Rivet analyses

e+e− mass distribution and form factor in J/ψ → e+e−π0

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/ψ → e+e−π0.

Source code:BESIII_2025_I2866232.cc

// -*- 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);

}