Rivet analyses

title: SND_2021_I1942539

Cross Section for $e^+e^-\to\eta\eta\gamma$ between 1.17 and 2 GeV

Experiment: SND (VEPP-2M)

Inspire ID: 1942539

Status: VALIDATED

Authors: - Peter Richardson

References: - arXiv: 2110.05845

Beams: e+ e-

Beam energies: ANY

Run details: - e+ e- to hadrons below 1.17 and 2 GeV

Cross Section for $e^+e^-\to\eta\eta\gamma$ between 1.17 and 2 GeV measured by the SND collaboration.

Source code:SND_2021_I1942539.cc

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

} ```