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 gamma gamma -> X(3915) -> J/psi omega
  class BABAR_2012_I1122034 : public Analysis {
  public:

    /// Constructor
    RIVET_DEFAULT_ANALYSIS_CTOR(BABAR_2012_I1122034);


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

    /// Book histograms and initialise projections before the run
    void init() {
      // Initialise and register projections
      declare(Beam(), "Beams");
      declare(FinalState(), "FS");
      declare(UnstableParticles(Cuts::pid == 223 or Cuts::pid == 443), "UFS");
      // histograms
      book(_h_mass, 1, 1, 1);
      for (unsigned int ix = 0; ix < 4; ++ix) {
        book(_h_angle1[ix], 2, 1, 1 + ix);
        if (ix < 3) book(_h_angle2[ix], 3, 1, 1 + ix);
      }
    }

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

    bool findScattered(Particle beam, double& q2) {
      bool found = false;
      Particle scat = beam;
      while (!scat.children().empty()) {
        found = false;
        for (const Particle& p : scat.children()) {
          if (p.pid() == scat.pid()) {
            scat = p;
            found = true;
            break;
          }
        }
        if (!found) break;
      }
      if (!found) return false;
      q2 = -(beam.mom() - scat.mom()).mass2();
      return true;
    }

    void findChildren(const Particle& p,
                      Particles& pim,
                      Particles& pip,
                      Particles& pi0,
                      unsigned int& ncount) {
      for (const Particle& child : p.children()) {
        if (child.pid() == PID::PIPLUS) {
          pip.push_back(child);
          ncount += 1;
        }
        else if (child.pid() == PID::PIMINUS) {
          pim.push_back(child);
          ncount += 1;
        }
        else if (child.pid() == PID::PI0) {
          pi0.push_back(child);
          ncount += 1;
        }
        else if (child.children().empty()) {
          ncount += 1;
        }
        else {
          findChildren(child, pim, pip, pi0, ncount);
        }
      }
    }

    /// Perform the per-event analysis
    void analyze(const Event& event) {
      // find scattered leptons and calc Q2
      const Beam& beams = apply<Beam>(event, "Beams");
      double q12 = -1, q22 = -1;
      if (!findScattered(beams.beams().first, q12)) vetoEvent;
      if (!findScattered(beams.beams().second, q22)) vetoEvent;
      // check the final state
      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;
      }
      // find the J/psi
      const FinalState& ufs = apply<FinalState>(event, "UFS");
      Particle omega, psi;
      bool found = false;
      for (const Particle& p1 : ufs.particles(Cuts::pid == 443)) {
        if (p1.children().empty()) continue;
        map<long, int> nRes = nCount;
        int ncount = ntotal;
        findChildren(p1, nRes, ntotal);
        for (const Particle& p2 : ufs.particles(Cuts::pid == 223)) {
          if (p2.children().empty()) continue;
          map<long, int> nRes2 = nRes;
          int ncount2 = ncount;
          findChildren(p2, nRes2, ncount2);
          found = true;
          for (const auto& val : nRes2) {
            if (abs(val.first) == 11) {
              if (val.second != 1) {
                found = false;
                break;
              }
            }
            else if (val.second != 0) {
              found = false;
              break;
            }
          }
          if (found) {
            psi = p1;
            omega = p2;
            break;
          }
        }
      }
      if (!found) vetoEvent;
      FourMomentum psum = omega.mom() + psi.mom();
      if (psum.pT() > 0.2) vetoEvent;
      // mass distribution
      _h_mass->fill(psum.mass());
      // from now on we need specific decay modes of J/psi and omega
      // first J/psi -> l+l-
      if (psi.children().size() != 2) vetoEvent;
      if (psi.children()[0].pid() != -psi.children()[1].pid()) vetoEvent;
      if (psi.children()[0].abspid() != 11 && psi.children()[0].abspid() != 13) vetoEvent;
      Particle ep = psi.children()[0];
      Particle em = psi.children()[1];
      if (ep.pid() > 0) swap(ep, em);
      // omega decay
      unsigned int ncount = 0;
      Particles pip, pim, pi0;
      findChildren(omega, pim, pip, pi0, ncount);
      if (ncount != 3 || !(pim.size() == 1 && pip.size() == 1 && pi0.size() == 1)) vetoEvent;
      // boost to gamma gamma frame
      LorentzTransform boostCMS = LorentzTransform::mkFrameTransformFromBeta(psum.betaVec());
      FourMomentum pPsi = boostCMS.transform(psi.mom());
      FourMomentum pOmega = boostCMS.transform(omega.mom());
      FourMomentum pLp = boostCMS.transform(ep.mom());
      FourMomentum pPip = boostCMS.transform(pip[0].mom());
      FourMomentum pPim = boostCMS.transform(pim[0].mom());
      Vector3 axis(0., 0., 1.);
      // lepton angle
      double cosL = pLp.p3().unit().dot(axis);
      _h_angle1[0]->fill(cosL);
      LorentzTransform boostOmega = LorentzTransform::mkFrameTransformFromBeta(pOmega.betaVec());
      pPip = boostOmega.transform(pPip);
      pPim = boostOmega.transform(pPim);
      // omega decay plane normal angle
      Vector3 axisOmega = pPip.p3().cross(pPim.p3()).unit();
      double cosN = axisOmega.dot(axis);
      _h_angle1[1]->fill(cosN);
      // angle lepton and omega
      _h_angle1[2]->fill(pLp.p3().unit().dot(axisOmega));
      // helicity angle
      _h_angle1[3]->fill(pPsi.p3().unit().dot(psum.p3().unit()));
      // now the new frame
      Vector3 axisZ = pOmega.p3().unit();
      Vector3 axisY = axisZ.cross(axisOmega);
      Vector3 axisX = axisY.cross(axisZ);
      // second set of angles
      LorentzTransform boostPsi = LorentzTransform::mkFrameTransformFromBeta(pPsi.betaVec());
      Vector3 axisL = boostPsi.transform(pLp).p3().unit();
      _h_angle2[0]->fill(axisZ.dot(axisOmega));
      _h_angle2[1]->fill(axisL.dot(pPsi.p3().unit()));
      axisZ *= -1.;
      axisX *= -1.;
      Vector3 axisnp = axisL.cross(axisZ).unit();
      double phiL = atan2(axisnp.dot(axisY), axisnp.dot(axisX));
      double phiN = atan2(axisOmega.dot(axisY), axisOmega.dot(axisX));
      _h_angle2[2]->fill(mapAngleMPiToPi(phiL - phiN) / M_PI * 180.);
    }


    /// Normalise histograms etc., after the run
    void finalize() {
      normalize(_h_mass, 1.0, false);
      normalize(_h_angle1, 1.0, false);
      normalize(_h_angle2, 1.0, false);
    }

    /// @}


    /// @name Histograms
    /// @{
    Histo1DPtr _h_mass, _h_angle1[4], _h_angle2[3];
    /// @}
  };


  RIVET_DECLARE_PLUGIN(BABAR_2012_I1122034);

}