Rivet analyses

title: BESIII_2016_I1419650

Mass and angular distributions in $J/\psi\to\gamma\phi\phi$ decays

Experiment: BESIII (BEPC)

Inspire ID: 1419650

Status: VALIDATED NOHEPDATA

Authors: - Peter Richardson

References: - Phys.Rev.D 93 (2016) 11, 112011

Beams: e- e+

Beam energies: (1.6, 1.6)GeV

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

Measurement of mass and angular distributions in $J/\psi\to\gamma\phi\phi$ decays

Source code:BESIII_2016_I1419650.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 phi phi class BESIII_2016_I1419650 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2016_I1419650);


/// @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::PHI);
  declare(PSI, "PSI");
  declare(Beam(), "Beams");
  // book histograms
  for (unsigned int ix = 0; ix < 5; ++ix) book(_h[ix], 1, 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 = {{333, 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& phi = PSI.decayProducts()[0].at(333);
  const Particle& gam = PSI.decayProducts()[0].at(22)[0];
  _h[0]->fill((phi[0].momentum() + phi[1].momentum()).mass());
  double cTheta = axis.dot(gam.p3().unit());
  if (vetoPhoton(abs(cTheta))) vetoEvent;
  _h[1]->fill(cTheta);
  // remaining angles
  LorentzTransform boost1 = LorentzTransform::mkFrameTransformFromBeta(
      PSI.decaying()[0].momentum().betaVec());
  FourMomentum pGamma = boost1.transform(gam.momentum());
  FourMomentum pPhiPhi = boost1.transform(phi[0].momentum() + phi[1].momentum());
  Vector3 e1z = pGamma.p3().unit();
  Vector3 e1y = e1z.cross(axis).unit();
  Vector3 e1x = e1y.cross(e1z).unit();
  LorentzTransform boost2 = LorentzTransform::mkFrameTransformFromBeta(pPhiPhi.betaVec());
  Vector3 axis2 = boost2.transform(boost1.transform(phi[0].momentum())).p3().unit();
  _h[2]->fill(e1z.dot(axis2));
  // now for the phi decays
  Particle Km[2], Kp[2];
  FourMomentum pKp[2], pPhi[2];
  for (unsigned int ix = 0; ix < 2; ++ix) {
    if (phi[ix].children().size() != 2 || phi[ix].children()[0].pid() != -phi[ix].children()[1].pid()
        || phi[ix].children()[0].abspid() != 321)
      vetoEvent;
    Km[ix] = phi[ix].children()[0];
    Kp[ix] = phi[ix].children()[1];
    if (Kp[ix].pid() < 0) swap(Km[ix], Kp[ix]);
    pKp[ix] = boost2.transform(boost1.transform(Kp[ix].momentum()));
    pPhi[ix] = boost2.transform(boost1.transform(phi[ix].momentum()));
    LorentzTransform boost3 = LorentzTransform::mkFrameTransformFromBeta(pPhi[ix].betaVec());
    pKp[ix] = boost3.transform(pKp[ix]);
  }
  double cK = axis2.dot(pKp[0].p3().unit());
  _h[3]->fill(cK);
  Vector3 Trans1 = pKp[0].p3() - cK * pKp[0].p3().mod() * axis2;
  Vector3 Trans2 = pKp[1].p3() - axis2.dot(pKp[1].p3()) * axis2;
  double chi = atan(Trans1.cross(Trans2).dot(axis2) / Trans1.dot(Trans2));
  _h[4]->fill(abs(chi) / M_PI * 180.);
}


/// Normalise histograms etc., after the run
void finalize() {
  for (unsigned int ix = 0; ix < 5; ++ix) normalize(_h[ix], 1., false);
}

/// @}


/// @name Histograms
/// @{
Histo1DPtr _h[5];
/// @}

};

RIVET_DECLARE_PLUGIN(BESIII_2016_I1419650);

} ```