Rivet analyses

title: BESIII_2016_I1437949

Mass distributions in $J/\psi\to\gamma\eta^\prime\pi^+\pi^-$

Experiment: BESIII (BEPC)

Inspire ID: 1437949

Status: VALIDATED NOHEPDATA

Authors: - Peter Richardson

References: - Phys.Rev.Lett. 117 (2016) 4, 042002

Beams: * *

Beam energies: ANY

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

Mass distributions in $J/\psi\to\gamma\eta^\prime\pi^+\pi^-$

Source code:BESIII_2016_I1437949.cc

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/DecayedParticles.hh"

include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {

/// @brief J/psi -> gamma eta' pi+pi- class BESIII_2016_I1437949 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2016_I1437949);


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

/// Book histograms and initialise projections before the run
void init() {
  // Initialise and register projections
  UnstableParticles ufs = UnstableParticles(Cuts::abspid == 443);
  declare(ufs, "UFS");
  DecayedParticles PSI(ufs);
  PSI.addStable(PID::ETA);
  PSI.addStable(PID::ETAPRIME);
  declare(PSI, "PSI");
  // histos
  for (unsigned int ix = 0; ix < 2; ++ix) book(_h[ix], 1, 1, 1 + ix);
  book(_h[2], 2, 1, 1);
}


/// Perform the per-event analysis
void analyze(const Event& event) {
  // find the J/psi decays
  static const map<PdgId, unsigned int>& mode = {{22, 1}, {331, 1}, {211, 1}, {-211, 1}};
  DecayedParticles PSI = apply<DecayedParticles>(event, "PSI");
  for (unsigned int ix = 0; ix < PSI.decaying().size(); ++ix) {
    if (!PSI.modeMatches(ix, 4, mode)) continue;
    const Particle& gam = PSI.decayProducts()[ix].at(22)[0];
    double mass = (PSI.decaying()[ix].momentum() - gam.momentum()).mass();
    for (unsigned int ix = 0; ix < 3; ++ix) _h[ix]->fill(mass);
  }
}


/// Normalise histograms etc., after the run
void finalize() {
  for (unsigned int ix = 0; ix < 3; ++ix) normalize(_h[ix], 1., false);
}

/// @}


/// @name Histograms
/// @{
Histo1DPtr _h[3];
/// @}

};

RIVET_DECLARE_PLUGIN(BESIII_2016_I1437949);

} ```