Rivet analyses

title: BESIII_2012_I1084539

Angular distributions in $J/\psi\to\gamma\eta(1405)(\to f_0(980)\pi^0$

Experiment: BESIII (BEPC)

Inspire ID: 1084539

Status: VALIDATED NOHEPDATA

Authors: - Peter Richardson

References: - Phys.Rev.Lett. 108 (2012) 182001

Beams: e- e+

Beam energies: (1.6, 1.6)GeV

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

Measurement of angular distributions in $J/\psi\to\gamma\eta(1405)(\to f_0(980)\pi^0$ by BESIII.

Source code:BESIII_2012_I1084539.cc

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/Beam.hh"

include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {

/// @brief J/psi -> eta(1405) class BESIII_2012_I1084539 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2012_I1084539);


/// @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");
  declare(Beam(), "Beams");
  // histos
  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();
  Particles psi = apply<UnstableParticles>(event, "UFS").particles();
  if (psi.size() != 1 || psi[0].children().size() != 2) vetoEvent;
  Particle gamma, eta;
  if (psi[0].children()[0].pid() == PID::GAMMA && psi[0].children()[1].pid() == 9020221) {
    gamma = psi[0].children()[0];
    eta = psi[0].children()[1];
  }
  else if (psi[0].children()[1].pid() == PID::GAMMA && psi[0].children()[0].pid() == 9020221) {
    gamma = psi[0].children()[1];
    eta = psi[0].children()[0];
  }
  else
    vetoEvent;
  if (eta.children().size() != 2) vetoEvent;
  // eta(1405) -> f0(980) pi0
  Particle f0, pi0;
  if (eta.children()[0].pid() == PID::PI0 && eta.children()[1].pid() == 9010221) {
    pi0 = eta.children()[0];
    f0 = eta.children()[1];
  }
  else if (eta.children()[1].pid() == PID::PI0 && eta.children()[0].pid() == 9010221) {
    pi0 = eta.children()[1];
    f0 = eta.children()[0];
  }
  else
    vetoEvent;
  double cTheta = axis.dot(gamma.p3().unit());
  _h[1]->fill(cTheta);
  LorentzTransform boost1 = LorentzTransform::mkFrameTransformFromBeta(psi[0].momentum().betaVec());
  FourMomentum pGamma = boost1.transform(gamma.momentum());
  Vector3 axis1 = pGamma.p3().unit();
  FourMomentum pEta = boost1.transform(eta.momentum());
  LorentzTransform boost2 = LorentzTransform::mkFrameTransformFromBeta(pEta.betaVec());
  FourMomentum pf0 = boost2.transform(boost1.transform(f0.momentum()));
  _h[0]->fill(pf0.p3().unit().dot(axis1));
}


/// 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_I1084539);

} ```