Rivet analyses

title: BESIII_2022_I2512962

Cross section for $e^+e^-\to\eta\Lambda\bar{\Lambda}$ from 3.51 to 4.7 GeV

Experiment: BESIII (BEPC)

Inspire ID: 2512962

Status: VALIDATED NOHEPDATA

Authors: - Peter Richardson

References: - arXiv: 2211.10755

Beams: e+ e-

Beam energies: (1.8, 1.8); (1.9, 1.9); (1.9, 1.9); (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); (2.1, 2.1); (2.1, 2.1); (2.1, 2.1); (2.1, 2.1); (2.1, 2.1); (2.1, 2.1); (2.1, 2.1); (2.2, 2.2); (2.2, 2.2); (2.2, 2.2); (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); (2.3, 2.3); (2.3, 2.3); (2.3, 2.3)GeV

Run details: - e+ e- to hadrons

Measurement of the cross section for $e^+e^-\to\eta\Lambda\bar{\Lambda}$ from 3.51 to 4.7 GeV by the BESIII collaboration.

Source code:BESIII_2022_I2512962.cc

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/FinalState.hh"

include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {

/// @brief e+e- -> eta lambda lambda bar class BESIII_2022_I2512962 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2022_I2512962);


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

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

  // Initialise and register projections
  declare(FinalState(), "FS");
  declare(UnstableParticles(), "UFS");
  // counter
  book(_sigma, 1, 1, 1);

  for (const string& en : _sigma.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& eta : ufs.particles(Cuts::pid == 221)) {
    bool matched = false;
    map<long, int> nRes = nCount;
    int ncount = ntotal;
    findChildren(eta, nRes, ncount);
    // then Lambda baryons
    for (const Particle& lambda : ufs.particles(Cuts::pid == 3122)) {
      map<long, int> nResB = nRes;
      int ncountB = ncount;
      findChildren(lambda, nResB, ncountB);
      for (const Particle& lambar : ufs.particles(Cuts::pid == -3122)) {
        map<long, int> nResC = nResB;
        int ncountC = ncountB;
        findChildren(lambar, nResC, ncountC);
        matched = true;
        for (const auto& val : nResC) {
          if (val.second != 0) {
            matched = false;
            break;
          }
        }
        if (matched) {
          _sigma->fill(_sqs);
          break;
        }
      }
      if (matched) break;
    }
    if (matched) break;
  }
}


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

/// @}


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

};

RIVET_DECLARE_PLUGIN(BESIII_2022_I2512962);

} ```