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(auto const & 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);

}