the paper and are not corrected for efficiency/acceptance, although the backgrounds given in the paper were subtracted.

Source code:BESIII_2015_I1373915.cc

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/DecayedParticles.hh"

include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {

/// @brief J/psi -> gamma pi0 pi0 class BESIII_2015_I1373915 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2015_I1373915);


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

/// Book histograms and initialise projections before the run
void init() {
  UnstableParticles ufs = UnstableParticles(Cuts::abspid == 443);
  declare(ufs, "UFS");
  DecayedParticles PSI(ufs);
  PSI.addStable(PID::PI0);
  declare(PSI, "PSI");
  book(_h, 1, 1, 1);
}


/// Perform the per-event analysis
void analyze(const Event& event) {
  // find the J/psi decays
  static const map<PdgId, unsigned int>& mode = {{111, 2}, {22, 1}};
  DecayedParticles PSI = apply<DecayedParticles>(event, "PSI");
  if (PSI.decaying().size() != 1) vetoEvent;
  if (!PSI.modeMatches(0, 3, mode)) vetoEvent;
  // particles
  const Particles& pi0 = PSI.decayProducts()[0].at(111);
  double mpipi = (pi0[0].momentum() + pi0[1].momentum()).mass();
  _h->fill(mpipi);
}


/// Normalise histograms etc., after the run
void finalize() {
  normalize(_h, 1., false);
}

/// @}


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

};

RIVET_DECLARE_PLUGIN(BESIII_2015_I1373915);

} ```