Rivet analyses

title: BESIII_2017_I1509920

$\psi(2S)\to e^+e^-\chi_{c(1,2)}$ and $\chi_{c(1,2)}\to e^+e^- J/\psi$ decays

Experiment: BESIII (BEPC)

Inspire ID: 1509920

Status: VALIDATED NOHEPDATA

Authors: - Peter Richardson

References: - Phys.Rev.Lett. 118 (2017) 22, 221802

Beams: e- e+

Beam energies: (1.8, 1.8)GeV

Run details: - e+e- > psi 2s.

Measurement of the $e^+e^-$ mass distribution and the helicity angle in the decays $\psi(2S)\to e^+e^-\chi_{c(1,2)}$ and $\chi_{c(1,2)}\to e^+e^- J/\psi$ decays. Data was read from the plots in the paper and the mass distributions are not corrected, although the angle distributions are

Source code:BESIII_2017_I1509920.cc

```c++ // -- C++ --

include "Rivet/Analysis.hh"

include "Rivet/Projections/Beam.hh"

include "Rivet/Projections/DecayedParticles.hh"

include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {

/// @brief psi(2S) -> e+e- chi_c and chi_c -> e+e- Jpsi$ class BESIII_2017_I1509920 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2017_I1509920);


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

/// Book histograms and initialise projections before the run
void init() {
  // Initialise and register projections
  declare(Beam(), "Beams");
  UnstableParticles ufs_psi(Cuts::abspid == PID::PSI2S);
  DecayedParticles psi(ufs_psi);
  psi.addStable(20443);
  psi.addStable(445);
  declare(psi, "PSI");
  UnstableParticles ufs_chi(Cuts::abspid == 445 or Cuts::abspid == 20443);
  DecayedParticles chi(ufs_chi);
  chi.addStable(PID::JPSI);
  declare(chi, "CHI");

  // Histograms
  for (unsigned int ix = 0; ix < 2; ++ix)
    for (unsigned int iy = 0; iy < 4; ++iy) book(_h[ix][iy], 1 + ix, 1, 1 + iy);
}


/// 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();
  // decaying particles
  static const map<PdgId, unsigned int>& mode1 = {{20443, 1}, {11, 1}, {-11, 1}};
  static const map<PdgId, unsigned int>& mode2 = {{445, 1}, {11, 1}, {-11, 1}};
  static const map<PdgId, unsigned int>& mode3 = {{443, 1}, {11, 1}, {-11, 1}};
  unsigned int iproj = 0;
  for (const DecayedParticles& in :
       {apply<DecayedParticles>(event, "PSI"), apply<DecayedParticles>(event, "CHI")}) {
    for (unsigned int ix = 0; ix < in.decaying().size(); ++ix) {
      int imode = -1, ichi = 0;
      if (iproj == 0) {
        if (in.modeMatches(ix, 3, mode1)) {
          imode = 0;
          ichi = 20443;
        }
        else if (in.modeMatches(ix, 3, mode2)) {
          imode = 1;
          ichi = 445;
        }
        else
          continue;
      }
      else {
        if (in.modeMatches(ix, 3, mode3)) {
          imode = in.decaying()[ix].pid() == 20443 ? 2 : 3;
          ichi = 443;
        }
        else
          continue;
      }
      // extract particles
      const Particle& em = in.decayProducts()[ix].at(11)[0];
      const Particle& ep = in.decayProducts()[ix].at(-11)[0];
      const Particle& out = in.decayProducts()[ix].at(ichi)[0];
      FourMomentum qq = ep.momentum() + em.momentum();
      double q = qq.mass();
      _h[0][imode]->fill(q);
      if (iproj == 0) {
        _h[1][imode]->fill(axis.dot((em.momentum() + ep.momentum()).p3().unit()));
      }
      else if (iproj == 1) {
        // boost everything to decaying particle frame
        LorentzTransform boost1 = LorentzTransform::mkFrameTransformFromBeta(
            in.decaying()[ix].momentum().betaVec());
        FourMomentum pout = boost1.transform(out.momentum());
        FourMomentum pe = boost1.transform(em.momentum());
        qq = boost1.transform(qq);
        Vector3 axis1 = pout.p3().unit();
        LorentzTransform boost2 = LorentzTransform::mkFrameTransformFromBeta(qq.betaVec());
        pe = boost2.transform(pe);
        _h[1][imode]->fill(pe.p3().unit().dot(axis1));
      }
    }
    ++iproj;
  }
}


/// Normalise histograms etc., after the run
void finalize() {
  for (unsigned int ix = 0; ix < 2; ++ix)
    for (unsigned int iy = 0; iy < 4; ++iy) normalize(_h[ix][iy], 1., false);
}

/// @}


/// @name Histograms
/// @{
Histo1DPtr _h[2][4];
/// @}

};

RIVET_DECLARE_PLUGIN(BESIII_2017_I1509920);

} ```