Rivet analyses

Kinematic distributions in η → π+π−π0 and 3π0 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 η → π+π−π0 and 3π0 decays. The data were read from the plots in the paper

Source code:BESIII_2023_I2633025.cc

// -*- 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);

}