Rivet analyses

Form-factor for ω → π0e+e−

Experiment: CMD2 (VEPP-2M)

Inspire ID: 676548

Status: VALIDATED NOHEPDATA

Authors: - Peter Richardson

References: - Phys.Lett.B 613 (2005) 29-38

Beams: * *

Beam energies: ANY

Run details: - Any process producing omega mesons with both the dalitz decay and radiative decay for normalization, originally e+e-

Measurement of the form factor |Fωπ0(q2)| for the decay ω → π0e+e−.

Source code:CMD2_2005_I676548.cc

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

namespace Rivet {


  /// @brief omega -> pi0 e+e-
  class CMD2_2005_I676548 : public Analysis {
  public:

    /// Constructor
    RIVET_DEFAULT_ANALYSIS_CTOR(CMD2_2005_I676548);


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

    /// Book histograms and initialise projections before the run
    void init() {
      // Initialise and register projections
      declare(UnstableParticles(Cuts::pid == 223), "UFS");
      // Book histograms
      book(_h_omega, 2, 1, 1);
      book(_weight_omega, "TMP/weight_omega");
    }

    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 && p.children().empty()) {
          ++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) {
      const double me = 0.5109989461 * MeV;
      const double momega = 782.65 * MeV;
      const double mpi = 134.9770 * MeV;
      // Loop over eta and omega mesons
      for (const Particle& p : apply<UnstableParticles>(event, "UFS").particles()) {
        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 && nep == 1 && npi == 1) {
          const double q = ptot.mass();
          const double beta = sqrt(1. - 4 * sqr(me / q));
          const double p = sqrt(sqr(1. + sqr(q) / (sqr(momega) - sqr(mpi)))
                                - 4. * sqr(momega * q / (sqr(momega) - sqr(mpi))));
          const double fact = beta * GeV / q * (1. + 2. * sqr(me / q)) * pow(p, 3);
          _h_omega->fill(q / GeV, 1. / fact);
        }
        else if (nstable == 2 && ngamma == 1 && npi == 1) {
          _weight_omega->fill();
        }
      }
    }


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

    /// @}


    /// @name Histograms
    /// @{
    Histo1DPtr _h_omega;
    CounterPtr _weight_omega;
    /// @}
  };


  RIVET_DECLARE_PLUGIN(CMD2_2005_I676548);

}