Rivet analyses

e+e− → e+e−π0 via intermediate photons at 10.58 GeV

Experiment: BELLE (KEKB)

Inspire ID: 1114749

Status: VALIDATED

Authors: - Peter Richardson

References: - Phys.Rev.D 86 (2012) 092007

Beams: e+ e-

Beam energies: (5.3, 5.3)GeV

Run details: - e+ e- > e+e- meson via photon photon -> meson

Measurement of the cross sections for the production of π0 in photon-photon collisions, i.e. e+e− → γγe+e− followed by γγ → π0, by the BELLE experiment at 10.58 GeV

Source code:BELLE_2012_I1114749.cc

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

namespace Rivet {


  /// @brief BELLE  e+e- > e+e- pi0
  class BELLE_2012_I1114749 : public Analysis {
  public:

    /// Constructor
    RIVET_DEFAULT_ANALYSIS_CTOR(BELLE_2012_I1114749);


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

    /// Book histograms and initialise projections before the run
    void init() {

      // Initialise and register projections
      declare(Beam(), "Beams");
      declare(FinalState(), "FS");
      declare(UnstableParticles(), "UFS");
      // book the histograms
      book(_h_pi0, 1, 1, 1);
    }

    void findChildren(const Particle& p, map<long, int>& nRes, int& ncount) {
      for (const Particle& child : p.children()) {
        if (child.children().empty()) {
          --nRes[child.pid()];
          --ncount;
        }
        else {
          findChildren(child, nRes, ncount);
        }
      }
    }

    bool findScattered(Particle beam, double& q2) {
      bool found = false;
      Particle scat = beam;
      while (!scat.children().empty()) {
        found = false;
        for (const Particle& p : scat.children()) {
          if (p.pid() == scat.pid()) {
            scat = p;
            found = true;
            break;
          }
        }
        if (!found) break;
      }
      if (!found) return false;
      q2 = -(beam.momentum() - scat.momentum()).mass2();
      return true;
    }

    /// Perform the per-event analysis
    void analyze(const Event& event) {
      // find scattered leptons and calc Q2
      const Beam& beams = apply<Beam>(event, "Beams");
      double q12 = -1, q22 = -1;
      if (!findScattered(beams.beams().first, q12)) vetoEvent;
      if (!findScattered(beams.beams().second, q22)) vetoEvent;
      double scale = max(q12, q22);
      // check the final state
      const FinalState& fs = apply<FinalState>(event, "FS");
      map<long, int> nCount;
      int ntotal(0);
      for (const Particle& p : fs.particles()) {
        nCount[p.pid()] += 1;
        ++ntotal;
      }
      // find the meson
      const FinalState& ufs = apply<FinalState>(event, "UFS");
      for (const Particle& p : ufs.particles(Cuts::pid == 111)) {
        if (p.children().empty()) continue;
        map<long, int> nRes = nCount;
        int ncount = ntotal;
        findChildren(p, nRes, ncount);
        bool matched = true;
        for (const auto& val : nRes) {
          if (abs(val.first) == 11) {
            if (val.second != 1) {
              matched = false;
              break;
            }
          }
          else if (val.second != 0) {
            matched = false;
            break;
          }
        }
        if (matched) {
          _h_pi0->fill(scale);
          break;
        }
      }
    }


    /// Normalise histograms etc., after the run
    void finalize() {
      // normalize the cross sections
      scale(_h_pi0, crossSection() / femtobarn / sumW());
    }

    ///@}


    /// @name Histograms
    ///@{
    Histo1DPtr _h_pi0;
    ///@}
  };


  RIVET_DECLARE_PLUGIN(BELLE_2012_I1114749);

}