Rivet analyses

title: BESIII_2023_I2617922

Cross section for $e^+e^- \to \eta J/\psi$ at $\sqrt{s}=3.773$ GeV

Experiment: BESIII (BEPC)

Inspire ID: 2617922

Status: VALIDATED NOHEPDATA

Authors: - Peter Richardson

References: - Phys.Rev.D 102 (2020) 3, 031101

Beams: e+ e-

Beam energies: (1.9, 1.9)GeV

Run details: - e+ e- to hadrons

Cross section for $e^+e^-\to\eta J/\psi$ for $\sqrt{s}=3.773$ GeV measured by the BESIII collaboration.

Source code:BESIII_2023_I2617922.cc

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/FinalState.hh"

include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {

/// @brief e+e- -> eta J/psi class BESIII_2023_I2617922 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2023_I2617922);


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

/// Book histograms and initialise projections before the run
void init() {
  declare(FinalState(), "FS");
  declare(UnstableParticles(), "UFS");
  book(_nJPsi, 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;
  }
  const FinalState& ufs = apply<FinalState>(event, "UFS");
  for (const Particle& p : ufs.particles(Cuts::pid == PID::JPSI)) {
    if (p.children().empty()) continue;
    // find the J/psi
    map<long, int> nRes = nCount;
    int ncount = ntotal;
    findChildren(p, nRes, ncount);
    bool matched = false;
    for (const Particle& p2 : ufs.particles(Cuts::pid == PID::ETA)) {
      if (p2.children().empty()) continue;
      bool JpsiParent = false;
      Particle parent = p2;
      while (!parent.parents().empty()) {
        parent = parent.parents()[0];
        if (parent.pid() == PID::JPSI) {
          JpsiParent = true;
          break;
        }
      }
      if (JpsiParent) continue;
      map<long, int> nRes2 = nRes;
      int ncount2 = ncount;
      findChildren(p2, nRes2, ncount2);
      if (ncount2 != 0) continue;
      matched = true;
      for (const auto& val : nRes2) {
        if (val.second != 0) {
          matched = false;
          break;
        }
      }
      if (matched) {
        _nJPsi->fill("3.773"s);
        break;
      }
    }
    if (matched) break;
  }
}


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

/// @}


/// @name Histograms
/// @{
BinnedHistoPtr<string> _nJPsi;
/// @}

};

RIVET_DECLARE_PLUGIN(BESIII_2023_I2617922);

} ```