Rivet analyses

title: BESIII_2018_I1653121

Cross section for $e^+e^-\to K^+K^-J/\psi$ and $K^0_SK^0_SJ/\psi$ for energies between 4.2 and 4.6 GeV

Experiment: BESIII (BEPC)

Inspire ID: 1653121

Status: VALIDATED

Authors: - Peter Richardson

References: - Phys.Rev. D97 (2018) 071101

Beams: e+ e-

Beam energies: (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 pi0 and KS0 set stable

Cross section for $e^+e^-\to K^+K^-J/\psi$ and $K^0_SK^0_SJ/\psi$ for energies between 4.2 and 4.6 GeV measured by the BESIII collaboration.

Source code:BESIII_2018_I1653121.cc

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/FinalState.hh"

include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {

/// @brief e+e- > KK J/Psi class BESIII_2018_I1653121 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2018_I1653121);


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

/// Book histograms and initialise projections before the run
void init() {
  declare(FinalState(), "FS");
  declare(UnstableParticles(Cuts::pid == 443), "UFS");
  book(_nKp, 1, 1, 1);
  book(_nK0, 1, 1, 2);
  for (const string& en : _nKp.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()];
      --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;
    map<long, int> nRes = nCount;
    int ncount = ntotal;
    findChildren(p, nRes, ncount);
    // J/psi KK
    if (ncount != 2) continue;
    bool matched = true;
    for (const auto& val : nRes) {
      if (abs(val.first) == 321 || abs(val.first) == 310) {
        continue;
      }
      else if (val.second != 0) {
        matched = false;
        break;
      }
    }
    if (matched) {
      if (nRes[321] == 1 && nRes[-321] == 1) {
        _nKp->fill(_sqs);
        break;
      }
      else if (nRes[310] == 2) {
        _nK0->fill(_sqs);
        break;
      }
    }
  }
}

/// Normalise histograms etc., after the run
void finalize() {
  const double fact = crossSection() / sumOfWeights() / picobarn;
  scale(_nKp, fact);
  scale(_nK0, fact);
}
/// @}


/// @name Histograms
/// @{
BinnedHistoPtr<string> _nKp, _nK0;
string _sqs = "";
/// @}

};

RIVET_DECLARE_PLUGIN(BESIII_2018_I1653121);

} ```