Rivet analyses

title: BESIII_2021_I1857930

Cross section for $e^+e^-\to\eta\phi$ for centre-of-mass energies between 2 and 3.08 GeV

Experiment: BESIII (BEPC)

Inspire ID: 1857930

Status: VALIDATED

Authors: - Peter Richardson

References: - arXiv: 2104.05549

Beams: e+ e-

Beam energies: (1.0, 1.0); (1.0, 1.0); (1.1, 1.1); (1.1, 1.1); (1.1, 1.1); (1.1, 1.1); (1.1, 1.1); (1.1, 1.1); (1.2, 1.2); (1.2, 1.2); (1.2, 1.2); (1.2, 1.2); (1.3, 1.3); (1.3, 1.3); (1.4, 1.4); (1.5, 1.5); (1.5, 1.5); (1.5, 1.5); (1.5, 1.5); (1.5, 1.5); (1.5, 1.5); (1.5, 1.5)GeV

Run details: - e+ e- to hadrons

Measurement of the vross section for $e^+e^-\to\eta\phi$ for centre-of-mass energies between 2 and 3.08 GeV by the BESIII collaboration.

Source code:BESIII_2021_I1857930.cc

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/FinalState.hh"

include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {

/// @brief e+e- > phi eta class BESIII_2021_I1857930 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2021_I1857930);


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

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

  // Initialise and register projections
  declare(FinalState(), "FS");
  declare(UnstableParticles(), "UFS");
  book(_nPhiEta, 1, 1, 1);
  for (const string& en : _nPhiEta.binning().edges<0>()) {
    const double eval = stod(en) * GeV;
    if (isCompatibleWithSqrtS(eval)) {
      _sqs = en;
      break;
    }
  }
  raiseBeamErrorIf(_sqs.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()] -= 1;
      --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");

  // loop over eta mesons
  for (const Particle& p : ufs.particles(Cuts::pid == 221)) {
    if (p.children().empty()) continue;
    map<long, int> nRes = nCount;
    int ncount = ntotal;
    findChildren(p, nRes, ncount);
    // loop over phi mesons
    for (const Particle& p2 : ufs.particles(Cuts::pid == 333)) {
      if (p2.parents()[0].isSame(p)) continue;
      map<long, int> nResB = nRes;
      int ncountB = ncount;
      findChildren(p2, nResB, ncountB);
      if (ncountB != 0) continue;
      bool matched = true;
      for (const auto& val : nResB) {
        if (val.second != 0) {
          matched = false;
          break;
        }
      }
      if (matched) _nPhiEta->fill(_sqs);
    }
  }
}


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

///@}


/// @name Histograms
///@{
BinnedHistoPtr<string> _nPhiEta;
string _sqs = "";
///@}

};

RIVET_DECLARE_PLUGIN(BESIII_2021_I1857930);

} ```