Rivet analyses

ψ(2S) → e+e−χc(1, 2) and χc(1, 2) → e+e−J/ψ decays

Experiment: BESIII (BEPC)

Inspire ID: 1509920

Status: VALIDATED NOHEPDATA

Authors: - Peter Richardson

References: - Phys.Rev.Lett. 118 (2017) 22, 221802

Beams: e- e+

Beam energies: (1.8, 1.8)GeV

Run details: - e+e- > psi 2s.

Measurement of the e+e− mass distribution and the helicity angle in the decays ψ(2S) → e+e−χc(1, 2) and χc(1, 2) → e+e−J/ψ decays. Data was read from the plots in the paper and the mass distributions are not corrected, although the angle distributions are

Source code:BESIII_2017_I1509920.cc

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

namespace Rivet {


  /// @brief psi(2S) -> e+e- chi_c and chi_c  -> e+e- Jpsi$
  class BESIII_2017_I1509920 : public Analysis {
  public:

    /// Constructor
    RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2017_I1509920);


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

    /// Book histograms and initialise projections before the run
    void init() {
      // Initialise and register projections
      declare(Beam(), "Beams");
      UnstableParticles ufs_psi(Cuts::abspid == PID::PSI2S);
      DecayedParticles psi(ufs_psi);
      psi.addStable(20443);
      psi.addStable(445);
      declare(psi, "PSI");
      UnstableParticles ufs_chi(Cuts::abspid == 445 or Cuts::abspid == 20443);
      DecayedParticles chi(ufs_chi);
      chi.addStable(PID::JPSI);
      declare(chi, "CHI");

      // Histograms
      for (unsigned int ix = 0; ix < 2; ++ix)
        for (unsigned int iy = 0; iy < 4; ++iy) book(_h[ix][iy], 1 + ix, 1, 1 + iy);
    }


    /// 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();
      // decaying particles
      static const map<PdgId, unsigned int>& mode1 = {{20443, 1}, {11, 1}, {-11, 1}};
      static const map<PdgId, unsigned int>& mode2 = {{445, 1}, {11, 1}, {-11, 1}};
      static const map<PdgId, unsigned int>& mode3 = {{443, 1}, {11, 1}, {-11, 1}};
      unsigned int iproj = 0;
      for (const DecayedParticles& in :
           {apply<DecayedParticles>(event, "PSI"), apply<DecayedParticles>(event, "CHI")}) {
        for (unsigned int ix = 0; ix < in.decaying().size(); ++ix) {
          int imode = -1, ichi = 0;
          if (iproj == 0) {
            if (in.modeMatches(ix, 3, mode1)) {
              imode = 0;
              ichi = 20443;
            }
            else if (in.modeMatches(ix, 3, mode2)) {
              imode = 1;
              ichi = 445;
            }
            else
              continue;
          }
          else {
            if (in.modeMatches(ix, 3, mode3)) {
              imode = in.decaying()[ix].pid() == 20443 ? 2 : 3;
              ichi = 443;
            }
            else
              continue;
          }
          // extract particles
          const Particle& em = in.decayProducts()[ix].at(11)[0];
          const Particle& ep = in.decayProducts()[ix].at(-11)[0];
          const Particle& out = in.decayProducts()[ix].at(ichi)[0];
          FourMomentum qq = ep.momentum() + em.momentum();
          double q = qq.mass();
          _h[0][imode]->fill(q);
          if (iproj == 0) {
            _h[1][imode]->fill(axis.dot((em.momentum() + ep.momentum()).p3().unit()));
          }
          else if (iproj == 1) {
            // boost everything to decaying particle frame
            LorentzTransform boost1 = LorentzTransform::mkFrameTransformFromBeta(
                in.decaying()[ix].momentum().betaVec());
            FourMomentum pout = boost1.transform(out.momentum());
            FourMomentum pe = boost1.transform(em.momentum());
            qq = boost1.transform(qq);
            Vector3 axis1 = pout.p3().unit();
            LorentzTransform boost2 = LorentzTransform::mkFrameTransformFromBeta(qq.betaVec());
            pe = boost2.transform(pe);
            _h[1][imode]->fill(pe.p3().unit().dot(axis1));
          }
        }
        ++iproj;
      }
    }


    /// Normalise histograms etc., after the run
    void finalize() {
      for (unsigned int ix = 0; ix < 2; ++ix)
        for (unsigned int iy = 0; iy < 4; ++iy) normalize(_h[ix][iy], 1., false);
    }

    /// @}


    /// @name Histograms
    /// @{
    Histo1DPtr _h[2][4];
    /// @}
  };


  RIVET_DECLARE_PLUGIN(BESIII_2017_I1509920);

}