Rivet analyses

title: BESIII_2012_I1097066

Angular analysis of direct $\psi(2S)\to\gamma\gamma J\psi$

Experiment: BESIII (BEPC)

Inspire ID: 1097066

Status: VALIDATED NOHEPDATA

Authors: - Peter Richardson

References: - Phys.Rev.Lett. 109 (2012) 172002

Beams: e- e+

Beam energies: (1.8, 1.8)GeV

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

Measurement of angular distributions in direct $\psi(2S)\to\gamma\gamma J\psi$ decays

Source code:BESIII_2012_I1097066.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) -> J/psi gamma gamma class BESIII_2012_I1097066 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2012_I1097066);


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

/// Book histograms and initialise projections before the run
void init() {
  // Initialise and register projections
  UnstableParticles ufs = UnstableParticles(Cuts::abspid == 100443);
  declare(ufs, "UFS");
  DecayedParticles PSI(ufs);
  PSI.addStable(PID::JPSI);
  declare(PSI, "PSI");
  declare(Beam(), "Beams");
  // book histograms
  for (unsigned int ix = 0; ix < 2; ++ix) book(_h[ix], 1, 1, 1 + ix);
}


/// 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 = {{443, 1}, {22, 2}};
  DecayedParticles PSI = apply<DecayedParticles>(event, "PSI");
  if (PSI.decaying().size() != 1) vetoEvent;
  if (!PSI.modeMatches(0, 3, mode)) vetoEvent;
  if (PSI.decaying()[0].children().size() != 3) vetoEvent;
  // particles
  const Particle& JPsi = PSI.decayProducts()[0].at(443)[0];
  const Particles& gam = PSI.decayProducts()[0].at(22);
  Vector3 norm = gam[0].p3().cross(gam[1].p3()).unit();
  _h[0]->fill(abs(axis.dot(norm)));
  if (JPsi.children().size() != 2) vetoEvent;
  if (JPsi.children()[0].pid() != -JPsi.children()[1].pid()) vetoEvent;
  if (JPsi.children()[0].abspid() != PID::EMINUS && JPsi.children()[0].abspid() != PID::MUON) vetoEvent;
  Particle lm = JPsi.children()[0];
  Particle lp = JPsi.children()[1];
  if (lm.pid() < 0) swap(lm, lp);
  Vector3 axis2 = JPsi.momentum().p3().unit();
  LorentzTransform boost = LorentzTransform::mkFrameTransformFromBeta(JPsi.momentum().betaVec());
  Vector3 axis3 = boost.transform(lm.momentum()).p3().unit();
  _h[1]->fill(abs(axis2.dot(axis3)));
}


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

/// @}


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

};

RIVET_DECLARE_PLUGIN(BESIII_2012_I1097066);

} ```