Rivet analyses

title: CELLO_1992_I345437

$\gamma\gamma\to\pi^+\pi^-$ for centre-of-mass energies between 0.75 and 2 GeV

Experiment: CELLO (Petra)

Inspire ID: 345437

Status: VALIDATED

Authors: - Peter Richardson

References: - Z.Phys.C 56 (1992) 381-390

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:CELLO_1992_I345437.cc

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/FinalState.hh"

namespace Rivet {

/// @brief gamma gamma -> pi+pi- class CELLO_1992_I345437 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(CELLO_1992_I345437);


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

/// Book histograms and initialise projections before the run
void init() {
  // Final state
  declare(FinalState(), "FS");
  // check CMS energy in range
  if (sqrtS() < 0.75 * GeV || sqrtS() > 2 * GeV) throw Error("Invalid CMS energy for CELLO_1992_I345437");
  int ibin = (sqrtS() - 0.70) / 0.05;
  if (ibin > 0 && ibin < 19) book(_h_cTheta, 2, 1, ibin);
  if (inRange(sqrtS() / GeV, 0.85, .95))
    book(_h_cTheta2, 2, 1, 19);
  else if (inRange(sqrtS() / GeV, 1.15, 1.25))
    book(_h_cTheta2, 2, 1, 20);
  else if (inRange(sqrtS() / GeV, 1.25, 1.35))
    book(_h_cTheta2, 2, 1, 21);

  book(_cPi, 1, 1, 1);
  _thetaAxis = YODA::Axis<double>(16, 0.0, 0.8);
}


/// Perform the per-event analysis
void analyze(const Event& event) {

  if (ecmEdge == "") ecmEdge = _cPi->bin(ecmAxis.index(sqrtS() / GeV)).xEdge();
  if (_h_cTheta && thetaEdges[0].empty()) thetaEdges[0] = _h_cTheta->xEdges();
  if (_h_cTheta2 && thetaEdges[1].empty()) thetaEdges[1] = _h_cTheta2->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(ecmEdge);
  if (_h_cTheta) _h_cTheta->fill(map2string(cTheta, 0));
  if (_h_cTheta2) _h_cTheta2->fill(map2string(cTheta, 1));
}


/// 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));
    }
  }
  if (_h_cTheta2) {
    scale(_h_cTheta2, fact);
    for (auto& b : _h_cTheta2->bins()) {
      const size_t idx = b.index();
      b.scaleW(1. / _thetaAxis.width(idx));
    }
  }
  scale(_cPi, fact);
}

/// @}

string map2string(const double theta, const size_t alt) const {
  const size_t idx = _thetaAxis.index(theta) - 1;
  if (idx < thetaEdges[alt].size()) return thetaEdges[alt][idx];
  return "OTHER";
}

/// @name Histograms
/// @{
BinnedHistoPtr<string> _cPi, _h_cTheta, _h_cTheta2;
vector<string> thetaEdges[2];
YODA::Axis<double> _thetaAxis;
YODA::Axis<double> ecmAxis{0.75, 0.775, 0.8, 0.825, 0.85, 0.875, 0.9,     0.925, 0.95, 0.975, 1.0, 1.025,
                           1.05, 1.075, 1.1, 1.125, 1.15, 1.175, 1.2,     1.225, 1.25, 1.275, 1.3, 1.325,
                           1.35, 1.375, 1.4, 1.425, 1.45, 1.475, 1.51875, 1.6,   1.7,  1.8,   1.9, 2.0};
string ecmEdge = "";
/// @}

};

RIVET_DECLARE_PLUGIN(CELLO_1992_I345437);

} ```