Rivet analyses

title: BESIII_2018_I1689296

Radiative $J/\psi$ decays to $K^0_SK^0_S$

Experiment: BESIII (BEPC)

Inspire ID: 1689296

Status: VALIDATED NOHEPDATA

Authors: - Peter Richardson

References: - Phys.Rev.D 98 (2018) 7, 072003

Beams: e- e+

Beam energies: (1.6, 1.6)GeV

Run details: - e+e- > J/psi

Measurement of mass and angular distributions in the decay $J/\psi\to\gamma K^0_SK^0S$. Plots were read from the paper and are not corrected for efficiency/acceptance, although the paper states the backgrounds are small.

Source code:BESIII_2018_I1689296.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 J/psi -> gamma KS0 KS0 class BESIII_2018_I1689296 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2018_I1689296);


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

/// Book histograms and initialise projections before the run
void init() {
  // Initialise and register projections
  UnstableParticles ufs = UnstableParticles(Cuts::abspid == 443);
  declare(ufs, "UFS");
  DecayedParticles PSI(ufs);
  PSI.addStable(PID::K0S);
  declare(PSI, "PSI");
  declare(Beam(), "Beams");
  // hisotgrams
  for (unsigned int ix = 0; ix < 3; ++ix) {
    if (ix < 2) book(_h_mass[ix], 1, 1, 1 + ix);
    book(_h_angle[ix], 2, 1, 1 + ix);
  }
}

// angle cuts due regions of BES calorimeter
bool vetoPhoton(const double& cTheta) {
  return cTheta > 0.92 || (cTheta > 0.8 && cTheta < 0.86);
}

/// 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();
  // find the J/psi decays
  static const map<PdgId, unsigned int>& mode = {{310, 2}, {22, 1}};
  DecayedParticles PSI = apply<DecayedParticles>(event, "PSI");
  if (PSI.decaying().size() != 1) vetoEvent;
  if (!PSI.modeMatches(0, 3, mode)) vetoEvent;
  // particles
  const Particles& K0 = PSI.decayProducts()[0].at(310);
  const Particle& gam = PSI.decayProducts()[0].at(22)[0];
  double mKK = (K0[0].momentum() + K0[1].momentum()).mass();
  _h_mass[0]->fill(mKK);
  for (unsigned int ix = 0; ix < 2; ++ix) _h_mass[1]->fill((gam.momentum() + K0[ix].momentum()).mass());
  double cTheta = axis.dot(gam.p3().unit());
  if (vetoPhoton(abs(cTheta))) vetoEvent;
  _h_angle[0]->fill(cTheta);
  // remaining angles
  LorentzTransform boost1 = LorentzTransform::mkFrameTransformFromBeta(
      PSI.decaying()[0].momentum().betaVec());
  FourMomentum pGamma = boost1.transform(gam.momentum());
  FourMomentum pKK = boost1.transform(K0[0].momentum() + K0[1].momentum());
  Vector3 e1z = pGamma.p3().unit();
  Vector3 e1y = e1z.cross(axis).unit();
  Vector3 e1x = e1y.cross(e1z).unit();
  LorentzTransform boost2 = LorentzTransform::mkFrameTransformFromBeta(pKK.betaVec());
  Vector3 axis2 = boost2.transform(boost1.transform(K0[0].momentum())).p3().unit();
  _h_angle[1]->fill(e1z.dot(axis2));
  double phi = atan2(e1y.dot(axis2), e1x.dot(axis2));
  _h_angle[2]->fill(phi);
}


/// Normalise histograms etc., after the run
void finalize() {
  for (unsigned int ix = 0; ix < 3; ++ix) {
    if (ix < 2) normalize(_h_mass[ix], 1., false);
    normalize(_h_angle[ix], 1., false);
  }
}

/// @}


/// @name Histograms
/// @{
Histo1DPtr _h_mass[2], _h_angle[3];
/// @}

};

RIVET_DECLARE_PLUGIN(BESIII_2018_I1689296);

} ```