Rivet analyses

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

namespace Rivet {


  /// @brief JPsi > Lambda, Lambdabar with Lambda -> n gamma
  class BESIII_2022_I2099126 : public Analysis {
  public:

    /// Constructor
    RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2022_I2099126);


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

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

      // Initialise and register projections
      declare(Beam(), "Beams");
      declare(UnstableParticles(), "UFS");
      declare(FinalState(), "FS");
      for (unsigned int ix = 0; ix < 2; ++ix) {
        book(_n[ix], "TMP/n_" + toString(ix + 1));
        book(_t[ix], "TMP/t_" + toString(ix + 1));
        for (unsigned int iy = 0; iy < 2; ++iy) {
          book(_h_mu[ix][iy], 1, 1, 2 * ix + iy + 1);
        }
      }
      book(_n[2], "TMP/n_3");
      book(_t[2], "TMP/t_3");
    }

    void findChildren(const Particle& p, map<long, int>& nRes, int& ncount) {
      for (const Particle& child : p.children()) {
        if (child.children().empty()) {
          nRes[child.pid()] -= 1;
          --ncount;
        }
        else
          findChildren(child, nRes, ncount);
      }
    }

    /// 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();
      // types of final state particles
      const FinalState& fs = apply<FinalState>(event, "FS");
      map<long, int> nCount;
      int ntotal(0);
      for (const Particle& p : fs.particles()) {
        nCount[p.pid()] += 1;
        ++ntotal;
      }
      // loop over lambda0 baryons
      const UnstableParticles& ufs = apply<UnstableParticles>(event, "UFS");
      Particle Lambda, LamBar;
      bool matched(false);
      for (const Particle& p : ufs.particles(Cuts::abspid == 3122)) {
        if (p.children().empty()) continue;
        map<long, int> nRes = nCount;
        int ncount = ntotal;
        findChildren(p, nRes, ncount);
        matched = false;
        // check for antiparticle
        for (const Particle& p2 : ufs.particles(Cuts::pid == -p.pid())) {
          if (p2.children().empty()) continue;
          map<long, int> nRes2 = nRes;
          int ncount2 = ncount;
          findChildren(p2, nRes2, ncount2);
          if (ncount2 == 0) {
            matched = true;
            for (const auto& val : nRes2) {
              if (val.second != 0) {
                matched = false;
                break;
              }
            }
            // found baryon and antibaryon
            if (matched) {
              if (p.pid() > 0) {
                Lambda = p;
                LamBar = p2;
              }
              else {
                Lambda = p2;
                LamBar = p;
              }
              break;
            }
          }
        }
        if (matched) break;
      }
      if (!matched) vetoEvent;
      // check the Lambda decay mode
      bool radiative[2] = {false, false};
      // identifyt Lambda decay
      Particle baryon1;
      if ((Lambda.children()[0].pid() == PID::PROTON && Lambda.children()[1].pid() == PID::PIMINUS)) {
        radiative[0] = false;
        baryon1 = Lambda.children()[0];
      }
      else if ((Lambda.children()[1].pid() == PID::PROTON && Lambda.children()[0].pid() == PID::PIMINUS)) {
        radiative[0] = false;
        baryon1 = Lambda.children()[1];
      }
      else if ((Lambda.children()[0].pid() == PID::NEUTRON && Lambda.children()[1].pid() == PID::PHOTON)) {
        radiative[0] = true;
