Rivet analyses

title: SND_2025_I3066778

Cross section for $e^+e^-\to\eta\gamma$ for energies near the $\phi$ 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 $e^+e^-\to\eta\gamma$ for energies near the $\phi$ resonance

Source code:SND_2025_I3066778.cc

```c++ // -- 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);

} ```