Rivet analyses

title: BESIII_2024_I2753516

$\eta$ spectra for various energies between 2 and 3.671 GeV

Experiment: BESIII (BEPC)

Inspire ID: 2753516

Status: VALIDATED NOHEPDATA

Authors: - Peter Richardson

References: - Phys.Rev.Lett. 133 (2024) 2, 021901 - arXiv: 2401.17873

Beams: e+ e-

Beam energies: (1.0, 1.0); (1.1, 1.1); (1.2, 1.2); (1.3, 1.3); (1.4, 1.4); (1.5, 1.5); (1.8, 1.8); (1.8, 1.8)GeV

Run details: - e+e- > hadrons

Measurement of the momentum spectrum for $\eta$ production in $e^+e^-$ collisions at low energies by BES.

Source code:BESIII_2024_I2753516.cc

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {

/// @brief eta spectra class BESIII_2024_I2753516 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2024_I2753516);


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

/// Book histograms and initialise projections before the run
void init() {
  // Initialise and register projections
  declare(UnstableParticles(Cuts::pid == 221), "UFS");
  // histos
  size_t ih = 1;
  for (double eVal : allowedEnergies()) {

    const string en = toString(round(eVal / MeV));
    if (isCompatibleWithSqrtS(eVal)) _sqs = en;
    book(_c[en], "_aux_sigma_" + en);
    book(_h[en], 1, 1, ih);
    ++ih;
  }
  raiseBeamErrorIf(_sqs.empty());
}


/// Perform the per-event analysis
void analyze(const Event& event) {
  _c[_sqs]->fill();
  const UnstableParticles& ufs = apply<UnstableParticles>(event, "UFS");
  for (const Particle& p : ufs.particles()) {
    _h[_sqs]->fill(p.mom().p3().mod());
  }
}


/// Normalise histograms etc., after the run
void finalize() {
  scale(_h, crossSectionPerEvent());
  scale(_c, crossSectionPerEvent());
  for (const auto& item : _c) {
    if (isZero(item.second->sumW())) continue;
    scale(_h[item.first], 1.0 / item.second->sumW());
  }
}

/// @}


/// @name Histograms
/// @{
map<string, Histo1DPtr> _h;
map<string, CounterPtr> _c;
string _sqs = "";
/// @}

};

RIVET_DECLARE_PLUGIN(BESIII_2024_I2753516);

} ```