Rivet analyses

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

namespace Rivet {


  /// @brief e+e- > eta eta gamma cross section
  class SND_2021_I1942539 : public Analysis {
  public:

    /// Constructor
    RIVET_DEFAULT_ANALYSIS_CTOR(SND_2021_I1942539);


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

    /// Book histograms and initialise projections before the run
    void init() {
      // Initialise and register projections
      declare(FinalState(), "FS");
      declare(UnstableParticles(), "UFS");
      book(_numEtaEtaGamma, "TMP/EtaEtaGamma", refData(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);
      }
    }

    /// Perform the per-event analysis
    void analyze(const Event& event) {
      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;
      }

      Particles etas = apply<FinalState>(event, "UFS").particles(Cuts::pid == 221);
      // find the first eta
      for (unsigned int ix = 0; ix < etas.size(); ++ix) {
        bool matched = false;
        if (etas[ix].children().empty()) continue;
        map<long, int> nRes = nCount;
        int ncount = ntotal;
        findChildren(etas[ix], nRes, ncount);
        // find the second eta
        for (unsigned int iy = ix + 1; iy < etas.size(); ++iy) {
          if (etas[iy].children().empty()) continue;
          map<long, int> nResB = nRes;
          int ncountB = ncount;
          findChildren(etas[iy], nResB, ncountB);
          matched = true;
          for (const auto& val : nResB) {
            if (val.first == 22) {
              if (val.second != 1) {
                matched = false;
                break;
              }
            }
            else if (val.second != 0) {
              matched = false;
              break;
            }
          }
          if (matched) {
            _numEtaEtaGamma->fill(sqrtS() / GeV);
            break;
          }
        }
        if (matched) break;
      }
    }


    /// Normalise histograms etc., after the run
    void finalize() {
      scale(_numEtaEtaGamma, crossSection() / sumOfWeights() / picobarn);
      Estimate1DPtr mult;
      book(mult, 1, 1, 1);
      barchart(_numEtaEtaGamma, mult);
    }

    ///@}


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


  RIVET_DECLARE_PLUGIN(SND_2021_I1942539);

}