Rivet analyses

title: BESIII_2012_I1079921

Mass and angular distributions in $J/\psi\to\gamma p\bar{p}$ near the $p\bar{p}$ threshold

Experiment: BESIII (BEPC)

Inspire ID: 1079921

Status: VALIDATED NOHEPDATA

Authors: - Peter Richardson

References: - Phys.Rev.Lett. 108 (2012) 112003

Beams: e- e+

Beam energies: (1.6, 1.6); (1.8, 1.8)GeV

Run details: - e+e- > J/psi or psi(2S)

Mass and angular distributions in $J/\psi\to\gamma p\bar{p}$ near the $p\bar{p}$ threshold

Source code:BESIII_2012_I1079921.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 p pbar class BESIII_2012_I1079921 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2012_I1079921);


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

/// Book histograms and initialise projections before the run
void init() {
  // Initialise and register projections
  UnstableParticles ufs = UnstableParticles(Cuts::abspid == 443 or Cuts::abspid == 100443);
  declare(ufs, "UFS");
  DecayedParticles PSI(ufs);
  declare(PSI, "PSI");
  declare(Beam(), "Beams");

  // book histos
  ih = -1;
  if (isCompatibleWithSqrtS(3.686, 1e-3))
    ih = 1;
  else if (isCompatibleWithSqrtS(3.1, 1e-3))
    ih = 0;
  raiseBeamErrorIf(ih < 0);
  for (size_t ix = 0; ix < 4; ++ix) {
    book(_h[ix], 1, 1, 1 + ix);
  }
  book(_h[-1], 2, 1, 1);
}

// angle cuts due regions of BES calorimeter
bool vetoPhoton(const double& cTheta) const {
  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.mom().p3().unit();
  else
    axis = beams.second.mom().p3().unit();
  // find the J/psi decays
  static const map<PdgId, unsigned int>& mode = {{2212, 1}, {-2212, 1}, {22, 1}};
  DecayedParticles PSI = apply<DecayedParticles>(event, "PSI");
  if (PSI.decaying().size() != 1) vetoEvent;
  if (sqrtS() > 3.2 && PSI.decaying()[0].pid() == 443) vetoEvent;
  if (!PSI.modeMatches(0, 3, mode)) vetoEvent;
  const Particle& pp = PSI.decayProducts()[0].at(2212)[0];
  const Particle& pbar = PSI.decayProducts()[0].at(-2212)[0];
  const Particle& gam = PSI.decayProducts()[0].at(22)[0];
  double mass = (pp.mom() + pbar.mom()).mass() - pp.mass() - pbar.mass();
  _h[0 - ih]->fill(mass);
  if (ih > 0 || mass > 0.05) vetoEvent;
  double cTheta = axis.dot(gam.p3().unit());
  // photon angle
  if (vetoPhoton(abs(cTheta))) vetoEvent;
  _h[1]->fill(cTheta);
  // remaining angles
  LorentzTransform boost1 = LorentzTransform::mkFrameTransformFromBeta(PSI.decaying()[0].mom().betaVec());
  FourMomentum pGamma = boost1.transform(gam.mom());
  FourMomentum pppbar = boost1.transform(pp.mom() + pbar.mom());
  Vector3 e1z = pGamma.p3().unit();
  Vector3 e1y = e1z.cross(axis).unit();
  Vector3 e1x = e1y.cross(e1z).unit();
  LorentzTransform boost2 = LorentzTransform::mkFrameTransformFromBeta(pppbar.betaVec());
  Vector3 axis2 = boost2.transform(boost1.transform(pp.mom())).p3().unit();
  _h[2]->fill(e1z.dot(axis2));
  double phi = atan2(axis2.dot(e1y), axis2.dot(e1x)) / M_PI * 180.;
  _h[3]->fill(phi);
}


/// Normalise histograms etc., after the run
void finalize() {
  normalize(_h, 1.0, false);
}

/// @}


/// @name Histograms
/// @{
map<int, Histo1DPtr> _h;
int ih;
/// @}

};

RIVET_DECLARE_PLUGIN(BESIII_2012_I1079921);

} ```