Rivet analyses

Cross section for e+e− → π+π− 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− → π+π− 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++ -*-
#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);


}