Rivet analyses

γγ → π+π− 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 γγ → π+π− for 0.35GeV < W < 1.6GeV. 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

// -*- C++ -*-
#include "Rivet/Analysis.hh"
#include "Rivet/Projections/FinalState.hh"

namespace Rivet {


  /// @brief gamma gamma -> pi+ pi-
  class MARKII_1990_I304882 : public Analysis {
  public:

    /// Constructor
    RIVET_DEFAULT_ANALYSIS_CTOR(MARKII_1990_I304882);


    /// @name Analysis methods
    ///@{

    /// Book histograms and initialise projections before the run
    void init() {
      // Final state
      declare(FinalState(),"FS");
      // check CMS energy in range
      if (sqrtS()<0.35*GeV || sqrtS()>1.6*GeV)
        throw Error("Invalid CMS energy for MARKII_1990_I304882");
      for (unsigned int ix=0;ix<7;++ix) {
        std::ostringstream title;
        title << "/TMP/nPi_" << ix;
        book(_cPi[ix], title.str());
      }
    }


    /// Perform the per-event analysis
    void analyze(const Event& event) {
      Particles part = apply<FinalState>(event,"FS").particles();
      if (part.size()!=2) vetoEvent;
      double cTheta(0.);
      bool foundP(false),foundM(false);
      for (const Particle & p : part) {
        if (p.pid()==PID::PIPLUS) {
          foundP=true;
          cTheta = abs(p.momentum().z()/p.momentum().p3().mod());
        }
        else if (p.pid()==PID::PIMINUS)
          foundM=true;
      }
      if (!foundP || !foundM) vetoEvent;
      int ibin = cTheta/0.1;
      if (ibin>5) vetoEvent;
      _cPi[   0  ]->fill();
      _cPi[ibin+1]->fill();
    }


    /// Normalise histograms etc., after the run
    void finalize() {
      double fact = crossSection()/nanobarn/sumOfWeights();
      for (unsigned int ih=0;ih<7;++ih) {
        unsigned int ix=2,iy=ih;
        if (ih==0) {
          ix=1;
          iy=1;
        }
        double sigma = _cPi[ih]->val()*fact;
        double error = _cPi[ih]->err()*fact;
        Estimate1DPtr mult;
        book(mult, ix, 1, iy);
        for (auto& b : mult->bins()) {
          if (inRange(sqrtS(), b.xMin(), b.xMax())) {
            b.set(sigma, error);
          }
        }
      }
    }

    ///@}


    /// @name Histograms
    ///@{
    CounterPtr _cPi[7];
    ///@}


  };


  RIVET_DECLARE_PLUGIN(MARKII_1990_I304882);

}