Rivet analyses

Cross section for e+e− → ηγ for energies near the ϕ resonance

Experiment: SND (2670980)

Inspire ID: 3066778

Status: VALIDATED NOHEPDATA

Authors: none listed

References: - Phys.Rev.D 113 (2026) 5, 052003 - arXiv: 2510.09019

Beams: e+ e-

Beam energies: (0.5, 0.5); (0.5, 0.5); (0.5, 0.5); (0.5, 0.5); (0.5, 0.5); (0.5, 0.5); (0.5, 0.5); (0.5, 0.5); (0.5, 0.5); (0.5, 0.5); (0.5, 0.5); (0.5, 0.5); (0.5, 0.5); (0.5, 0.5); (0.5, 0.5); (0.5, 0.5); (0.5, 0.5); (0.5, 0.5); (0.5, 0.5); (0.5, 0.5); (0.5, 0.5)GeV

Run details: - e+ e- to hadrons

Cross section for e+e− → ηγ for energies near the ϕ resonance

Source code:SND_2025_I3066778.cc

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

namespace Rivet {


  /// @brief Cross section for e+e- > gamma eta
  class SND_2025_I3066778 : public Analysis {
  public:

    /// Constructor
    RIVET_DEFAULT_ANALYSIS_CTOR(SND_2025_I3066778);


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

    /// Book histograms and initialise projections before the run
    void init() {
      // Initialise and register projections
      declare(FinalState(), "FS");
      declare(UnstableParticles(), "UFS");
      for (size_t ix=0; ix<2; ++ix) {
        book(_numEtaGamma[ix], 1+ix, 1, 1);
        for (const string& en : _numEtaGamma[ix].binning().edges<0>()) {
          const double eval = stod(en)*MeV;
          if (isCompatibleWithSqrtS(eval)) {
            _sqs[ix] = en;
            break;
          }
        }
      }
      raiseBeamErrorIf(_sqs[0].empty() && _sqs[1].empty());
    }

    void findChildren(const Particle& p, map<long,int>& nRes, int& ncount) const {
      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;
      }

      const FinalState& ufs = apply<FinalState>(event, "UFS");
      for (const Particle& p : ufs.particles(Cuts::pid==221)) {
        if (p.children().empty()) continue;
        // find the eta
        map<long,int> nRes = nCount;
        int ncount = ntotal;
        findChildren(p,nRes,ncount);
        // eta gamma
        if (ncount!=1) continue;
        bool matched = true;
        for (const auto& val : nRes) {
          if (val.first==22) {
            if (val.second !=1) {
              matched = false;
              break;
            }
          }
          else if (val.second!=0) {
            matched = false;
            break;
          }
        }
        if (matched) {
          for (size_t ix=0; ix<2; ++ix) {
            if (_sqs[ix]!="") _numEtaGamma[ix]->fill(_sqs[ix]);
          }
        }
      }
    }


    /// Normalise histograms etc., after the run
    void finalize() {
      scale(_numEtaGamma, crossSection()/sumOfWeights()/nanobarn);
    }

    /// @}


    /// @name Histograms
    /// @{
    BinnedHistoPtr<string> _numEtaGamma[2];
    string _sqs[2] = {"",""};
    /// @}


  };


  RIVET_DECLARE_PLUGIN(SND_2025_I3066778);

}