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VENUS_1995_I392360

$\gamma\gamma\to\pi^+\pi^-$ for centre-of-mass energies between 1 and 1.5 GeV
Experiment: VENUS (Tristan)
Inspire ID: 392360
Status: VALIDATED
Authors:
  • Peter Richardson
References:
  • J.Phys.Soc.Jap. 64 (1995) 435-447
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: VENUS_1995_I392360.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 VENUS_1995_I392360 : public Analysis {
10  public:
11
12    /// Constructor
13    RIVET_DEFAULT_ANALYSIS_CTOR(VENUS_1995_I392360);
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      if(sqrtS()<1.*GeV || sqrtS()>1.5*GeV)
25	throw Error("Invalid CMS energy for VENUS_1995_I392360");
26      // bin for the angle plots
27      int ibin = (sqrtS()-1.0)/0.05 + 2;
28      book(_h_cTheta,ibin,1,1);
29      book(_cPi, "/TMP/nPi");
30      _cmax = ibin>2 ? 0.6 : 0.4;
31    }
32
33
34    /// Perform the per-event analysis
35    void analyze(const Event& event) {
36      Particles part = applyProjection<FinalState>(event,"FS").particles();
37      if(part.size()!=2) vetoEvent;
38      double cTheta(0.);
39      bool foundP(false),foundM(false);
40      for(const Particle & p : part) {
41	if(p.pid()==PID::PIPLUS) {
42	  foundP=true;
43	  cTheta = abs(p.momentum().z()/p.momentum().p3().mod());
44	}
45	else if(p.pid()==PID::PIMINUS)
46	  foundM=true;
47      }
48      if(!foundP || !foundM) vetoEvent;
49      if(cTheta<=_cmax)    _cPi->fill();
50      if(_h_cTheta ) _h_cTheta ->fill(cTheta);
51    }
52
53
54    /// Normalise histograms etc., after the run
55    void finalize() {
56      double fact = crossSection()/nanobarn/sumOfWeights();
57      if(_h_cTheta ) scale(_h_cTheta ,fact);
58      double sigma = _cPi->val()*fact;
59      double error = _cPi->err()*fact;
60      Scatter2D temphisto(refData(1, 1, 1));
61      Scatter2DPtr mult;
62      book(mult, 1, 1, 1);
63      for (size_t b = 0; b < temphisto.numPoints(); b++) {
64	const double x  = temphisto.point(b).x();
65	pair<double,double> ex = temphisto.point(b).xErrs();
66	pair<double,double> ex2 = ex;
67	if(ex2.first ==0.) ex2. first=0.0001;
68	if(ex2.second==0.) ex2.second=0.0001;
69	if (inRange(sqrtS(), x-ex2.first, x+ex2.second)) {
70	  mult->addPoint(x, sigma, ex, make_pair(error,error));
71	}
72	else {
73	  mult->addPoint(x, 0., ex, make_pair(0.,.0));
74	}
75      }
76    }
77
78    ///@}
79
80
81    /// @name Histograms
82    ///@{
83    Histo1DPtr _h_cTheta;
84    CounterPtr _cPi;
85    double _cmax;
86    ///@}
87
88
89  };
90
91
92  RIVET_DECLARE_PLUGIN(VENUS_1995_I392360);
93
94}