Rivet analyses

title: BESIII_2022_I2135117

Mass and angluar distributions in $J/\psi\to\gamma\eta\eta^\prime$

Experiment: BESIII (BEPC)

Inspire ID: 2135117

Status: VALIDATED NOHEPDATA

Authors: - Peter Richardson

References: - Phys.Rev.D 106 (2022) 7, 072012

Beams: * *

Beam energies: ANY

Run details: - Any process producing J/psi, originally e+e-

Measurement of mass and angluar distributions in $J/\psi\to\gamma\eta\eta^\prime$ by BESIII. The background subtracted data were read from the plots in the paper.

Source code:BESIII_2022_I2135117.cc

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/DecayedParticles.hh"

include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {

/// @brief J/psi -> gamma eta eta' class BESIII_2022_I2135117 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2022_I2135117);


/// @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::ETA);
  PSI.addStable(PID::ETAPRIME);
  declare(PSI, "PSI");
  // histos
  for (unsigned int ix = 0; ix < 3; ++ix) {
    book(_h_angle[ix], 2, 1, 1 + ix);
    book(_h_mass[ix], 1, 1, 1 + ix);
  }
  book(_h_angle[3], 1, 1, 4);
}


/// Perform the per-event analysis
void analyze(const Event& event) {
  // find the J/psi decays
  static const map<PdgId, unsigned int>& mode = {{22, 1}, {221, 1}, {331, 1}};
  DecayedParticles PSI = apply<DecayedParticles>(event, "PSI");
  for (unsigned int ix = 0; ix < PSI.decaying().size(); ++ix) {
    if (!PSI.modeMatches(ix, 3, mode)) continue;
    const Particle& eta = PSI.decayProducts()[ix].at(221)[0];
    const Particle& etap = PSI.decayProducts()[ix].at(331)[0];
    const Particle& gam = PSI.decayProducts()[ix].at(22)[0];
    double mEE = (eta.momentum() + etap.momentum()).mass();
    double mEG = (gam.momentum() + eta.momentum()).mass();
    if (abs(mEG - 1.019461) < 0.04) continue;
    _h_mass[0]->fill(mEE);
    _h_mass[1]->fill(mEG);
    _h_mass[2]->fill((gam.momentum() + etap.momentum()).mass());
    // angles
    LorentzTransform boost1 = LorentzTransform::mkFrameTransformFromBeta(
        PSI.decaying()[0].momentum().betaVec());
    FourMomentum pGamma = boost1.transform(gam.momentum());
    FourMomentum pEE = boost1.transform(eta.momentum() + etap.momentum());
    LorentzTransform boost2 = LorentzTransform::mkFrameTransformFromBeta(pEE.betaVec());
    Vector3 axis2 = boost2.transform(boost1.transform(eta.momentum())).p3().unit();
    double cTheta = pGamma.p3().unit().dot(axis2);
    _h_angle[3]->fill(cTheta);
    if (mEE > 1.5 && mEE < 1.7)
      _h_angle[0]->fill(cTheta);
    else if (mEE > 1.7 && mEE < 2.)
      _h_angle[1]->fill(cTheta);
    else if (mEE > 2. && mEE < 3.2)
      _h_angle[2]->fill(cTheta);
  }
}


/// Normalise histograms etc., after the run
void finalize() {
  for (unsigned int ix = 0; ix < 4; ++ix) {
    normalize(_h_angle[ix], 1., false);
    if (ix < 3) normalize(_h_mass[ix], 1., false);
  }
}

/// @}


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

};

RIVET_DECLARE_PLUGIN(BESIII_2022_I2135117);

} ```