Rivet analyses

Kinematic distributions in the decay η → π+π−π0 and η(′) → π0π0π0

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 η → π+π−π0 and η(′) → π0π0π0. The data was read from the plots in the paper and the backgrounds subtract in the η′ decays.

Source code:BESIII_2015_I1376484.cc

// -*- 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);

}