1// -*- C++ -*-
  2#include "Rivet/Analysis.hh"
  3#include "Rivet/Projections/Beam.hh"
  4#include "Rivet/Projections/UnstableParticles.hh"
  5#include "Rivet/Projections/DecayedParticles.hh"
  6
  7namespace Rivet {
  8
  9
 10  /// @brief J/psi -> gamma KS0 KS0
 11  class BESIII_2018_I1689296 : public Analysis {
 12  public:
 13
 14    /// Constructor
 15    RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2018_I1689296);
 16
 17
 18    /// @name Analysis methods
 19    /// @{
 20
 21    /// Book histograms and initialise projections before the run
 22    void init() {
 23      // Initialise and register projections
 24      UnstableParticles ufs = UnstableParticles(Cuts::abspid==443);
 25      declare(ufs, "UFS");
 26      DecayedParticles PSI(ufs);
 27      PSI.addStable(PID::K0S);
 28      declare(PSI, "PSI");
 29      declare(Beam(), "Beams");
 30      // hisotgrams
 31      for(unsigned int ix=0;ix<3;++ix) {
 32	if(ix<2) book(_h_mass[ix],1,1,1+ix);
 33	book(_h_angle[ix],2,1,1+ix);
 34      }
 35    }
 36
 37    // angle cuts due regions of BES calorimeter
 38    bool vetoPhoton(const double & cTheta) {
 39      return cTheta>0.92 || (cTheta>0.8 && cTheta<0.86);
 40    }
 41
 42    /// Perform the per-event analysis
 43    void analyze(const Event& event) {
 44      // get the axis, direction of incoming electron
 45      const ParticlePair& beams = apply<Beam>(event, "Beams").beams();
 46      Vector3 axis;
 47      if(beams.first.pid()>0)
 48	axis = beams.first .momentum().p3().unit();
 49      else
 50	axis = beams.second.momentum().p3().unit();
 51      // find the J/psi decays
 52      static const map<PdgId,unsigned int> & mode = { { 310,2},{ 22,1}};
 53      DecayedParticles PSI = apply<DecayedParticles>(event, "PSI");
 54      if( PSI.decaying().size()!=1) vetoEvent;
 55      if(!PSI.modeMatches(0,3,mode)) vetoEvent;
 56      // particles
 57      const Particles & K0  = PSI.decayProducts()[0].at(310);
 58      const Particle  & gam = PSI.decayProducts()[0].at( 22)[0];
 59      double mKK = (K0[0].momentum()+K0[1].momentum()).mass();
 60      _h_mass[0]->fill(mKK);
 61      for(unsigned int ix=0;ix<2;++ix)
 62	_h_mass[1]->fill((gam.momentum()+K0[ix].momentum()).mass());
 63      double cTheta = axis.dot(gam.p3().unit());
 64      if(vetoPhoton(abs(cTheta))) vetoEvent;
 65      _h_angle[0]->fill(cTheta);
 66      // remaining angles
 67      LorentzTransform boost1 = LorentzTransform::mkFrameTransformFromBeta(PSI.decaying()[0].momentum().betaVec());
 68      FourMomentum pGamma = boost1.transform(gam.momentum());
 69      FourMomentum pKK    = boost1.transform(K0[0].momentum()+K0[1].momentum());
 70      Vector3 e1z = pGamma.p3().unit();
 71      Vector3 e1y = e1z.cross(axis).unit();
 72      Vector3 e1x = e1y.cross(e1z).unit();
 73      LorentzTransform boost2 = LorentzTransform::mkFrameTransformFromBeta(pKK.betaVec());
 74      Vector3 axis2 = boost2.transform(boost1.transform(K0[0].momentum())).p3().unit();
 75      _h_angle[1]->fill(e1z.dot(axis2));
 76      double phi = atan2(e1y.dot(axis2),e1x.dot(axis2));
 77      _h_angle[2]->fill(phi);
 78    }
 79
 80
 81    /// Normalise histograms etc., after the run
 82    void finalize() {
 83      for(unsigned int ix=0;ix<3;++ix) {
 84	if(ix<2) normalize(_h_mass[ix],1.,false);
 85	normalize(_h_angle[ix],1.,false);
 86      }
 87    }
 88
 89    /// @}
 90
 91
 92    /// @name Histograms
 93    /// @{
 94    Histo1DPtr _h_mass[2], _h_angle[3];
 95    /// @}
 96
 97
 98  };
 99
100
101  RIVET_DECLARE_PLUGIN(BESIII_2018_I1689296);
102
103}