Rivet analyses

γγ → π+π− for centre-of-mass energies between 1 and 1.5 GeV

Experiment: VENUS (Tristan)

Inspire ID: 392360

Status: VALIDATED

Authors: - Peter Richardson

References: - J.Phys.Soc.Jap. 64 (1995) 435-447

Beams: 22 22

Beam energies: ANY

Run details: - gamma gamma to hadrons

Measurement of the differential cross section for γγ → π+π−. Both the cross section as a function of the centre-of-mass energy of the photonic collision, and the differential cross section with respect to the pion scattering angle are measured.

Source code:VENUS_1995_I392360.cc

// -*- C++ -*-
#include "Rivet/Analysis.hh"
#include "Rivet/Projections/FinalState.hh"

namespace Rivet {


  /// @brief gamma gamma -> pi+pi-
  class VENUS_1995_I392360 : public Analysis {
  public:

    /// Constructor
    RIVET_DEFAULT_ANALYSIS_CTOR(VENUS_1995_I392360);


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

    /// Book histograms and initialise projections before the run
    void init() {
      // Final state
      declare(FinalState(), "FS");
      // check CMS energy in range
      if (!inRange(sqrtS(), 1 * GeV, 1.5 * GeV)) {
        throw Error("Invalid CMS energy for VENUS_1995_I392360");
      }
      // bin for the angle plots
      int ibin = (sqrtS() - 1.0) / 0.05 + 2;
      book(_h_cTheta, ibin, 1, 1);
      book(_cPi, 1, 1, 1);
      _cmax = ibin > 2 ? 0.6 : 0.4;
      if (ibin == 2)
        _axis = YODA::Axis<double>(4, 0.0, 0.4);
      else
        _axis = YODA::Axis<double>(6, 0.0, 0.6);
      _ecms = _cPi.binning().edges<0>()[ibin - 2];
    }


    /// Perform the per-event analysis
    void analyze(const Event& event) {
      if (_edges.empty()) _edges = _h_cTheta->xEdges();
      Particles part = apply<FinalState>(event, "FS").particles();
      if (part.size() != 2) vetoEvent;
      double cTheta(0.);
      bool foundP(false), foundM(false);
      for (const Particle& p : part) {
        if (p.pid() == PID::PIPLUS) {
          foundP = true;
          cTheta = abs(p.momentum().z() / p.momentum().p3().mod());
        }
        else if (p.pid() == PID::PIMINUS)
          foundM = true;
      }
      if (!foundP || !foundM) vetoEvent;
      if (cTheta <= _cmax) _cPi->fill(_ecms);
      if (_h_cTheta) _h_cTheta->fill(map2string(cTheta));
    }

    string map2string(const double value) const {
      const size_t idx = _axis.index(value) - 1;
      if (idx < _edges.size()) return _edges[idx];
      return "OTHER";
    }

    /// Normalise histograms etc., after the run
    void finalize() {
      const double fact = crossSection() / nanobarn / sumOfWeights();
      if (_h_cTheta) {
        scale(_h_cTheta, fact);
        for (auto& b : _h_cTheta->bins()) {
          const size_t idx = b.index();
          b.scaleW(1. / _axis.width(idx));
        }
      }
      scale(_cPi, fact);
    }

    ///@}


    /// @name Histograms
    ///@{
    BinnedHistoPtr<string> _h_cTheta, _cPi;
    double _cmax;
    YODA::Axis<double> _axis;
    vector<string> _edges;
    string _ecms;
    ///@}
  };


  RIVET_DECLARE_PLUGIN(VENUS_1995_I392360);

}