Rivet analyses

Form factor for the decay π0 → γe+e−

Experiment: A2 (MAMI)

Inspire ID: 1498079

Status: VALIDATED NOHEPDATA

Authors: - Peter Richardson

References: - Phys.Rev.C 95 (2017) 2, 025202

Beams: * *

Beam energies: ANY

Run details: - pi0 production with both the dalitz decay and radiative decay for normalization

Measurement of the form factor |Fπ0(q2)| for the decay π0 → γe+e− by the A2 experiment at MAMI.

Source code:A2_2017_I1498079.cc

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

namespace Rivet {


  /// @brief form factors in pi0->gamma gamma(e+e-)
  class A2_2017_I1498079 : public Analysis {
  public:

    /// Constructor
    RIVET_DEFAULT_ANALYSIS_CTOR(A2_2017_I1498079);


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

    /// Book histograms and initialise projections before the run
    void init() {
      // Initialise and register projections
      declare(UnstableParticles(), "UFS");
      book(_h_pi0, 1, 1, 1);
      book(_weight_pi0, "TMP/weight_pi0");
    }

    void findDecayProducts(const Particle& mother,
                           unsigned int& nstable,
                           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::GAMMA && p.children().empty()) {
          ++ngamma;
          ++nstable;
        }
        else if (!p.children().empty()) {
          findDecayProducts(p, nstable, nep, nem, ngamma, ptot);
        }
        else
          ++nstable;
      }
    }

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

      // Loop over pi0 and omega mesons
      for (const Particle& p : apply<UnstableParticles>(event, "UFS").particles(Cuts::pid == 111)) {
        unsigned nstable(0), nep(0), nem(0), ngamma(0);
        FourMomentum ptot;
        findDecayProducts(p, nstable, nep, nem, ngamma, ptot);
        if (nstable == 3 && nem == 1 && nem == 1 && ngamma == 1) {
          double q = ptot.mass();
          double beta = sqrt(1. - 4 * sqr(me / q));
          double p = 1. - sqr(q / mpi);
          double fact = beta * MeV / q * (1. + 2. * sqr(me / q)) * pow(p, 3);
          _h_pi0->fill(q / MeV, 1. / fact);
        }
        else if (nstable == 2 && ngamma == 2) {
          _weight_pi0->fill();
        }
      }
    }


    /// Normalise histograms etc., after the run
    void finalize() {
      static double alpha = 7.2973525664e-3;
      scale(_h_pi0, 0.75 * M_PI / alpha / *_weight_pi0);
    }

    ///@}


    /// @name Histograms
    ///@{
    Histo1DPtr _h_pi0;
    CounterPtr _weight_pi0;

    ///@}
  };


  RIVET_DECLARE_PLUGIN(A2_2017_I1498079);

}