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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
 1// -*- C++ -*-
 2#include "Rivet/Analysis.hh"
 3#include "Rivet/Projections/FinalState.hh"
 4
 5namespace Rivet {
 6
 7
 8  /// @brief gamma gamma -> pi+pi-
 9  class BELLE_2007_I749358 : public Analysis {
10  public:
11
12    /// Constructor
13    RIVET_DEFAULT_ANALYSIS_CTOR(BELLE_2007_I749358);
14
15
16    /// @name Analysis methods
17    ///@{
18
19    /// Book histograms and initialise projections before the run
20    void init() {
21      // Final state
22      declare(FinalState(),"FS");
23      // check CMS energy in range
24      _energyAxis = YODA::Axis<double>(140, 0.8, 1.5);
25      _thetaAxis = YODA::Axis<double>(12, 0.0, 0.6);
26      if (sqrtS()<0.8*GeV || sqrtS()>1.5*GeV)
27        throw Error("Invalid CMS energy for BELLE_2007_I749358");
28      eIndex = _energyAxis.index(sqrtS()/GeV) - 1;
29
30      // bin for the angle plots
31      int ibin = (sqrtS()-0.8)/0.005 + 2;
32      book(_h_cTheta,ibin,1,1);
33      book(_cPi, "/TMP/nPi");
34    }
35
36
37    /// Perform the per-event analysis
38    void analyze(const Event& event) {
39
40      if (_EcmEdges.empty())  _EcmEdges = _cPi->xEdges();
41      if (_thetaEdges.empty())  _thetaEdges = _h_cTheta->xEdges();
42
43      Particles part = apply<FinalState>(event,"FS").particles();
44      if (part.size()!=2) vetoEvent;
45      double cTheta(0.);
46      bool foundP(false),foundM(false);
47      for (const Particle& p : part) {
48        if (p.pid()==PID::PIPLUS) {
49          foundP=true;
50          cTheta = abs(p.momentum().z()/p.momentum().p3().mod());
51        }
52        else if (p.pid()==PID::PIMINUS)
53          foundM=true;
54      }
55      if (!foundP || !foundM)  vetoEvent;
56      if (cTheta<=0.6)  _cPi->fill(_EcmEdges[eIndex]);
57      if (_h_cTheta ) {
58        size_t idx = _thetaAxis.index(cTheta) - 1;
59        _h_cTheta->fill(_thetaEdges[idx]);
60      }
61    }
62
63
64    /// Normalise histograms etc., after the run
65    void finalize() {
66      const double fact = crossSection()/nanobarn/sumOfWeights();
67      if (_h_cTheta ) scale(_h_cTheta ,fact);
68      scale(_cPi, fact);
69    }
70
71    ///@}
72
73
74    /// @name Histograms
75    ///@{
76    BinnedHistoPtr<string> _h_cTheta, _cPi;
77    YODA::Axis<double> _energyAxis;
78    YODA::Axis<double> _thetaAxis;
79    vector<string> _EcmEdges, _thetaEdges;
80    size_t eIndex;
81    ///@}
82
83
84  };
85
86
87  RIVET_DECLARE_PLUGIN(BELLE_2007_I749358);
88
89}