Rivet analyses

title: BESIII_2015_I1376484

Kinematic distributions in the decay $\eta\to\pi^+\pi^-\pi^0$ and $\eta^{(\prime)}\to\pi^0\pi^0\pi^0$

Experiment: BESIII (BEPC)

Inspire ID: 1376484

Status: VALIDATED NOHEPDATA

Authors: - Peter Richardson

References: - Phys.Rev.D 92 (2015) 012014

Beams: * *

Beam energies: ANY

Run details: - Any process producing eta/etaprime mesons

Measurement of kinematic distributions in the decays $\eta\to\pi^+\pi^-\pi^0$ and $\eta^{(\prime)}\to\pi^0\pi^0\pi^0$. The data was read from the plots in the paper and the backgrounds subtract in the $\eta^\prime$ decays.

Source code:BESIII_2015_I1376484.cc

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/DecayedParticles.hh"

include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {

/// @brief eta -> pi+pi-pi0 and eta' -> pi0pi0pi0$ class BESIII_2015_I1376484 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2015_I1376484);


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

/// Book histograms and initialise projections before the run
void init() {
  // Initialise and register projections
  UnstableParticles ufs = UnstableParticles(Cuts::pid == PID::ETA or Cuts::pid == PID::ETAPRIME);
  declare(ufs, "UFS");
  DecayedParticles ETA(ufs);
  ETA.addStable(PID::PI0);
  ETA.addStable(PID::K0S);
  ETA.addStable(PID::ETA);
  declare(ETA, "ETA");
  for (unsigned int ix = 0; ix < 2; ++ix) {
    book(_h[ix], 1, 1, 1 + ix);
    book(_h[ix + 2], 2 + ix, 1, 1);
  }
}


/// Perform the per-event analysis
void analyze(const Event& event) {
  static const map<PdgId, unsigned int>& mode1 = {{211, 1}, {-211, 1}, {111, 1}};
  static const map<PdgId, unsigned int>& mode2 = {{111, 3}};
  DecayedParticles ETA = apply<DecayedParticles>(event, "ETA");
  // loop over particles
  for (unsigned int ix = 0; ix < ETA.decaying().size(); ++ix) {
    // select right decay mode
    if (ETA.decaying()[ix].pid() == PID::ETA && ETA.modeMatches(ix, 3, mode1)) {
      const Particle& pi0 = ETA.decayProducts()[ix].at(111)[0];
      const Particle& pip = ETA.decayProducts()[ix].at(211)[0];
      const Particle& pim = ETA.decayProducts()[ix].at(-211)[0];
      double s1 = (pi0.momentum() + pim.momentum()).mass2();
      double s2 = (pi0.momentum() + pip.momentum()).mass2();
      double s3 = (pip.momentum() + pim.momentum()).mass2();
      double mOut = pi0.mass() + pip.mass() + pim.mass();
      double Q = ETA.decaying()[ix].mass() - mOut;
      double X = sqrt(3.) / 2. / ETA.decaying()[ix].mass() / Q * (s1 - s2);
      double Y = 3. * (sqr(ETA.decaying()[ix].mass() - pi0.mass()) - s3) / 2. / ETA.decaying()[ix].mass()
              / Q
          - 1.;
      _h[0]->fill(X);
      _h[1]->fill(Y);
    }
    else if (ETA.modeMatches(ix, 3, mode2)) {
      const Particles& pi0 = ETA.decayProducts()[ix].at(111);
      double s1 = (pi0[2].momentum() + pi0[1].momentum()).mass2();
      double s2 = (pi0[2].momentum() + pi0[0].momentum()).mass2();
      double s3 = (pi0[0].momentum() + pi0[1].momentum()).mass2();
      double mOut = pi0[2].mass() + pi0[0].mass() + pi0[1].mass();
      double Q = ETA.decaying()[ix].mass() - mOut;
      double X = sqrt(3.) / 2. / ETA.decaying()[ix].mass() / Q * (s1 - s2);
      double Y = 3. * (sqr(ETA.decaying()[ix].mass() - pi0[2].mass()) - s3) / 2.
              / ETA.decaying()[ix].mass() / Q
          - 1.;
      double Z = sqr(X) + sqr(Y);
      if (ETA.decaying()[ix].pid() == PID::ETA)
        _h[2]->fill(Z);
      else
        _h[3]->fill(Z);
    }
  }
}


/// Normalise histograms etc., after the run
void finalize() {
  for (unsigned int ix = 0; ix < 4; ++ix) normalize(_h[ix]);
}

/// @}


/// @name Histograms
/// @{
Histo1DPtr _h[4];
/// @}

};

RIVET_DECLARE_PLUGIN(BESIII_2015_I1376484);

} ```