Rivet analyses

title: BELLE_2009_I815978

$\gamma\gamma\to \pi^0\pi^0$ for centre-of-mass energies between 0.6 and 4.1 GeV

Experiment: BELLE (KEKB)

Inspire ID: 815978

Status: VALIDATED

Authors: - Peter Richardson

References: - Phys.Rev.D 79 (2009) 052009, 2009

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 $0.6 \text{GeV} < W < 4.1 \text{GeV}$. 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 kaon scattering angle are measured.

Source code:BELLE_2009_I815978.cc

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/FinalState.hh"

namespace Rivet {

/// @brief gamma gamma -> pi0 pi0 class BELLE_2009_I815978 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(BELLE_2009_I815978);


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

/// Book histograms and initialise projections before the run
void init() {
  // Initialise and register projections
  declare(FinalState(), "FS");
  // histos
  if (inRange(sqrtS() / GeV, 0.6, 3.3) || inRange(sqrtS() / GeV, 3.6, 4.1)) {
    if (sqrtS() > 0.72) book(_sigmapipi[0], "TMP/npipi_1", refData(31, 1, 1));
    book(_sigmapipi[1], "TMP/npipi_2", refData(31, 1, 2));
    double sMin = 0.6, step = 0.02;
    unsigned int ihist = 1, iy = 1;
    while (sMin < 4.1) {
      if (inRange(sqrtS() / GeV, sMin, sMin + step)) {
        break;
      }
      sMin += step;
      iy += 1;
      if (iy == 4) {
        ihist += 1;
        iy = 1;
      }
      if (fuzzyEquals(1.8, sMin))
        step = 0.04;
      else if (fuzzyEquals(2.4, sMin))
        step = 0.1;
      else if (fuzzyEquals(3.2, sMin))
        sMin = 3.6;
    }
    if (!inRange(sqrtS() / GeV, 3.2, 3.6)) book(_h_cTheta, ihist, 1, iy);
  }
  else {
    throw Error("Invalid CMS energy for BELLE_2009_I815978");
  }
}


/// 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;
  }
  double cTheta = abs(part[0].mom().z() / part[0].mom().p3().mod());
  if (cTheta <= 0.6 && _sigmapipi[0]) _sigmapipi[0]->fill(sqrtS());
  if (cTheta <= 0.8 && _sigmapipi[1]) _sigmapipi[1]->fill(sqrtS());
  if (_h_cTheta) _h_cTheta->fill(cTheta);
}


/// Normalise histograms etc., after the run
void finalize() {
  double fact = crossSection() / nanobarn / sumOfWeights();
  if (_h_cTheta) scale(_h_cTheta, fact);
  for (unsigned int ix = 0; ix < 2; ++ix) {
    if (!_sigmapipi[ix]) continue;
    scale(_sigmapipi[ix], fact);
    Estimate1DPtr tmp;
    book(tmp, 31, 1, 1 + ix);
    barchart(_sigmapipi[ix], tmp);
  }
}

/// @}


/// @name Histograms
/// @{
Histo1DPtr _sigmapipi[2], _h_cTheta;
/// @}

};

RIVET_DECLARE_PLUGIN(BELLE_2009_I815978);

} ```