Rivet analyses

title: BESIII_2018_I1641075

$\eta^\prime\to \pi^+\pi^-\gamma$ decays

Experiment: BESIII (BEPC)

Inspire ID: 1641075

Status: VALIDATED

Authors: - Peter Richardson

References: - Phys.Rev.Lett. 120 (2018) 242003, 2018

Beams: * *

Beam energies: ANY

Run details: - Events with eta prime decays, either particle guns or collisions.

Differential decay rates for $\eta^\prime\to \pi^+\pi^-\gamma$ decays. Data from HEPData has been manipulated to remove the background, correct for the efficiency, and normalise the distribution.

Source code:BESIII_2018_I1641075.cc

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/DecayedParticles.hh"

include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {

/// @brief eta' -> pi+pi- gamma decay class BESIII_2018_I1641075 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2018_I1641075);


/// @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);
  declare(ETA, "ETA");

  // Book histograms
  book(_h_m, 1, 1, 5);
}

/// Perform the per-event analysis
void analyze(const Event& event) {
  static const map<PdgId, unsigned int>& mode = {{PID::PIPLUS, 1}, {PID::PIMINUS, 1}, {PID::GAMMA, 1}};
  DecayedParticles ETA = apply<DecayedParticles>(event, "ETA");
  // loop over particles
  for (unsigned int ix = 0; ix < ETA.decaying().size(); ++ix) {
    if (!ETA.modeMatches(ix, 3, mode)) continue;
    const Particle& pip = ETA.decayProducts()[ix].at(PID::PIPLUS)[0];
    const Particle& pim = ETA.decayProducts()[ix].at(PID::PIMINUS)[0];
    _h_m->fill((pip.momentum() + pim.momentum()).mass());
  }
}


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

/// @}


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

};

RIVET_DECLARE_PLUGIN(BESIII_2018_I1641075);

} ```