Rivet analyses

title: CMD3_2023_I2634277

Cross section for $e^+e^-\to\pi^+\pi^-$ between threshold and 1.2 GeV

Experiment: CMD3 (VEPP-2M)

Inspire ID: 2634277

Status: VALIDATED NOHEPDATA

Authors: - Peter Richardson

References: - arXiv: 2302.08834

Beams: e+ e-

Beam energies: ANY

Run details: - e+ e- to hadrons

Cross section for $e^+e^-\to\pi^+\pi^-$ between threshold and 1.2 GeV. data on $|F_\pi|^2$ extracted from tables in the paper

Source code:CMD3_2023_I2634277.cc

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/FinalState.hh"

namespace Rivet {

/// @brief e+e- > pi+ pi- class CMD3_2023_I2634277 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(CMD3_2023_I2634277);


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

/// Book histograms and initialise projections before the run
void init() {
  // Initialise and register projections
  declare(FinalState(), "FS");

  // Book histograms
  for (size_t ix = 0; ix < 3; ++ix) {
    book(_sigma[ix], 1, 1 + ix, 1);
    for (const string& en : _sigma[ix].binning().edges<0>()) {
      const double eval = stod(en) * MeV;
      if (isCompatibleWithSqrtS(eval)) {
        _sqs[ix] = en;
        break;
      }
    }
  }
  raiseBeamErrorIf(_sqs[0].empty() && _sqs[1].empty() && _sqs[2].empty());
}


/// Perform the per-event analysis
void analyze(const Event& event) {
  const FinalState& fs = apply<FinalState>(event, "FS");
  if (fs.particles().size() != 2) vetoEvent;
  for (const Particle& p : fs.particles()) {
    if (p.abspid() != PID::PIPLUS) vetoEvent;
  }
  for (size_t ix = 0; ix < 3; ++ix) {
    if (!_sqs[ix].empty()) _sigma[ix]->fill(_sqs[ix]);
  }
}


/// Normalise histograms etc., after the run
void finalize() {
  scale(_sigma, crossSection() / sumOfWeights() / nanobarn);
  const double gev2nb = 0.389e6;
  const double alpha = 1. / 137.035999084;
  for (size_t ix = 0; ix < 3; ++ix) {
    for (auto& b : _sigma[ix]->bins()) {
      const double eval = stod(b.xEdge()) * MeV;
      const double beta = sqrt(1. - 4. * sqr(0.13957039 / eval));
      const double sigma0 = gev2nb * M_PI * sqr(alpha) * beta * sqr(beta) / 3 / sqr(eval);
      b.scaleW(1.0 / sigma0);
    }
  }
}

/// @}


/// @name Histograms
/// @{
BinnedHistoPtr<string> _sigma[3];
string _sqs[3];
/// @}

};

RIVET_DECLARE_PLUGIN(CMD3_2023_I2634277); } ```