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MARKII_1990_I304882

$\gamma\gamma\to\pi^+\pi^-$ for centre-of-mass energies between 0.35 and 1.6 GeV
Experiment: MARKII (PEP)
Inspire ID: 304882
Status: VALIDATED
Authors:
  • Peter Richardson
References:
  • Phys.Rev.D 42 (1990) 1350-1367
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 $0.35 \text{GeV} < W < 1.6 \text{GeV}$. 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: MARKII_1990_I304882.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 MARKII_1990_I304882 : public Analysis {
 10  public:
 11
 12    /// Constructor
 13    RIVET_DEFAULT_ANALYSIS_CTOR(MARKII_1990_I304882);
 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()<0.35*GeV || sqrtS()>1.6*GeV)
 25	throw Error("Invalid CMS energy for MARKII_1990_I304882");
 26      for(unsigned int ix=0;ix<7;++ix) {
 27	std::ostringstream title;
 28	title << "/TMP/nPi_" << ix;
 29	book(_cPi[ix], title.str());
 30      }
 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      int ibin = cTheta/0.1;
 50      if(ibin>5) vetoEvent;
 51      _cPi[   0  ]->fill();
 52      _cPi[ibin+1]->fill();
 53    }
 54
 55
 56    /// Normalise histograms etc., after the run
 57    void finalize() {
 58      double fact = crossSection()/nanobarn/sumOfWeights();
 59      for(unsigned int ih=0;ih<7;++ih) {
 60	unsigned int ix=2,iy=ih;
 61	if(ih==0) {
 62	  ix=1;
 63	  iy=1;
 64	}
 65	double sigma = _cPi[ih]->val()*fact;
 66	double error = _cPi[ih]->err()*fact;
 67	Scatter2D temphisto(refData(ix, 1, iy));
 68	Scatter2DPtr mult;
 69	book(mult, ix, 1, iy);
 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
 89    /// @name Histograms
 90    ///@{
 91    CounterPtr _cPi[7];
 92    ///@}
 93
 94
 95  };
 96
 97
 98  RIVET_DECLARE_PLUGIN(MARKII_1990_I304882);
 99
100}