        baryon1 = Lambda.children()[0];
      }
      else if ((Lambda.children()[1].pid() == PID::NEUTRON && Lambda.children()[0].pid() == PID::PHOTON)) {
        radiative[0] = true;
        baryon1 = Lambda.children()[1];
      }
      else
        vetoEvent;
      Particle baryon2;
      if ((LamBar.children()[0].pid() == PID::ANTIPROTON && LamBar.children()[1].pid() == PID::PIPLUS)) {
        radiative[1] = false;
        baryon2 = LamBar.children()[0];
      }
      else if ((LamBar.children()[1].pid() == PID::ANTIPROTON && LamBar.children()[0].pid() == PID::PIPLUS)) {
        radiative[1] = false;
        baryon2 = LamBar.children()[1];
      }
      else if ((LamBar.children()[0].pid() == PID::ANTINEUTRON
                && LamBar.children()[1].pid() == PID::PHOTON)) {
        radiative[1] = true;
        baryon2 = LamBar.children()[0];
      }
      else if ((LamBar.children()[1].pid() == PID::ANTINEUTRON
                && LamBar.children()[0].pid() == PID::PHOTON)) {
        radiative[1] = true;
        baryon2 = LamBar.children()[1];
      }
      else
        vetoEvent;
      if (radiative[0] == radiative[1]) vetoEvent;
      // boost to the Lambda rest frame
      LorentzTransform boost1 = LorentzTransform::mkFrameTransformFromBeta(Lambda.momentum().betaVec());
      Vector3 e1z = Lambda.momentum().p3().unit();
      Vector3 e1y = e1z.cross(axis).unit();
      Vector3 e1x = e1y.cross(e1z).unit();
      Vector3 axis1 = boost1.transform(baryon1.momentum()).p3().unit();
      double n1x(e1x.dot(axis1)), n1y(e1y.dot(axis1)), n1z(e1z.dot(axis1));
      // boost to the Lambda bar
      LorentzTransform boost2 = LorentzTransform::mkFrameTransformFromBeta(LamBar.momentum().betaVec());
      Vector3 axis2 = boost2.transform(baryon2.momentum()).p3().unit();
      double n2x(e1x.dot(axis2)), n2y(e1y.dot(axis2)), n2z(e1z.dot(axis2));
      double cosL = axis.dot(Lambda.momentum().p3().unit());
      double sinL = sqrt(1. - sqr(cosL));
      double T1 = sqr(sinL) * n1x * n2x + sqr(cosL) * n1z * n2z;
      // lambda -> n gamma
      if (radiative[0]) {
        _h_mu[0][0]->fill(cosL, n2y);
        _h_mu[0][1]->fill(cosL, n1y);
        _n[0]->fill();
        _n[2]->fill();
        _t[0]->fill(T1);
        _t[2]->fill(T1);
      }
      // lambdabar -> nbar gamma
      else {
        _h_mu[1][0]->fill(cosL, n1y);
        _h_mu[1][1]->fill(cosL, n2y);
        _n[1]->fill();
        _n[2]->fill();
        _t[1]->fill(T1);
        _t[2]->fill(T1);
      }
    }


    /// Normalise histograms etc., after the run
    void finalize() {
      // values of constants
      double aPsi = 0.461;
      double aPlus = -0.758;
      double factor = 45. * (3. + aPsi) / (11. + 5. * aPsi) / aPlus;
      // plots
      for (unsigned int ix = 0; ix < 2; ++ix) {
        for (unsigned int iy = 0; iy < 2; ++iy) {
          scale(_h_mu[ix][iy], 10. * 0.2 / *_n[ix]);
        }
      }
      // alpha from the moments
      for (unsigned int ix = 0; ix < 3; ++ix) {
        double value = _t[ix]->val() / _n[ix]->val();
        double error = _t[ix]->err() / _n[ix]->val();
        value *= factor;
        error *= abs(factor);
        if (ix == 1) value *= -1.;
        Estimate0DPtr alpha;
        book(alpha, 2, 1, 1 + ix);
        alpha->set(value, error);
      }
    }

    /// @}


    /// @name Histograms
    /// @{
    Histo1DPtr _h_mu[2][2];
    CounterPtr _n[3];
    Histo1DPtr _h_ctheta[3];
    CounterPtr _t[3];
    /// @}
  };


  RIVET_DECLARE_PLUGIN(BESIII_2022_I2099126);

}