Rivet analyses

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

namespace Rivet {


  /// @brief D0 -> Kbar0 pi- e+ nu_e
  class BESIII_2018_I1705754 : public Analysis {
  public:

    /// Constructor
    RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2018_I1705754);


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

    /// Book histograms and initialise projections before the run
    void init() {

      // Initialise and register projections
      UnstableParticles ufs = UnstableParticles(Cuts::pid == 421);
      declare(ufs, "UFS");
      DecayedParticles D0(ufs);
      D0.addStable(PID::PI0);
      D0.addStable(PID::K0S);
      D0.addStable(PID::ETA);
      D0.addStable(PID::ETAPRIME);
      declare(D0, "D0");

      // Book histograms
      for (unsigned int ix = 0; ix < 5; ++ix) book(_h[ix], 1, 1, 1 + ix);
    }


    /// Perform the per-event analysis
    void analyze(const Event& event) {
      static const map<PdgId, unsigned int>& mode1 = {{310, 1}, {-211, 1}, {-11, 1}, {12, 1}};
      static const map<PdgId, unsigned int>& mode2 = {{130, 1}, {-211, 1}, {-11, 1}, {12, 1}};
      static const map<PdgId, unsigned int>& mode3 = {{-311, 1}, {-211, 1}, {-11, 1}, {12, 1}};
      DecayedParticles D0 = apply<DecayedParticles>(event, "D0");
      // loop over particles
      for (unsigned int ix = 0; ix < D0.decaying().size(); ++ix) {
        Particle K0;
        if (D0.modeMatches(ix, 4, mode1))
          K0 = D0.decayProducts()[ix].at(310)[0];
        else if (D0.modeMatches(ix, 4, mode2))
          K0 = D0.decayProducts()[ix].at(130)[0];
        else if (D0.modeMatches(ix, 4, mode3))
          K0 = D0.decayProducts()[ix].at(-311)[0];
        else
          continue;
        const Particle& pim = D0.decayProducts()[ix].at(-211)[0];
        const Particle& ep = D0.decayProducts()[ix].at(-11)[0];
        const Particle& nue = D0.decayProducts()[ix].at(12)[0];
        FourMomentum pKstar = K0.momentum() + pim.momentum();
        _h[0]->fill(pKstar.mass());
        FourMomentum qq = D0.decaying()[ix].momentum() - pKstar;
        _h[1]->fill(qq.mass2());
        // boost momenta to D0 rest frame
        LorentzTransform boost = LorentzTransform::mkFrameTransformFromBeta(
            D0.decaying()[ix].momentum().betaVec());
        FourMomentum pKS = boost.transform(pKstar);
        Matrix3 ptoz(-pKS.p3().unit(), Vector3(0, 0, 1));
        boost.preMult(ptoz);
        // the momenta in frane to W along z
        FourMomentum pD = boost.transform(D0.decaying()[ix].momentum());
        FourMomentum pK = boost.transform(K0.momentum());
        FourMomentum ppi = boost.transform(pim.momentum());
        FourMomentum pe = boost.transform(ep.momentum());
        FourMomentum pnu = boost.transform(nue.momentum());
        pKstar = pK + ppi;
        qq = pD - pKstar;
        LorentzTransform boostK = LorentzTransform::mkFrameTransformFromBeta(pKstar.betaVec());
        Vector3 axisK = boostK.transform(pK).p3().unit();
        _h[3]->fill(axisK.dot(pKstar.p3().unit()));
        LorentzTransform boostW = LorentzTransform::mkFrameTransformFromBeta(qq.betaVec());
        Vector3 axisE = boostW.transform(pe).p3().unit();
        _h[2]->fill(axisE.dot(qq.p3().unit()));
        axisK.setZ(0.);
        axisE.setZ(0.);
        double chi = atan2(axisE.cross(axisK).dot(qq.p3().unit()), axisE.dot(axisK));
        _h[4]->fill(chi);
      }
    }


    /// Normalise histograms etc., after the run
    void finalize() {
      for (unsigned int ix = 0; ix < 5; ++ix) normalize(_h[ix]);
    }

    /// @}


    /// @name Histograms
    /// @{
    Histo1DPtr _h[5];
    /// @}
  };


  RIVET_DECLARE_PLUGIN(BESIII_2018_I1705754);

}