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 psi(2S) -> J/psi gamma gamma
  class BESIII_2012_I1097066 : public Analysis {
  public:

    /// Constructor
    RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2012_I1097066);


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

    /// Book histograms and initialise projections before the run
    void init() {
      // Initialise and register projections
      UnstableParticles ufs = UnstableParticles(Cuts::abspid == 100443);
      declare(ufs, "UFS");
      DecayedParticles PSI(ufs);
      PSI.addStable(PID::JPSI);
      declare(PSI, "PSI");
      declare(Beam(), "Beams");
      // book histograms
      for (unsigned int ix = 0; ix < 2; ++ix) book(_h[ix], 1, 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 = {{443, 1}, {22, 2}};
      DecayedParticles PSI = apply<DecayedParticles>(event, "PSI");
      if (PSI.decaying().size() != 1) vetoEvent;
      if (!PSI.modeMatches(0, 3, mode)) vetoEvent;
      if (PSI.decaying()[0].children().size() != 3) vetoEvent;
      // particles
      const Particle& JPsi = PSI.decayProducts()[0].at(443)[0];
      const Particles& gam = PSI.decayProducts()[0].at(22);
      Vector3 norm = gam[0].p3().cross(gam[1].p3()).unit();
      _h[0]->fill(abs(axis.dot(norm)));
      if (JPsi.children().size() != 2) vetoEvent;
      if (JPsi.children()[0].pid() != -JPsi.children()[1].pid()) vetoEvent;
      if (JPsi.children()[0].abspid() != PID::EMINUS && JPsi.children()[0].abspid() != PID::MUON) vetoEvent;
      Particle lm = JPsi.children()[0];
      Particle lp = JPsi.children()[1];
      if (lm.pid() < 0) swap(lm, lp);
      Vector3 axis2 = JPsi.momentum().p3().unit();
      LorentzTransform boost = LorentzTransform::mkFrameTransformFromBeta(JPsi.momentum().betaVec());
      Vector3 axis3 = boost.transform(lm.momentum()).p3().unit();
      _h[1]->fill(abs(axis2.dot(axis3)));
    }


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

    /// @}


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


  RIVET_DECLARE_PLUGIN(BESIII_2012_I1097066);

}