Rivet analyses

title: VENUS_1995_I392360

$\gamma\gamma\to\pi^+\pi^-$ 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 $\gamma\gamma\to\pi^+\pi^-$. 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++ // -- 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);

} ```