Rivet analyses

title: BESIII_2019_I1623214

Cross section for $e^+e^-\to \eta Y(2175)$ for energies above 3.7 GeV

Experiment: BESIII (BEPC)

Inspire ID: 1623214

Status: VALIDATED NOHEPDATA

Authors: - Peter Richardson

References: - Phys.Rev.D 99 (2019) 1, 012014

Beams: e+ e-

Beam energies: (1.8, 1.8); (1.9, 1.9); (2.0, 2.0); (2.1, 2.1); (2.1, 2.1); (2.2, 2.2); (2.2, 2.2); (2.3, 2.3)GeV

Run details: - e+ e- to hadrons

Measurement of the cross section for $e^+e^-\to \eta Y(2175)$ times the branching ratio $Y(2175)\to\phi f_0(f_0\to\pi^+\pi^-)$ for energies above 3.7 GeV. The PDG code for the $Y(2175)$ is not defined but the most probable assignment is that it is the $\phi$ member of the $^3D_1$ multiplet so we take its PDG code to be 30333, although this can be changed using the PID option if a different code is used by the event generator performing the simulation.

Source code:BESIII_2019_I1623214.cc

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/FinalState.hh"

include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {

/// @brief e+ e- > eta Y(2175) class BESIII_2019_I1623214 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2019_I1623214);


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

/// Book histograms and initialise projections before the run
void init() {
  // set the PDG code
  _pid = getOption<double>("PID", 30333);
  // 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()];
      --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 any eta mesons
  for (const Particle& eta : ufs.particles(Cuts::pid == 221)) {
    bool matched = false;
    if (eta.children().empty()) continue;
    map<long, int> nRes = nCount;
    int ncount = ntotal;
    findChildren(eta, nRes, ncount);
    for (const Particle& phi : ufs.particles(Cuts::pid == 333)) {
      if (phi.children().empty()) continue;
      map<long, int> nRes2 = nRes;
      int ncount2 = ncount;
      findChildren(phi, nRes2, ncount2);
      matched = true;
      // required eta phi pi+ pi- final state
      for (const auto& val : nRes2) {
        if (abs(val.first) == 211) {
          if (val.second != 1) {
            matched = false;
            break;
          }
        }
        else if (val.second != 0) {
          matched = false;
          break;
        }
      }
      if (!matched) continue;
      matched = false;
      // finally check we have f0
      for (const Particle& f0 : ufs.particles(Cuts::pid == 9010221)) {
        if (f0.children().empty()) continue;
        map<long, int> nRes3 = nRes2;
        int ncount3 = ncount2;
        findChildren(f0, nRes3, ncount3);
        matched = true;
        for (const auto& val : nRes3) {
          if (val.second != 0) {
            matched = false;
            break;
          }
        }
        if (matched) break;
      }
      if (matched) break;
    }
    if (!matched) continue;
    // finally check phi(3D1) present
    for (const Particle& phi : ufs.particles(Cuts::pid == _pid)) {
      if (phi.children().empty()) continue;
      map<long, int> nRes2 = nRes;
      int ncount2 = ncount;
      findChildren(phi, nRes2, ncount2);
      matched = true;
      // required eta phi(3D1) final state
      for (const auto& val : nRes2) {
        if (val.second != 0) {
          matched = false;
          break;
        }
      }
      if (matched) break;
    }
    if (matched) {
      _sigma->fill(_sqs);
      break;
    }
  }
}


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

/// @}


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

};

RIVET_DECLARE_PLUGIN(BESIII_2019_I1623214);

} ```