Rivet analyses

Form factor for the decay ϕ → π0e+e−

Experiment: KLOE2 (DAPHNE)

Inspire ID: 1416825

Status: VALIDATED

Authors: - Peter Richardson

References: - Phys.Lett. B757 (2016) 362-367, 2016

Beams: * *

Beam energies: ANY

Run details: - phi production with both the dalitz decay and radiative decay for normalization

Measurement of the form factor |Fϕπ0(q2)| for the decay ϕ → π0e+e− by the KLOE experiment at DAPHNE

Source code:KLOE2_2016_I1416825.cc

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

namespace Rivet {


  /// @brief form factor for phi-> pi0
  class KLOE2_2016_I1416825 : public Analysis {
  public:

    /// Constructor
    RIVET_DEFAULT_ANALYSIS_CTOR(KLOE2_2016_I1416825);


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

    /// Book histograms and initialise projections before the run
    void init() {
      // Initialise and register projections
      declare(UnstableParticles(), "UFS");
      // Book histograms
      book(_h_m, 1, 1, 1);
      book(_weight, "TMP/weight");
    }

    void findDecayProducts(const Particle& mother,
                           unsigned int& nstable,
                           unsigned int& npi,
                           unsigned int& nep,
                           unsigned int& nem,
                           unsigned int& ngamma,
                           FourMomentum& ptot) {
      for (const Particle& p : mother.children()) {
        int id = p.pid();
        if (id == PID::EMINUS) {
          ++nem;
          ++nstable;
          ptot += p.momentum();
        }
        else if (id == PID::EPLUS) {
          ++nep;
          ++nstable;
          ptot += p.momentum();
        }
        else if (id == PID::PI0) {
          ++npi;
          ++nstable;
        }
        else if (id == PID::GAMMA) {
          ++ngamma;
          ++nstable;
        }
        else if (!p.children().empty()) {
          findDecayProducts(p, nstable, npi, nep, nem, ngamma, ptot);
        }
        else
          ++nstable;
      }
    }

    /// Perform the per-event analysis
    void analyze(const Event& event) {
      static double me = 0.5109989461 * MeV;
      static double mphi = 1019.461 * MeV;
      static double mpi = 134.9770 * MeV;

      // Loop over phi mesons
      for (const Particle& p : apply<UnstableParticles>(event, "UFS").particles(Cuts::pid == 333)) {
        unsigned nstable(0), npi(0), nep(0), nem(0), ngamma(0);
        FourMomentum ptot;
        findDecayProducts(p, nstable, npi, nep, nem, ngamma, ptot);
        if (nstable == 3 && nem == 1 && nem == 1 && npi == 1) {
          double q = ptot.mass();
          double beta = sqrt(1. - 4 * sqr(me / q));
          double p = sqrt(sqr(1. + sqr(q) / (sqr(mphi) - sqr(mpi)))
                          - 4. * sqr(mphi * q / (sqr(mphi) - sqr(mpi))));
          double fact = beta * MeV / q * (1. + 2. * sqr(me / q)) * pow(p, 3);
          _h_m->fill(q / MeV, 1. / fact);
        }
        else if (nstable == 2 && ngamma == 1 && npi == 1) {
          _weight->fill();
        }
      }
    }


    /// Normalise histograms etc., after the run
    void finalize() {
      static double alpha = 7.2973525664e-3;
      scale(_h_m, 1.5 * M_PI / alpha / *_weight);
    }

    /// @}


    /// @name Histograms
    /// @{
    Histo1DPtr _h_m;
    CounterPtr _weight;
    /// @}
  };


  RIVET_DECLARE_PLUGIN(KLOE2_2016_I1416825);


}