Rivet analyses

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

namespace Rivet {


  /// @brief psi(2S) -> J/Psi pi+pi-
  class BES_1999_I507637 : public Analysis {
  public:

    /// Constructor
    RIVET_DEFAULT_ANALYSIS_CTOR(BES_1999_I507637);


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

    /// Book histograms and initialise projections before the run
    void init() {
      // Initialise and register projections
      declare(UnstableParticles(), "UFS");
      book(_h_mpipi, 1, 1, 1);
      book(_h_cosl, 1, 1, 2);
      book(_h_cosX, 1, 1, 3);
      book(_h_cospi, 1, 1, 4);
    }

    void findDecayProducts(const Particle& mother,
                           unsigned int& nstable,
                           Particles& pip,
                           Particles& pim,
                           Particles& onium) {
      for (const Particle& p : mother.children()) {
        int id = p.pid();
        if (id == PID::PIMINUS) {
          pim.push_back(p);
          ++nstable;
        }
        else if (id == PID::PIPLUS) {
          pip.push_back(p);
          ++nstable;
        }
        else if (id == 443) {
          onium.push_back(p);
          ++nstable;
        }
        else if (!p.children().empty()) {
          findDecayProducts(p, nstable, pip, pim, onium);
        }
        else
          ++nstable;
      }
    }

    void findLeptons(const Particle& mother, unsigned int& nstable, Particles& lp, Particles& lm) {
      for (const Particle& p : mother.children()) {
        int id = p.pid();
        if (id == 11 || id == 13) {
          lm.push_back(p);
          ++nstable;
        }
        else if (id == -11 || id == -13) {
          lp.push_back(p);
          ++nstable;
        }
        else if (!p.children().empty()) {
          findLeptons(p, nstable, lp, lm);
        }
        else
          ++nstable;
      }
    }

    /// Perform the per-event analysis
    void analyze(const Event& event) {
      // loop over unstable particles
      for (const Particle& psi : apply<UnstableParticles>(event, "UFS").particles(Cuts::pid == 100443)) {
        unsigned int nstable(0);
        Particles pip, pim, onium;
        findDecayProducts(psi, nstable, pip, pim, onium);
        // check for onium
        if (onium.size() != 1 || nstable != 3 || pip.size() != 1 || pim.size() != 1) continue;
        FourMomentum q = pip[0].momentum() + pim[0].momentum();
        _h_mpipi->fill(q.mass());
        _h_cosX->fill(cos(q.polarAngle()));
        // leptons from J/psi decay
        nstable = 0;
        Particles lp, lm;
        findLeptons(onium[0], nstable, lp, lm);
        if (nstable == 2 && lp.size() == 1 && lm.size() == 1) {
          LorentzTransform boost = LorentzTransform::mkFrameTransformFromBeta(onium[0].momentum().betaVec());
          FourMomentum pl = boost.transform(lp[0].momentum());
          _h_cosl->fill(cos(pl.polarAngle()));
        }
        // pions in rest frame
        LorentzTransform boost = LorentzTransform::mkFrameTransformFromBeta(q.betaVec());
        FourMomentum ppi = boost.transform(pip[0].momentum());
        Vector3 axis1 = q.p3().unit();
        _h_cospi->fill(axis1.dot(ppi.p3().unit()));
      }
    }


    /// Normalise histograms etc., after the run
    void finalize() {
      normalize(_h_mpipi);
      normalize(_h_cosl, 1., false);
      normalize(_h_cosX);
      normalize(_h_cospi);
    }

    /// @}


    /// @name Histograms
    /// @{
    Histo1DPtr _h_mpipi, _h_cosl, _h_cosX, _h_cospi;
    /// @}
  };


  RIVET_DECLARE_PLUGIN(BES_1999_I507637);

}