Rivet analyses

Radiative J/ψ decays to KS0KS0

Experiment: BESIII (BEPC)

Inspire ID: 1689296

Status: VALIDATED NOHEPDATA

Authors: - Peter Richardson

References: - Phys.Rev.D 98 (2018) 7, 072003

Beams: e- e+

Beam energies: (1.6, 1.6)GeV

Run details: - e+e- > J/psi

Measurement of mass and angular distributions in the decay J/ψ → γKS0K0S. Plots were read from the paper and are not corrected for efficiency/acceptance, although the paper states the backgrounds are small.

Source code:BESIII_2018_I1689296.cc

// -*- C++ -*-
#include "Rivet/Analysis.hh"
#include "Rivet/Projections/Beam.hh"
#include "Rivet/Projections/DecayedParticles.hh"
#include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {


  /// @brief J/psi -> gamma KS0 KS0
  class BESIII_2018_I1689296 : public Analysis {
  public:

    /// Constructor
    RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2018_I1689296);


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

    /// Book histograms and initialise projections before the run
    void init() {
      // Initialise and register projections
      UnstableParticles ufs = UnstableParticles(Cuts::abspid == 443);
      declare(ufs, "UFS");
      DecayedParticles PSI(ufs);
      PSI.addStable(PID::K0S);
      declare(PSI, "PSI");
      declare(Beam(), "Beams");
      // hisotgrams
      for (unsigned int ix = 0; ix < 3; ++ix) {
        if (ix < 2) book(_h_mass[ix], 1, 1, 1 + ix);
        book(_h_angle[ix], 2, 1, 1 + ix);
      }
    }

    // angle cuts due regions of BES calorimeter
    bool vetoPhoton(const double& cTheta) {
      return cTheta > 0.92 || (cTheta > 0.8 && cTheta < 0.86);
    }

    /// Perform the per-event analysis
    void analyze(const Event& event) {
      // get the axis, direction of incoming electron
      const ParticlePair& beams = apply<Beam>(event, "Beams").beams();
      Vector3 axis;
      if (beams.first.pid() > 0)
        axis = beams.first.momentum().p3().unit();
      else
        axis = beams.second.momentum().p3().unit();
      // find the J/psi decays
      static const map<PdgId, unsigned int>& mode = {{310, 2}, {22, 1}};
      DecayedParticles PSI = apply<DecayedParticles>(event, "PSI");
      if (PSI.decaying().size() != 1) vetoEvent;
      if (!PSI.modeMatches(0, 3, mode)) vetoEvent;
      // particles
      const Particles& K0 = PSI.decayProducts()[0].at(310);
      const Particle& gam = PSI.decayProducts()[0].at(22)[0];
      double mKK = (K0[0].momentum() + K0[1].momentum()).mass();
      _h_mass[0]->fill(mKK);
      for (unsigned int ix = 0; ix < 2; ++ix) _h_mass[1]->fill((gam.momentum() + K0[ix].momentum()).mass());
      double cTheta = axis.dot(gam.p3().unit());
      if (vetoPhoton(abs(cTheta))) vetoEvent;
      _h_angle[0]->fill(cTheta);
      // remaining angles
      LorentzTransform boost1 = LorentzTransform::mkFrameTransformFromBeta(
          PSI.decaying()[0].momentum().betaVec());
      FourMomentum pGamma = boost1.transform(gam.momentum());
      FourMomentum pKK = boost1.transform(K0[0].momentum() + K0[1].momentum());
      Vector3 e1z = pGamma.p3().unit();
      Vector3 e1y = e1z.cross(axis).unit();
      Vector3 e1x = e1y.cross(e1z).unit();
      LorentzTransform boost2 = LorentzTransform::mkFrameTransformFromBeta(pKK.betaVec());
      Vector3 axis2 = boost2.transform(boost1.transform(K0[0].momentum())).p3().unit();
      _h_angle[1]->fill(e1z.dot(axis2));
      double phi = atan2(e1y.dot(axis2), e1x.dot(axis2));
      _h_angle[2]->fill(phi);
    }


    /// Normalise histograms etc., after the run
    void finalize() {
      for (unsigned int ix = 0; ix < 3; ++ix) {
        if (ix < 2) normalize(_h_mass[ix], 1., false);
        normalize(_h_angle[ix], 1., false);
      }
    }

    /// @}


    /// @name Histograms
    /// @{
    Histo1DPtr _h_mass[2], _h_angle[3];
    /// @}
  };


  RIVET_DECLARE_PLUGIN(BESIII_2018_I1689296);

}