Rivet analyses

title: CRYSTAL_BALL_1990_I294492

$\gamma\gamma\to \pi^0\pi^0$ for centre-of-mass energies between 0.25 and 1.95 GeV

Experiment: CRYSTAL_BALL (DORIS)

Inspire ID: 294492

Status: UNVALIDATED

Authors: - Peter Richardson

References: - Phys.Rev.D 41 (1990) 3324, 1990

Beams: 22 22

Beam energies: ANY

Run details: - gamma gamma to hadrons, pi0 must be set stable

Measurement of the differential cross section for $\gamma\gamma\to \pi^0\pi^0$ for $0.25 \text{GeV} < W < 1.95 \text{GeV}$. 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:CRYSTAL_BALL_1990_I294492.cc

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/FinalState.hh"

namespace Rivet {

/// @brief gamma gamma -> pi0pi0 class CRYSTAL_BALL_1990_I294492 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(CRYSTAL_BALL_1990_I294492);


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

/// Book histograms and initialise projections before the run
void init() {
  // Initialise and register projections
  declare(FinalState(), "FS");
  // histos
  const double eval = sqrtS() / GeV;
  raiseBeamErrorIf(!inRange(eval, 0.25, 1.95));

  size_t ih = 0;
  for (double emax : {0.5, 0.7, 0.9, 1.1, 1.3, 1.7}) {
    string label = toString(1000 * emax);
    book(_h[label], 2, 1, ++ih);
    if (_sqs.empty() && eval < emax) _sqs = label;
  }

  if (inRange(eval, 1.1, 1.5, RangeBoundary::CLOSED, RangeBoundary::CLOSED)) {
    _sqs15 = "1500"s;
  }
  book(_h["1500"], 2, 1, 7);
  _edges = _h["1500"].binning().edges<0>();

  book(_h["npipi"], 1, 1, 1);
  for (const string& en : _h["npipi"].binning().edges<0>()) {
    if (isCompatibleWithSqrtS(stod(en))) {
      _edge = en;
      break;
    }
  }
  _axis = YODA::Axis<double>({0., 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8});
}


/// Perform the per-event analysis
void analyze(const Event& event) {
  Particles part = apply<FinalState>(event, "FS").particles();
  if (part.size() != 2) vetoEvent;
  for (const Particle& p : part) {
    if (p.pid() != PID::PI0) vetoEvent;
  }
  const double cTheta = abs(part[0].mom().z() / part[0].mom().p3().mod());
  if (cTheta <= 0.8) _h["npipi"]->fill(_edge);
  if (!_sqs.empty()) _h[_sqs]->fill(disc(cTheta));
  if (!_sqs15.empty()) _h["1500"]->fill(disc(cTheta));
}


string disc(const double value) const {
  size_t idx = _axis.index(value);
  if (0 < idx && idx <= _edges.size()) return _edges[idx - 1];
  return "OTHER"s;
}


/// Normalise histograms etc., after the run
void finalize() {
  scale(_h, crossSection() / nanobarn / sumOfWeights());
  for (auto& item : _h) {
    if (item.first == "npipi"s) continue;
    for (auto& b : item.second->bins()) {
      const size_t idx = b.index();
      b.scaleW(1. / _axis.width(idx));
    }
  }
}

/// @}


/// @name Histograms
/// @{
map<string, BinnedHistoPtr<string>> _h;
YODA::Axis<double> _axis;
vector<string> _edges;
string _sqs = "", _sqs15 = "", _edge = "";
/// @}

};

RIVET_DECLARE_PLUGIN(CRYSTAL_BALL_1990_I294492);

} ```