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 J/psi -> gamma eta eta
  class BESIII_2013_I1209121 : public Analysis {
  public:

    /// Constructor
    RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2013_I1209121);


    /// @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::ETA);
      declare(PSI, "PSI");
      declare(Beam(), "Beams");
      // book histograms
      for (unsigned int ix = 0; ix < 4; ++ix) book(_h[ix], 1, 1, 1 + ix);
    }

    // angle cuts due regions of BES calorimeter
    bool vetoPhoton(const double& cTheta) {
      return cTheta > 0.92 || (cTheta > 0.8 && cTheta < 0.86);
    }

    /// 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 = {{221, 2}, {22, 1}};
      DecayedParticles PSI = apply<DecayedParticles>(event, "PSI");
      if (PSI.decaying().size() != 1) vetoEvent;
      if (!PSI.modeMatches(0, 3, mode)) vetoEvent;
      // particles
      const Particles& eta = PSI.decayProducts()[0].at(221);
      const Particle& gam = PSI.decayProducts()[0].at(22)[0];
      _h[0]->fill((eta[0].momentum() + eta[1].momentum()).mass());
      double cTheta = axis.dot(gam.p3().unit());
      if (vetoPhoton(abs(cTheta))) vetoEvent;
      _h[1]->fill(cTheta);
      // remaining angles
      LorentzTransform boost1 = LorentzTransform::mkFrameTransformFromBeta(
          PSI.decaying()[0].momentum().betaVec());
      FourMomentum pGamma = boost1.transform(gam.momentum());
      FourMomentum pEtaEta = boost1.transform(eta[0].momentum() + eta[1].momentum());
      Vector3 e1z = pGamma.p3().unit();
      Vector3 e1y = e1z.cross(axis).unit();
      Vector3 e1x = e1y.cross(e1z).unit();
      LorentzTransform boost2 = LorentzTransform::mkFrameTransformFromBeta(pEtaEta.betaVec());
      Vector3 axis2 = boost2.transform(boost1.transform(eta[0].momentum())).p3().unit();
      _h[2]->fill(e1z.dot(axis2));
      _h[3]->fill(atan2(e1y.dot(axis2), e1x.dot(axis2)));
    }


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

    /// @}


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


  RIVET_DECLARE_PLUGIN(BESIII_2013_I1209121);

}