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);

}