Rivet analyses

Photon and Light Meson Production in Hadronic Z0 Decays

Experiment: OPAL (LEP 1)

Inspire ID: 470419

Status: VALIDATED

Authors: - Peter Richardson

References: - Eur.Phys.J.C5:411-437,1998 - hep-ex/9805011

Beams: e+ e-

Beam energies: (45.6, 45.6)GeV

Run details: - Hadronic Z decay events generated on the Z pole ($\sqrt{s} = 91.2$ GeV)

The inclusive production rates and differential cross sections of photons and mesons with a final state containing photons have been measured with the OPAL detector at LEP. The light mesons covered by the measurements are the π0, η, ρ(770)±, ω(782), η′(958) and a0(980)±.

Source code:OPAL_1998_I470419.cc

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

namespace Rivet {


  /// @brief OPAL photon/light meson paper
  ///
  /// @author Peter Richardson
  class OPAL_1998_I470419 : public Analysis {
  public:

    RIVET_DEFAULT_ANALYSIS_CTOR(OPAL_1998_I470419);


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

    void init() {
      declare(Beam(), "Beams");
      declare(ChargedFinalState(), "FS");
      declare(UnstableParticles(), "UFS");
      book(_histXePhoton, 2, 1, 1);
      book(_histXiPhoton, 3, 1, 1);
      book(_histXePi, 4, 1, 1);
      book(_histXiPi, 5, 1, 1);
      book(_histXeEta, 6, 1, 1);
      book(_histXiEta, 7, 1, 1);
      book(_histXeRho, 8, 1, 1);
      book(_histXiRho, 9, 1, 1);
      book(_histXeOmega, 10, 1, 1);
      book(_histXiOmega, 11, 1, 1);
      book(_histXeEtaPrime, 12, 1, 1);
      book(_histXiEtaPrime, 13, 1, 1);
      book(_histXeA0, 14, 1, 1);
      book(_histXiA0, 15, 1, 1);
    }


    void analyze(const Event& e) {
      // First, veto on leptonic events by requiring at least 4 charged FS particles
      const FinalState& fs = apply<FinalState>(e, "FS");
      const size_t numParticles = fs.particles().size();

      // Even if we only generate hadronic events, we still need a cut on numCharged >= 2.
      if (numParticles < 2) {
        MSG_DEBUG("Failed leptonic event cut");
        vetoEvent;
      }
      MSG_DEBUG("Passed leptonic event cut");

      // Get beams and average beam momentum
      const ParticlePair& beams = apply<Beam>(e, "Beams").beams();
      const double meanBeamMom = (beams.first.p3().mod() + beams.second.p3().mod()) / 2.0;
      MSG_DEBUG("Avg beam momentum = " << meanBeamMom);

      // Final state of unstable particles to get particle spectra
      const UnstableParticles& ufs = apply<UnstableParticles>(e, "UFS");

      for (const Particle& p : ufs.particles()) {
        const int id = p.abspid();
        double xi = -log(p.p3().mod() / meanBeamMom);
        double xE = p.E() / meanBeamMom;
        switch (id) {
          case 22: // Photons
            _histXePhoton->fill(xE);
            _histXiPhoton->fill(xi);
            break;
          case 111: // Neutral pions
            _histXePi->fill(xE);
            _histXiPi->fill(xi);
            break;
          case 221: // eta
            _histXeEta->fill(xE);
            _histXiEta->fill(xi);
            break;
          case 213: // Charged rho (770)
            _histXeRho->fill(xE);
            _histXiRho->fill(xi);
            break;
          case 223: // omega (782)
            _histXeOmega->fill(xE);
            _histXiOmega->fill(xi);
            break;
          case 331: // eta' (958)
            _histXeEtaPrime->fill(xE);
            _histXiEtaPrime->fill(xi);
            break;
          case 9000211: // Charged a_0 (980)
            _histXeA0->fill(xE);
            _histXiA0->fill(xi);
            break;
        }
      }
    }


    /// Finalize
    void finalize() {
      scale(_histXePhoton, 1. / sumOfWeights());
      scale(_histXiPhoton, 1. / sumOfWeights());
      scale(_histXePi, 1. / sumOfWeights());
      scale(_histXiPi, 1. / sumOfWeights());
      scale(_histXeEta, 1. / sumOfWeights());
      scale(_histXiEta, 1. / sumOfWeights());
      scale(_histXeRho, 1. / sumOfWeights());
      scale(_histXiRho, 1. / sumOfWeights());
      scale(_histXeOmega, 1. / sumOfWeights());
      scale(_histXiOmega, 1. / sumOfWeights());
      scale(_histXeEtaPrime, 1. / sumOfWeights());
      scale(_histXiEtaPrime, 1. / sumOfWeights());
      scale(_histXeA0, 1. / sumOfWeights());
      scale(_histXiA0, 1. / sumOfWeights());
    }

    /// @}


  private:

    /// @{
    Histo1DPtr _histXePhoton;
    Histo1DPtr _histXiPhoton;
    Histo1DPtr _histXePi;
    Histo1DPtr _histXiPi;
    Histo1DPtr _histXeEta;
    Histo1DPtr _histXiEta;
    Histo1DPtr _histXeRho;
    Histo1DPtr _histXiRho;
    Histo1DPtr _histXeOmega;
    Histo1DPtr _histXiOmega;
    Histo1DPtr _histXeEtaPrime;
    Histo1DPtr _histXiEtaPrime;
    Histo1DPtr _histXeA0;
    Histo1DPtr _histXiA0;
    /// @}
  };


  RIVET_DECLARE_ALIASED_PLUGIN(OPAL_1998_I470419, OPAL_1998_S3749908);

}

Aliases: - OPAL_1998_S3749908