Rivet analyses

title: BESIII_2017_I1596897

Cross section for $e^+e^- \to \eta h_c$ for energies between 4.085 and 4.6 GeV

Experiment: BESIII (BEPC)

Inspire ID: 1596897

Status: VALIDATED

Authors: - Peter Richardson

References: - Phys.Rev. D96 (2017) no.1, 012001

Beams: e+ e-

Beam energies: (2.0, 2.0); (2.1, 2.1); (2.1, 2.1); (2.1, 2.1); (2.1, 2.1); (2.1, 2.1); (2.1, 2.1); (2.2, 2.2); (2.2, 2.2); (2.2, 2.2); (2.2, 2.2); (2.2, 2.2); (2.3, 2.3); (2.3, 2.3); (2.3, 2.3)GeV

Run details: - e+ e- to hadrons.

Cross section for $e^+e^-\to\eta h_c$ for energies between 4.085 and 4.6 GeV measured by the BESIII collaboration.

Source code:BESIII_2017_I1596897.cc

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/FinalState.hh"

include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {

/// @brief e+ e- > eta h_c class BESIII_2017_I1596897 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2017_I1596897);


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

/// Book histograms and initialise projections before the run
void init() {
  declare(FinalState(), "FS");
  declare(UnstableParticles(), "UFS");
  book(_nhc, 1, 1, 1);
  for (const string& en : _nhc.binning().edges<0>()) {
    const double eval = stod(en) * MeV;
    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()];
      --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 h_c
  for (const Particle& hc : ufs.particles(Cuts::pid == 10443)) {
    if (hc.children().empty()) continue;
    map<long, int> nRes = nCount;
    int ncount = ntotal;
    findChildren(hc, nRes, ncount);
    bool matched = false;
    // loop over eta
    for (const Particle& eta : ufs.particles(Cuts::pid == 221)) {
      // check eta not child of h_c
      Particle parent = eta;
      while (!parent.parents().empty()) {
        parent = parent.parents()[0];
        if (parent.abspid() == 10443) break;
      }
      if (fuzzyEquals(parent.momentum(), hc.momentum())) continue;
      map<long, int> nRes2 = nRes;
      int ncount2 = ncount;
      findChildren(eta, nRes2, ncount2);
      if (ncount2 != 0) continue;
      matched = true;
      for (const auto& val : nRes2) {
        if (val.second != 0) {
          matched = false;
          break;
        }
      }
      if (matched) {
        _nhc->fill(_sqs);
        break;
      }
    }
    if (matched) break;
  }
}


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

/// @}


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

};

RIVET_DECLARE_PLUGIN(BESIII_2017_I1596897);

} ```