Rivet analyses

title: BESIII_2014_I1287631

$e^+e^-$ mass distribution in $J/\psi\to e^+e^-$ $+\eta^\prime,$ $\eta$ or $\pi^0$

Experiment: BESIII (BEPC)

Inspire ID: 1287631

Status: VALIDATED NOHEPDATA

Authors: - Peter Richardson

References: - Phys.Rev.D 89 (2014) 9, 092008

Beams: * *

Beam energies: ANY

Run details: - Any process producing J/psi

$e^+e^-$ mass distribution in $J/\psi\to e^+e^-$ $+\eta^\prime,$ $\eta$ or $\pi^0$. Data read from plots in paper.Mass distributions are not efficiency corrected.

Source code:BESIII_2014_I1287631.cc

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/DecayedParticles.hh"

include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {

/// @brief J/psi -> eta', eta, pi0 e+e- class BESIII_2014_I1287631 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2014_I1287631);


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

/// Book histograms and initialise projections before the run
void init() {
  UnstableParticles ufs = UnstableParticles(Cuts::abspid == PID::JPSI);
  declare(ufs, "UFS");
  DecayedParticles psi(ufs);
  psi.addStable(PID::PI0);
  psi.addStable(PID::ETA);
  psi.addStable(PID::ETAPRIME);
  declare(psi, "PSI");
  for (unsigned int ix = 0; ix < 5; ++ix) book(_h[ix], 1, 1, ix + 1);
  book(_h[5], 2, 1, 1);
  book(_c, "TMP/den");
}


/// Perform the per-event analysis
void analyze(const Event& event) {
  // define the decay mode
  static const map<PdgId, unsigned int>& mode0 = {{PID::ETAPRIME, 1}, {PID::PHOTON, 1}};
  static const map<PdgId, unsigned int>& mode1 = {{PID::ETAPRIME, 1}, {11, 1}, {-11, 1}};
  static const map<PdgId, unsigned int>& mode2 = {{PID::ETA, 1}, {11, 1}, {-11, 1}};
  static const map<PdgId, unsigned int>& mode3 = {{PID::PI0, 1}, {11, 1}, {-11, 1}};
  DecayedParticles psi = apply<DecayedParticles>(event, "PSI");
  // loop over particles
  for (unsigned int ix = 0; ix < psi.decaying().size(); ++ix) {
    unsigned int imode = 0;
    if (psi.modeMatches(ix, 2, mode0)) {
      _c->fill();
      continue;
    }
    else if (psi.modeMatches(ix, 3, mode1)) {
      imode = 1;
    }
    else if (psi.modeMatches(ix, 3, mode2)) {
      imode = 2;
    }
    else if (psi.modeMatches(ix, 3, mode3)) {
      imode = 3;
    }
    else
      continue;
    // e+ e-
    const Particle& em = psi.decayProducts()[ix].at(11)[0];
    const Particle& ep = psi.decayProducts()[ix].at(-11)[0];
    double q = (ep.momentum() + em.momentum()).mass();
    if (imode == 1) {
      _h[0]->fill(q);
      _h[1]->fill(q);
      double me = em.mass();
      double beta = sqrt(1. - 4. * sqr(me / q));
      double mJpsi = psi.decaying()[ix].mass();
      double mEta = psi.decayProducts()[ix].at(331)[0].mass();
      double p = sqrt(sqr(1. + sqr(q) / (sqr(mJpsi) - sqr(mEta)))
                      - 4. * sqr(mJpsi * q / (sqr(mJpsi) - sqr(mEta))));
      double fact = beta * GeV / q * (1. + 2. * sqr(me / q)) * pow(p, 3);
      _h[5]->fill(q, 1. / fact);
    }
    else if (imode == 2) {
      _h[2]->fill(q);
      _h[3]->fill(q);
    }
    else {
      _h[4]->fill(q);
    }
  }
}


/// Normalise histograms etc., after the run
void finalize() {
  for (unsigned int ix = 0; ix < 5; ++ix) normalize(_h[ix], 1., false);
  static double alpha = 7.2973525664e-3;
  scale(_h[5], 1.5 * M_PI / alpha / *_c);
}

/// @}


/// @name Histograms
/// @{
Histo1DPtr _h[6];
CounterPtr _c;
/// @}

};

RIVET_DECLARE_PLUGIN(BESIII_2014_I1287631);

} ```