Rivet analyses

title: BESIII_2016_I1385603

Cross section for $e^+e^-\to \pi^+\pi^-$ between 0.6 and 0.9 GeV

Experiment: BES III (BEPC II)

Inspire ID: 1385603

Status: VALIDATED

Authors: - Peter Richardson

References: - Phys. Lett. B753 (2016) 629-638

Beams: e+ e-

Beam energies: ANY

Run details: - e+ e- to hadrons below 0.6 and 0.9 GeV

Measurement of the cross section for $e^+e^-\to \pi^+\pi^-$ at energies between 0.6 and 0.9 GeV. Useful for comparing models of the pion form factor.

Source code:BESIII_2016_I1385603.cc

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/FinalState.hh"

namespace Rivet {

/// @brief Cross section for $e^+e^-\to \pi^+\pi^-$ between 0.6 and 0.9 GeV class BESIII_2016_I1385603 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2016_I1385603);


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

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

  // Initialise and register projections
  declare(FinalState(), "FS");

  // Book histograms
  book(_npion, "TMP/pion");
}


/// 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 (abs(p.pid()) != PID::PIPLUS) vetoEvent;
  }
  _npion->fill();
}


/// Normalise histograms etc., after the run
void finalize() {
  double sigma = _npion->val();
  double error = _npion->err();
  sigma *= crossSection() / sumOfWeights() / nanobarn;
  error *= crossSection() / sumOfWeights() / nanobarn;
  Estimate1DPtr mult;
  book(mult, 1, 1, 1);
  for (auto& b : mult->bins()) {
    if (inRange(sqrtS() / GeV, b.xMin(), b.xMax())) {
      b.set(sigma, error);
    }
  }
}

/// @}


/// @name Histograms
/// @{
CounterPtr _npion;
/// @}

};

RIVET_DECLARE_PLUGIN(BESIII_2016_I1385603);

} ```