Rivet analyses

title: CMD2_1999_I503154

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

Experiment: (VEPP-2M)

Inspire ID: 503154

Status: VALIDATED

Authors: - Peter Richardson

References: - Phys.Lett. B460 (1999) 242-247, 1999

Beams: e+ e-

Beam energies: ANY

Run details: - e+ e- to hadrons below 0.6 and 1.38 GeV

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

Source code:CMD2_1999_I503154.cc

```c++ // -- C++ --

include "Rivet/Analysis.hh"

include "Rivet/Projections/FinalState.hh"

include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {

/// @brief Add a short analysis description here class CMD2_1999_I503154 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(CMD2_1999_I503154);


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

/// Book histograms and initialise projections before the run
void init() {

  // Initialise and register projections
  declare(FinalState(), "FS");
  declare(UnstableParticles(), "UFS");
  book(_numEtaGamma, "TMP/EtaGamma");
}


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;
  }

  const FinalState& ufs = apply<FinalState>(event, "UFS");
  for (const Particle& p : ufs.particles()) {
    if (p.children().empty()) continue;
    // find the omega
    if (p.pid() == 221) {
      map<long, int> nRes = nCount;
      int ncount = ntotal;
      findChildren(p, nRes, ncount);
      // eta pi+pi-
      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) _numEtaGamma->fill();
    }
  }
}


/// Normalise histograms etc., after the run
void finalize() {
  double sigma = _numEtaGamma->val();
  double error = _numEtaGamma->err();
  sigma *= crossSection() / sumOfWeights() / nanobarn;
  error *= crossSection() / sumOfWeights() / nanobarn;
  Estimate1DPtr mult;
  book(mult, 1, 1, 1);
  for (auto& b : mult->bins()) {
    if (inRange(sqrtS() / MeV, b.xMin(), b.xMax())) {
      b.set(sigma, error);
    }
  }
}

/// @}


/// @name Histograms
/// @{
CounterPtr _numEtaGamma;
/// @}

};

RIVET_DECLARE_PLUGIN(CMD2_1999_I503154);

} ```