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);


}