Rivet analyses

Form factors for the decays χc(1, 2) → J/ψμ+μ−

Experiment: BESIII (BEPC)

Inspire ID: 1716256

Status: VALIDATED NOHEPDATA

Authors: - Peter Richardson

References: - Phys.Rev.D 99 (2019) 5, 051101

Beams: * *

Beam energies: ANY

Run details: none listed

Measurement of the form factors and differential branching ratios for the decays χc(1, 2) → J/ψμ+μ− by the BESIII collaboration.

Source code:BESIII_2019_I1716256.cc

// -*- C++ -*-
#include "Rivet/Analysis.hh"
#include "Rivet/Projections/DecayedParticles.hh"
#include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {


  /// @brief chi_c -> J/psi mu+mu-
  class BESIII_2019_I1716256 : public Analysis {
  public:

    /// Constructor
    RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2019_I1716256);


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

    /// Book histograms and initialise projections before the run
    void init() {
      UnstableParticles ufs = UnstableParticles(Cuts::pid == 20443 || Cuts::pid == 445);
      declare(ufs, "UFS");
      DecayedParticles chi(ufs);
      chi.addStable(PID::JPSI);
      declare(chi, "chi");
      // Initialise and register projections
      for (unsigned int ix = 0; ix < 2; ++ix) {
        for (unsigned int iy = 0; iy < 2; ++iy) {
          book(_h[ix][iy], 1 + ix, 1, 1 + iy);
          book(_n[ix][iy], "TMP/n_" + toString(ix + 1) + "_" + toString(iy + 1));
        }
      }
    }


    /// Perform the per-event analysis
    void analyze(const Event& event) {
      static double alpha = 7.2973525664e-3;
      static const map<PdgId, unsigned int>& mode1 = {{443, 1}, {22, 1}};
      static const map<PdgId, unsigned int>& mode2 = {{443, 1}, {13, 1}, {-13, 1}};
      DecayedParticles chi = apply<DecayedParticles>(event, "chi");
      // loop over particles
      for (unsigned int ix = 0; ix < chi.decaying().size(); ++ix) {
        unsigned int iloc = chi.decaying()[ix].pid() == 20443 ? 0 : 1;
        _n[iloc][0]->fill();
        if (chi.modeMatches(ix, 2, mode1)) {
          _n[iloc][1]->fill();
        }
        else if (chi.modeMatches(ix, 3, mode2)) {
          const Particle& mum = chi.decayProducts()[ix].at(13)[0];
          const Particle& mup = chi.decayProducts()[ix].at(-13)[0];
          const Particle& Jpsi = chi.decayProducts()[ix].at(443)[0];
          double q = (mum.momentum() + mup.momentum()).mass();
          // br
          _h[iloc][0]->fill(q);
          double M2 = chi.decaying()[ix].mass2();
          double m = Jpsi.mass();
          // factor for form factor (eqn 2 arXiv:1709.02444)
          double fact = alpha / 3. / M_PI / sqr(q) * sqrt((1. - sqr(m + q) / M2) * (1. - sqr(m - q) / M2))
              / (1. - sqr(m) / M2) * (1. + 2. * sqr(mum.mass() / q)) * sqrt(1. - 4. * sqr(mum.mass() / q));
          // additional factor of 2q from dq^2 = 2q dq
          _h[iloc][1]->fill(q, 1. / fact / 2 / q);
        }
      }
    }


    /// Normalise histograms etc., after the run
    void finalize() {
      for (unsigned int ix = 0; ix < 2; ++ix) {
        for (unsigned int iy = 0; iy < 2; ++iy) {
          scale(_h[ix][iy], 1. / *_n[ix][iy]);
        }
        scale(_h[ix][0], 1e5);
      }
    }
    /// @}

    /// @name Histograms
    ///@{
    Histo1DPtr _h[2][2];
    CounterPtr _n[2][2];

    ///@}
  };


  RIVET_DECLARE_PLUGIN(BESIII_2019_I1716256);

}