Rivet analyses

γγ → π0π0 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 γγ → π0π0 for 0.6GeV < W < 4.1GeV. 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++ -*-
#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);

}