Rivet analyses

title: BESIII_2015_I1356733

Cross section for $e^+e^-\to\pi^+\pi^-X(3823)(\to\gamma\chi_{c1})$ and $\pi^+\pi^-\psi(2S)$ for $\sqrt{s}=4.23$ to $4.6$ GeV

Experiment: BESIII (BEPC)

Inspire ID: 1356733

Status: VALIDATED NOHEPDATA SINGLEWEIGHT

Authors: - Peter Richardson

References: - Phys.Rev.Lett. 115 (2015) 1, 011803

Beams: e+ e-

Beam energies: (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, pi0 set stable

Measurement of the cross section for $e^+e^-\to\pi^+\pi^-X(3823)(\to\gamma\chi_{c1})$ and $\pi^+\pi^-\psi(2S)$ for $\sqrt{s}=4.23$ to $4.6$ GeV. The $X(3823)$ is consistent with the $\psi(1^3D_2)$ state and therefore we use the PDG code 20445, although this can be changed using the PID option.

Source code:BESIII_2015_I1356733.cc

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/FinalState.hh"

include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {

/// @brief e+e- to hadrons class BESIII_2015_I1356733 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2015_I1356733);


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

/// Book histograms and initialise projections before the run
void init() {
  // set the PDG code
  _pid = getOption<double>("PID", 20445);
  // projections
  declare(FinalState(), "FS");
  declare(UnstableParticles(Cuts::pid == 20443 || Cuts::pid == 100443), "UFS");
  // histograms
  for (size_t ix = 0; ix < 2; ++ix) {
    book(_sigma[ix], 1, 1, 1 + ix);
  }

  for (const string& en : _sigma[0].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;
  }
  for (const Particle& psi : apply<UnstableParticles>(event, "UFS").particles()) {
    if (psi.children().empty()) continue;
    map<long, int> nRes = nCount;
    int ncount = ntotal;
    findChildren(psi, nRes, ncount);
    int nphoton = psi.pid() == 100443 ? 0 : 1;
    if (ncount != 2 + nphoton) continue;
    bool matched = true;
    for (const auto& val : nRes) {
      if (abs(val.first) == 211) {
        if (val.second != 1) {
          matched = false;
          break;
        }
      }
      else if (val.first == 22) {
        if (val.second != nphoton) {
          matched = false;
          break;
        }
      }
      else if (val.second != 0) {
        matched = false;
        break;
      }
    }
    if (matched) {
      if (nphoton == 0) {
        _sigma[1]->fill(_sqs);
        break;
      }
      else {
        Particle parent = psi.parents()[0];
        if (parent.pid() == _pid && parent.children().size() == 2
            && (parent.children()[0].pid() == 22 || parent.children()[1].pid() == 22)) {
          _sigma[0]->fill(_sqs);
          break;
        }
      }
    }
  }
}


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

/// @}


/// @name Histograms
/// @{
BinnedHistoPtr<string> _sigma[2];
string _sqs = "";
int _pid;
/// @}

};

RIVET_DECLARE_PLUGIN(BESIII_2015_I1356733);

} ```