Rivet analyses

title: BELLE_2007_I749358

$\gamma\gamma\to\pi^+\pi^-$ for centre-of-mass energies between 0.8 and 1.5 GeV

Experiment: BELLE (KEKB)

Inspire ID: 749358

Status: VALIDATED

Authors: - Peter Richardson

References: - J.Phys.Soc.Jap. 76 (2007) 074102

Beams: 22 22

Beam energies: ANY

Run details: - gamma gamma to hadrons

Measurement of the differential cross section for $\gamma\gamma\to\pi^+\pi^-$ for $0.8 \text{GeV} < W < 1.5 \text{GeV}$. 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:BELLE_2007_I749358.cc

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/FinalState.hh"

namespace Rivet {

/// @brief gamma gamma -> pi+pi- class BELLE_2007_I749358 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(BELLE_2007_I749358);


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

/// Book histograms and initialise projections before the run
void init() {
  // Final state
  declare(FinalState(), "FS");
  // check CMS energy in range
  _energyAxis = YODA::Axis<double>(140, 0.8, 1.5);
  _thetaAxis = YODA::Axis<double>(12, 0.0, 0.6);
  if (sqrtS() < 0.8 * GeV || sqrtS() > 1.5 * GeV)
    throw Error("Invalid CMS energy for BELLE_2007_I749358");
  eIndex = _energyAxis.index(sqrtS() / GeV) - 1;

  // bin for the angle plots
  int ibin = (sqrtS() - 0.8) / 0.005 + 2;
  book(_h_cTheta, ibin, 1, 1);
  book(_cPi, 1, 1, 1);
}


/// Perform the per-event analysis
void analyze(const Event& event) {
  if (_EcmEdges.empty()) _EcmEdges = _cPi->xEdges();
  if (_thetaEdges.empty()) _thetaEdges = _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 <= 0.6) _cPi->fill(_EcmEdges[eIndex]);
  if (_h_cTheta) {
    size_t idx = _thetaAxis.index(cTheta) - 1;
    _h_cTheta->fill(idx < _thetaEdges.size() ? _thetaEdges[idx] : "OTHER"s);
  }
}


/// 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. / _thetaAxis.width(idx));
    }
  }
  scale(_cPi, fact);
}

///@}


/// @name Histograms
///@{
BinnedHistoPtr<string> _h_cTheta, _cPi;
YODA::Axis<double> _energyAxis;
YODA::Axis<double> _thetaAxis;
vector<string> _EcmEdges, _thetaEdges;
size_t eIndex;
///@}

};

RIVET_DECLARE_PLUGIN(BELLE_2007_I749358);

} ```