Rivet analyses

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

namespace Rivet {


  /// @brief Jpsi -> gamma gamma phi
  class BESIII_2018_I1646687 : public Analysis {
  public:

    /// Constructor
    RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2018_I1646687);


    /// @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::PHI);
      declare(PSI, "PSI");
      declare(Beam(), "Beams");
      book(_h_mass, 1, 1, 1);
      for (unsigned int ix = 0; ix < 2; ++ix) {
        book(_h_angle[ix], 2, 1, 1 + ix);
      }
    }


    /// 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 = {{22, 2}, {333, 1}};
      DecayedParticles PSI = apply<DecayedParticles>(event, "PSI");
      if (PSI.decaying().size() != 1) vetoEvent;
      if (!PSI.modeMatches(0, 3, mode)) vetoEvent;
      // particles
      const Particle& phi = PSI.decayProducts()[0].at(333)[0];
      const Particles& gam = PSI.decayProducts()[0].at(22);
      for (unsigned int ix = 0; ix < 2; ++ix) {
        double mPhiGamma = (gam[ix].momentum() + phi.momentum()).mass();
        _h_mass->fill(mPhiGamma);
        double cTheta = axis.dot(gam[ix == 0 ? 1 : 0].p3().unit());
        if (mPhiGamma > 1.4 && mPhiGamma < 1.6)
          _h_angle[0]->fill(cTheta);
        else if (mPhiGamma > 1.75 && mPhiGamma < 1.9)
          _h_angle[1]->fill(cTheta);
      }
    }


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

    /// @}


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


  RIVET_DECLARE_PLUGIN(BESIII_2018_I1646687);

}