Rivet analyses

γγ → ηπ0 for centre-of-mass energies between 0.84 and 4.0 GeV

Experiment: BELLE (KEKB)

Inspire ID: 822474

Status: VALIDATED

Authors: - Peter Richardson

References: - Phys.Rev.D 80 (2009) 032001, 2009

Beams: 22 22

Beam energies: ANY

Run details: - gamma gamma to hadrons, pi0 must be set stable

Measurement of the differential cross section for γγ → ηπ0 for 0.84GeV < W < 4.0GeV. Both the cross section as a function of the centre-of-mass energy of the photonic collision, and the differential cross section with respect to the pion scattering angle are measured.

Source code:BELLE_2009_I822474.cc

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

namespace Rivet {


  /// @brief gamma gamma -> eta pi0
  class BELLE_2009_I822474 : public Analysis {
  public:

    /// Constructor
    RIVET_DEFAULT_ANALYSIS_CTOR(BELLE_2009_I822474);


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

    /// Book histograms and initialise projections before the run
    void init() {
      // Initialise and register projections
      declare(FinalState(), "FS");
      declare(UnstableParticles(), "UFS");
      // book histos
      if (inRange(sqrtS() / GeV, 0.84, 4.)) {
        book(_sigmaEtaPi, "TMP/nEtaPi", refData(1, 1, 1));
        double sMin = 0.84, step = 0.02;
        unsigned int ihist = 2;
        while (sMin < 4.) {
          if (inRange(sqrtS() / GeV, sMin, sMin + step)) {
            break;
          }
          sMin += step;
          ihist += 1;
          if (fuzzyEquals(1.6, sMin))
            step = 0.04;
          else if (fuzzyEquals(2.4, sMin))
            step = 0.1;
        }
        book(_h_cTheta, ihist, 1, 1);
      }
      else {
        throw Error("Invalid CMS energy for BELLE_2009_I822474");
      }
    }

    void findChildren(const Particle& p, map<long, int>& nRes, int& ncount) {
      for (const Particle& child : p.children()) {
        if (child.children().empty()) {
          nRes[child.pid()] -= 1;
          --ncount;
        }
        else {
          findChildren(child, nRes, ncount);
        }
      }
    }

    /// Perform the per-event analysis
    void analyze(const Event& event) {
      const FinalState& fs = apply<FinalState>(event, "FS");
      // find the final-state particles
      map<long, int> nCount;
      int ntotal(0);
      for (const Particle& p : fs.particles()) {
        nCount[p.pid()] += 1;
        ++ntotal;
      }
      const FinalState& ufs = apply<FinalState>(event, "UFS");
      for (const Particle& p : ufs.particles(Cuts::pid == PID::ETA)) {
        if (p.children().empty()) continue;
        map<long, int> nRes = nCount;
        int ncount = ntotal;
        findChildren(p, nRes, ncount);
        if (ncount != 1) continue;
        bool matched = true;
        for (const auto& val : nRes) {
          if (val.first == PID::PI0) {
            if (val.second != 1) {
              matched = false;
              break;
            }
          }
          else if (val.second != 0) {
            matched = false;
            break;
          }
        }
        if (matched) {
          double cTheta = abs(p.momentum().z() / p.momentum().p3().mod());
          if (cTheta <= 0.8) _sigmaEtaPi->fill(sqrtS());
          if (_h_cTheta) _h_cTheta->fill(cTheta);
          break;
        }
      }
    }


    /// Normalise histograms etc., after the run
    void finalize() {
      const double fact = crossSection() / nanobarn / sumOfWeights();
      if (_h_cTheta) scale(_h_cTheta, fact);
      scale(_sigmaEtaPi, fact);
      Estimate1DPtr tmp;
      book(tmp, 1, 1, 1);
      barchart(_sigmaEtaPi, tmp);
    }

    /// @}


    /// @name Histograms
    /// @{
    Histo1DPtr _sigmaEtaPi, _h_cTheta;
    /// @}
  };


  RIVET_DECLARE_PLUGIN(BELLE_2009_I822474);

}