Rivet analyses

title: BESIII_2023_I2633025

Kinematic distributions in $\eta\to\pi^+\pi^-\pi^0$ and $3\pi^0$ decays

Experiment: BESIII (BEPC)

Inspire ID: 2633025

Status: VALIDATED NOHEPDATA

Authors: - Peter Richardson

References: - arXiv: 2302.08282

Beams: * *

Beam energies: ANY

Run details: - Any process producing eta mesons, originally e+e-

Kinematic distributions in $\eta\to\pi^+\pi^-\pi^0$ and $3\pi^0$ decays. The data were read from the plots in the paper

Source code:BESIII_2023_I2633025.cc

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/DecayedParticles.hh"

include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {

/// @brief eta -> 3 pions class BESIII_2023_I2633025 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2023_I2633025);


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

/// Book histograms and initialise projections before the run
void init() {
  // Initialise and register projections
  UnstableParticles ufs = UnstableParticles(Cuts::pid == PID::ETA);
  declare(ufs, "UFS");
  DecayedParticles ETA(ufs);
  ETA.addStable(PID::PI0);
  ETA.addStable(PID::K0S);
  declare(ETA, "ETA");
  vector<double> bins = {-1., -0.9, -0.8, -0.7, -0.6, -0.5, -0.4, -0.3, -0.2, -0.1,
                         0.,  0.1,  0.2,  0.3,  0.4,  0.5,  0.6,  0.7,  0.8,  0.9};
  // histograms
  for (unsigned int ix = 0; ix < 2; ++ix) {
    book(_d[ix], bins);
    book(_c[ix], "TMP/c_" + toString(ix));
    for (unsigned int iy = 0; iy < 19; ++iy) {
      book(_d[ix]->bin(iy + 1), 3 + ix, 1, 1 + iy);
    }
  }
}


/// Perform the per-event analysis
void analyze(const Event& event) {
  DecayedParticles ETA = apply<DecayedParticles>(event, "ETA");
  // loop over particles
  for (unsigned int ix = 0; ix < ETA.decaying().size(); ++ix) {
    if (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.mom() + pim.mom()).mass2();
      double s2 = (pi0.mom() + pip.mom()).mass2();
      double s3 = (pip.mom() + pim.mom()).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.;
      _d[0]->fill(Y, X);
      _c[0]->fill();
    }
    else if (ETA.modeMatches(ix, 3, mode2)) {
      const Particles& pi0 = ETA.decayProducts()[ix].at(111);
      double s1 = (pi0[2].mom() + pi0[1].mom()).mass2();
      double s2 = (pi0[2].mom() + pi0[0].mom()).mass2();
      double s3 = (pi0[0].mom() + pi0[1].mom()).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.;
      _d[1]->fill(Y, X);
      _c[1]->fill();
    }
  }
}


/// Normalise histograms etc., after the run
void finalize() {
  for (unsigned int ix = 0; ix < 2; ++ix) {
    scale(_d[ix], 1. / *_c[ix]);
    divByGroupWidth(_d[ix]);
  }
}

/// @}


/// @name Histograms
/// @{
Histo1DGroupPtr _d[2];
CounterPtr _c[2];
const map<PdgId, unsigned int> mode1 = {{211, 1}, {-211, 1}, {111, 1}};
const map<PdgId, unsigned int> mode2 = {{111, 3}};
/// @}

};

RIVET_DECLARE_PLUGIN(BESIII_2023_I2633025);

} ```