Rivet analyses

Form factors for the decays ω → π0e+e− and η → γe+e−

Experiment: A2 (MAMI)

Inspire ID: 1486671

Status: VALIDATED NOHEPDATA

Authors: - Peter Richardson

References: - Phys.Rev. C95 (2017) no.3, 035208

Beams: * *

Beam energies: ANY

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

Measurement of the form factors |Fωπ0(q2)| for the decay ω → π0e+e− and |Fη(q2)| for the decay η → γe+e− by the A2 experiment at MAMI.

Source code:A2_2017_I1486671.cc

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

namespace Rivet {


  /// @brief form factors for omega->pi and eta->gamma
  class A2_2017_I1486671 : public Analysis {
  public:

    /// Constructor
    RIVET_DEFAULT_ANALYSIS_CTOR(A2_2017_I1486671);


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

    /// Book histograms and initialise projections before the run
    void init() {
      // Initialise and register projections
      declare(UnstableParticles(), "UFS");
      // Book histograms
      book(_h_eta  , 1, 1, 1);
      book(_h_omega, 2, 1, 1);
      book(_weight_eta  ,"TMP/weight_eta");
      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) {
      static double me     = 0.5109989461*MeV;
      static double momega = 782.65*MeV;
      static double meta   = 547.862 *MeV;
      static double mpi    = 134.9770*MeV;

      // Loop over eta and omega mesons
      for( const Particle& p : apply<UnstableParticles>(event, "UFS").particles(Cuts::pid==223 or Cuts::pid==221)) {
        unsigned nstable(0),npi(0),nep(0),nem(0),ngamma(0);
        FourMomentum ptot;
        findDecayProducts(p,nstable,npi,nep,nem,ngamma,ptot);
    if(p.pid()==221) {
      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/meta);
        double fact = beta*MeV/q*(1.+2.*sqr(me/q))*pow(p,3);
        _h_eta->fill(q/MeV,1./fact);
      }
      else if(nstable==2 && ngamma==2) {
        _weight_eta->fill();
      }
    }
    else {
      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(momega)-sqr(mpi)))-4.*sqr(momega*q/(sqr(momega)-sqr(mpi))));
        double fact = beta*MeV/q*(1.+2.*sqr(me/q))*pow(p,3);
        _h_omega->fill(q/MeV,1./fact);
      }
      else if(nstable==2 && ngamma ==1 && npi==1) {
        _weight_omega->fill();
      }
    }
      }
    }


    /// Normalise histograms etc., after the run
    void finalize() {
      static double alpha= 7.2973525664e-3;
      scale(_h_eta  , 0.75*M_PI/alpha/ *_weight_eta  );
      scale(_h_omega, 1.5 *M_PI/alpha/ *_weight_omega);
    }

    /// @}


    /// @name Histograms
    /// @{
    Histo1DPtr _h_eta,_h_omega;
    CounterPtr _weight_eta,_weight_omega;
    /// @}


  };


  RIVET_DECLARE_PLUGIN(A2_2017_I1486671);


}