Rivet analyses

title: BESIII_2025_I2866232

$e^+e^-$ mass distribution and form factor in $J/\psi\to e^+e^-\pi^0$

Experiment: BESIII (BEPC)

Inspire ID: 2866232

Status: VALIDATED NOHEPDATA

Authors: - Peter Richardson

References: - arXiv: 2501.04344

Beams: * *

Beam energies: ANY

Run details: - Any process producing J/psi

$e^+e^-$ mass distribution in $J/\psi\to e^+e^-\pi^0$.

Source code:BESIII_2025_I2866232.cc

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/DecayedParticles.hh"

include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {

/// @brief Add a short analysis description here class BESIII_2025_I2866232 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2025_I2866232);


/// @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);
  declare(psi, "PSI");
  book(_h[0], 1, 1, 2);
  book(_h[1], 1, 1, 4);
  book(_c[0], "TMP/den1");
  book(_c[1], "TMP/den2");
}


/// Perform the per-event analysis
void analyze(const Event& event) {
  // define the decay mode
  static const map<PdgId, unsigned int>& mode0 = {{PID::PI0, 1}, {PID::PHOTON, 1}};
  static const map<PdgId, unsigned int>& mode1 = {{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) {
    _c[0]->fill();
    if (psi.modeMatches(ix, 2, mode0)) {
      _c[1]->fill();
      continue;
    }
    else if (psi.modeMatches(ix, 3, mode1)) {
      // 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();
      _h[0]->fill(q);
      double me = em.mass();
      double beta = sqrt(1. - 4. * sqr(me / q));
      double mJpsi = psi.decaying()[ix].mass();
      double mPi0 = psi.decayProducts()[ix].at(111)[0].mass();
      double p = sqrt(sqr(1. + sqr(q) / (sqr(mJpsi) - sqr(mPi0)))
                      - 4. * sqr(mJpsi * q / (sqr(mJpsi) - sqr(mPi0))));
      double fact = beta * GeV / q * (1. + 2. * sqr(me / q)) * pow(p, 3);
      _h[1]->fill(q, 1. / fact);
    }
  }
}


/// Normalise histograms etc., after the run
void finalize() {
  scale(_h[0], 1e8 / *_c[0]);
  static double alpha = 7.2973525664e-3;
  scale(_h[1], 1.5 * M_PI / alpha / *_c[1]);
}

/// @}


/// @name Histograms
/// @{
Histo1DPtr _h[2];
CounterPtr _c[3];
/// @}

};

RIVET_DECLARE_PLUGIN(BESIII_2025_I2866232);

} ```