Rivet analyses

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

namespace Rivet {


  /// @brief D0 -> omega phi
  class BESIII_2022_I1900094 : public Analysis {
  public:

    /// Constructor
    RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2022_I1900094);


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

    /// Book histograms and initialise projections before the run
    void init() {
      // Initialise and register projections
      declare(UnstableParticles(Cuts::abspid == PID::D0), "UFS");
      for (unsigned int ix = 0; ix < 2; ++ix) book(_h[ix], 1, 1, 1 + ix);
    }

    void findDecayProducts(const Particle& mother,
                           unsigned int& nstable,
                           Particles& pip,
                           Particles& pim,
                           Particles& pi0) {
      for (const Particle& p : mother.children()) {
        int id = p.pid();
        if (id == PID::KPLUS || id == PID::KMINUS || id == PID::K0S || id == PID::K0L) {
          ++nstable;
        }
        else if (id == PID::PIPLUS) {
          pip.push_back(p);
          ++nstable;
        }
        else if (id == PID::PIMINUS) {
          pim.push_back(p);
          ++nstable;
        }
        else if (id == PID::PI0) {
          pi0.push_back(p);
          ++nstable;
        }
        else if (!p.children().empty()) {
          findDecayProducts(p, nstable, pip, pim, pi0);
        }
        else
          ++nstable;
      }
    }

    /// Perform the per-event analysis
    void analyze(const Event& event) {
      for (const Particle& D0 : apply<UnstableParticles>(event, "UFS").particles()) {
        Particle omega, phi;
        if (D0.children()[0].pid() == PID::OMEGA && D0.children()[1].pid() == PID::PHI) {
          omega = D0.children()[0];
          phi = D0.children()[1];
        }
        else if (D0.children()[1].pid() == PID::OMEGA && D0.children()[0].pid() == PID::PHI) {
          omega = D0.children()[1];
          phi = D0.children()[0];
        }
        else
          continue;
        LorentzTransform boost = LorentzTransform::mkFrameTransformFromBeta(D0.momentum().betaVec());
        // first the phi
        if (phi.children().size() == 2 && phi.children()[0].abspid() == PID::KPLUS
            && phi.children()[1].abspid() == PID::KPLUS) {
          // first boost all relevant momenta to D0 rest frame
          FourMomentum pD0 = boost.transform(D0.momentum());
          FourMomentum pphi = boost.transform(phi.momentum());
          FourMomentum pK = boost.transform(phi.children()[0].momentum());
          LorentzTransform boost2 = LorentzTransform::mkFrameTransformFromBeta(pphi.betaVec());
          Vector3 axis1 = boost2.transform(pD0).p3().unit();
          Vector3 axis2 = boost2.transform(pK).p3().unit();
          _h[1]->fill(abs(axis1.dot(axis2)));
        }
        // then the omega
        unsigned int nstable(0);
        Particles pip, pim, pi0;
        findDecayProducts(omega, nstable, pip, pim, pi0);
        if (nstable == 3 && pip.size() == 1 && pip.size() == 1 && pi0.size() == 1) {
          FourMomentum pD0 = boost.transform(D0.momentum());
          FourMomentum pomega = boost.transform(omega.momentum());
          FourMomentum ppip = boost.transform(pip[0].momentum());
          FourMomentum ppim = boost.transform(pim[0].momentum());
          LorentzTransform boost2 = LorentzTransform::mkFrameTransformFromBeta(pomega.betaVec());
          Vector3 axis = boost2.transform(pD0).p3().unit();
          Vector3 pp = boost2.transform(ppip).p3();
          Vector3 pm = boost2.transform(ppim).p3();
          Vector3 axis2 = pp.cross(pm).unit();
          _h[0]->fill(abs(axis.dot(axis2)));
        }
      }
    }


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

    /// @}


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


  RIVET_DECLARE_PLUGIN(BESIII_2022_I1900094);

}