Rivet analyses

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

namespace Rivet {


  /// @brief psi(2S) -> gamma chi_c1,2
  class BESIII_2017_I1624548 : public Analysis {
  public:

    /// Constructor
    RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2017_I1624548);


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

    /// Book histograms and initialise projections before the run
    void init() {
      // Initialise and register projections
      declare(Beam(), "Beams");
      declare(UnstableParticles(Cuts::pid==445), "UFS");
      declare(FinalState(), "FS");
      for(unsigned int ix=0;ix<3;++ix)
    book(_h[ix],1,1,1+ix);
    }

    void findChildren(const Particle & p,map<long,int> & nRes, int &ncount) {
      for( const Particle &child : p.children()) {
    if(child.children().empty()) {
      nRes[child.pid()]-=1;
      --ncount;
    }
    else
      findChildren(child,nRes,ncount);
      }
    }

    /// 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();
      // types of final state particles
      const FinalState& fs = apply<FinalState>(event, "FS");
      map<long,int> nCount;
      int ntotal(0);
      for (const Particle& p :  fs.particles()) {
    nCount[p.pid()] += 1;
    ++ntotal;
      }
      // loop over chi_c states
      Particle chi;
      bool matched = false;
      const UnstableParticles & ufs = apply<UnstableParticles>(event, "UFS");
      for (const Particle& p :  ufs.particles()) {
        if(p.children().empty()) continue;
        map<long,int> nRes=nCount;
        int ncount = ntotal;
        findChildren(p,nRes,ncount);
    if(ncount==1) {
      matched = true;
      for(auto const & val : nRes) {
        if(val.first==PID::PHOTON) {
          if(val.second!=1) {
          matched = false;
          break;
          }
        }
        else if(val.second!=0) {
          matched = false;
          break;
        }
      }
      if(matched) {
        chi=p;
        break;
      }
    }
      }
      if(!matched) vetoEvent;
      // have chi_c find psi2S 
      if(chi.parents().empty() || chi.children().size()!=2) vetoEvent;
      Particle psi2S = chi.parents()[0];
      if(psi2S.pid()!=100443 || psi2S.children().size()!=2) vetoEvent;
      // then the first photon
      Particle gamma1;
      if(psi2S.children()[0].pid()==PID::PHOTON)
    gamma1 = psi2S.children()[0];
      else if(psi2S.children()[1].pid()==PID::PHOTON)
    gamma1 = psi2S.children()[1];
      else
    vetoEvent;
      // and second photon
      Particle gamma2;
      if(chi.children()[0].pid()==PID::PHOTON &&
     chi.children()[1].pid()==PID::PHOTON) {
    gamma2 = chi.children()[0];
      }
      else
    vetoEvent;
      // first angle of gamma1 w.r.t beam
      _h[0]->fill(axis.dot(gamma1.momentum().p3().unit()));
      // axis in the chi frame
      LorentzTransform boost1 = LorentzTransform::mkFrameTransformFromBeta(chi.momentum().betaVec());
      Vector3 e1z = gamma1.momentum().p3().unit();
      Vector3 e1y = e1z.cross(axis).unit();
      Vector3 e1x = e1y.cross(e1z).unit();
      // cos theta_2 and phi 2 distributions
      FourMomentum pGamma2 = boost1.transform(gamma2.momentum());
      Vector3 axis1 = pGamma2.p3().unit();
      _h[1]->fill(e1z.dot(axis1));
      double phi2 = atan2(e1y.dot(axis1),e1x.dot(axis1));
      if(phi2<0) phi2+=2.*M_PI;
      _h[2]->fill(phi2);
    }


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

    /// @}


    /// @name Histograms
    /// @{
    Histo1DPtr _h[3];
    /// @}


  };


  RIVET_DECLARE_PLUGIN(BESIII_2017_I1624548);

}