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CELLO_1992_I345437

$\gamma\gamma\to\pi^+\pi^-$ for centre-of-mass energies between 0.75 and 2 GeV
Experiment: CELLO (Petra)
Inspire ID: 345437
Status: VALIDATED
Authors:
  • Peter Richardson
References:
  • Z.Phys.C 56 (1992) 381-390
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: CELLO_1992_I345437.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 CELLO_1992_I345437 : public Analysis {
 10  public:
 11
 12    /// Constructor
 13    RIVET_DEFAULT_ANALYSIS_CTOR(CELLO_1992_I345437);
 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()<0.75*GeV || sqrtS()>2.*GeV)
 25	throw Error("Invalid CMS energy for CELLO_1992_I345437");
 26      int ibin = (sqrtS()-0.70)/0.05;
 27      if(ibin>0&&ibin<19)
 28	book(_h_cTheta,2,1,ibin);
 29      if(inRange(sqrtS()/GeV,0.85,.95))
 30	book(_h_cTheta2,2,1,19);
 31      else if(inRange(sqrtS()/GeV,1.15,1.25))
 32	book(_h_cTheta2,2,1,20);
 33      else if(inRange(sqrtS()/GeV,1.25,1.35))
 34	book(_h_cTheta2,2,1,21);
 35      book(_cPi, "/TMP/nPi");
 36    }
 37
 38
 39    /// Perform the per-event analysis
 40    void analyze(const Event& event) {
 41      Particles part = applyProjection<FinalState>(event,"FS").particles();
 42      if(part.size()!=2) vetoEvent;
 43      double cTheta(0.);
 44      bool foundP(false),foundM(false);
 45      for(const Particle & p : part) {
 46	if(p.pid()==PID::PIPLUS) {
 47	  foundP=true;
 48	  cTheta = abs(p.momentum().z()/p.momentum().p3().mod());
 49	}
 50	else if(p.pid()==PID::PIMINUS)
 51	  foundM=true;
 52      }
 53      if(!foundP || !foundM) vetoEvent;
 54      if(cTheta<=0.6)    _cPi->fill();
 55      if(_h_cTheta ) _h_cTheta ->fill(cTheta);
 56      if(_h_cTheta2) _h_cTheta2->fill(cTheta);
 57    }
 58
 59
 60    /// Normalise histograms etc., after the run
 61    void finalize() {
 62      double fact = crossSection()/nanobarn/sumOfWeights();
 63      if(_h_cTheta ) scale(_h_cTheta ,fact);
 64      if(_h_cTheta2) scale(_h_cTheta2,fact);
 65      double sigma = _cPi->val()*fact;
 66      double error = _cPi->err()*fact;
 67      Scatter2D temphisto(refData(1, 1, 1));
 68      Scatter2DPtr mult;
 69      book(mult, 1, 1, 1);
 70      for (size_t b = 0; b < temphisto.numPoints(); b++) {
 71	const double x  = temphisto.point(b).x();
 72	pair<double,double> ex = temphisto.point(b).xErrs();
 73	pair<double,double> ex2 = ex;
 74	if(ex2.first ==0.) ex2. first=0.0001;
 75	if(ex2.second==0.) ex2.second=0.0001;
 76	if (inRange(sqrtS(), x-ex2.first, x+ex2.second)) {
 77	  mult->addPoint(x, sigma, ex, make_pair(error,error));
 78	}
 79	else {
 80	  mult->addPoint(x, 0., ex, make_pair(0.,.0));
 81	}
 82      }
 83    }
 84
 85    ///@}
 86
 87
 88    /// @name Histograms
 89    ///@{
 90    Histo1DPtr _h_cTheta,_h_cTheta2;
 91    CounterPtr _cPi;
 92    ///@}
 93
 94
 95  };
 96
 97
 98  RIVET_DECLARE_PLUGIN(CELLO_1992_I345437);
 99
100}