Rivet analyses

title: BESIII_2018_I1697377

$J/\psi\to e^+e^-\eta$ decays

Experiment: BESIII ()

Inspire ID: 1697377

Status: VALIDATED

Authors: - Peter Richardson

References: - Phys.Rev. D99 (2019) no.1, 012006

Beams: * *

Beam energies: ANY

Run details: - Events with J/psi decays, either particle guns or collisions.

Differential decay rates for $J/\psi\to e^+e^-\eta$ decays.

Source code:BESIII_2018_I1697377.cc

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {

/// @brief $J/\psi\to e^+e^-\eta$ decays class BESIII_2018_I1697377 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2018_I1697377);


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

/// Book histograms and initialise projections before the run
void init() {
  // Initialise and register projections
  declare(UnstableParticles(), "UFS");
  // Book histograms
  book(_h_m, 1, 1, 1);
}


void findDecayProducts(const Particle& mother,
                       unsigned int& nstable,
                       unsigned int& neta,
                       unsigned int& nep,
                       unsigned int& nem,
                       FourMomentum& ptot) {
  for (const Particle& p : mother.children()) {
    int id = p.pid();
    if (id == PID::EMINUS) {
      ++nem;
      ++nstable;
      ptot += p.momentum();
    }
    else if (id == PID::EPLUS) {
      ++nep;
      ++nstable;
      ptot += p.momentum();
    }
    else if (id == PID::ETA) {
      ++neta;
      ++nstable;
    }
    else if (!p.children().empty()) {
      findDecayProducts(p, nstable, neta, nep, nem, ptot);
    }
    else
      ++nstable;
  }
}


/// Perform the per-event analysis
void analyze(const Event& event) {

  // Loop over J/psi mesons
  for (const Particle& p : apply<UnstableParticles>(event, "UFS").particles(Cuts::pid == 443)) {
    unsigned nstable(0), neta(0), nep(0), nem(0);
    FourMomentum ptot;
    findDecayProducts(p, nstable, neta, nep, nem, ptot);
    if (nstable == 3 && nem == 1 && nem == 1 && neta == 1) {
      _h_m->fill(ptot.mass());
    }
  }
}


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

/// @}


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

};

RIVET_DECLARE_PLUGIN(BESIII_2018_I1697377);

} ```