MEPP2GammaGamma.h

// -*- C++ -*-
//
// MEPP2GammaGamma.h is a part of Herwig - A multi-purpose Monte Carlo event generator
// Copyright (C) 2002-2019 The Herwig Collaboration
//
// Herwig is licenced under version 3 of the GPL, see COPYING for details.
// Please respect the MCnet academic guidelines, see GUIDELINES for details.
//
#ifndef HERWIG_MEPP2GammaGamma_H
#define HERWIG_MEPP2GammaGamma_H
//
// This is the declaration of the MEPP2GammaGamma class.
//
 
#include "Herwig/MatrixElement/HwMEBase.h"
#include "ThePEG/Helicity/Vertex/AbstractFFVVertex.h"
#include "Herwig/MatrixElement/ProductionMatrixElement.h"
#include "ThePEG/Helicity/WaveFunction/SpinorWaveFunction.h"
#include "ThePEG/Helicity/WaveFunction/VectorWaveFunction.h"
#include "ThePEG/Helicity/WaveFunction/SpinorBarWaveFunction.h"
 
namespace Herwig {
 
using namespace ThePEG;
using namespace ThePEG::Helicity;
 
class MEPP2GammaGamma: public HwMEBase {
 
public:
 
  MEPP2GammaGamma() : _maxflavour(5),_process(0), scalePreFactor_(1.) {
    massOption(vector<unsigned int>(2,0));
  }
  
  virtual unsigned int orderInAlphaS() const;
 
  virtual unsigned int orderInAlphaEW() const;
 
  virtual double me2() const;
 
  virtual Energy2 scale() const;
 
  virtual void getDiagrams() const;
 
  virtual Selector<DiagramIndex> diagrams(const DiagramVector & dv) const;
 
  virtual Selector<const ColourLines *>
  colourGeometries(tcDiagPtr diag) const;
 
  virtual void constructVertex(tSubProPtr);
 
 
public:
 
  void persistentOutput(PersistentOStream & os) const;
 
  void persistentInput(PersistentIStream & is, int version);
 
  static void Init();
 
protected:
 
  virtual IBPtr clone() const;
 
  virtual IBPtr fullclone() const;
 
protected:
 
  virtual void doinit();
 
private:
 
  double qqbarME(vector<SpinorWaveFunction> & fin, vector<SpinorBarWaveFunction> & ain,
                 vector<VectorWaveFunction> & p1 , vector<VectorWaveFunction>    & p2 ,
                 bool me) const;
 
  double ggME(vector<VectorWaveFunction> & g1 , vector<VectorWaveFunction>    & g2 ,
              vector<VectorWaveFunction> & p1 , vector<VectorWaveFunction>    & p2 ,
              bool me) const;
 
  Complex ggme(Energy2 s,Energy2 t,Energy2 u) const {
    double ltu(log(abs(t/u)));
    double frac1((t-u)/s),frac2((sqr(t)+sqr(u))/sqr(s));
    double thetatu = (t/u<0) ? 0 : 1;
    double thetat  = (t<ZERO)   ? 0 : 1;
    double thetau  = (u<ZERO)   ? 0 : 1;
    using Constants::pi;
    return Complex(1.+frac1*ltu+0.5*frac2*(sqr(ltu)+sqr(pi)*thetatu),
                   -pi*(thetat-thetau)*(frac1+frac2*ltu));
  }
 
private:
 
  MEPP2GammaGamma & operator=(const MEPP2GammaGamma &) = delete;
 
private:
 
  AbstractFFVVertexPtr _photonvertex;
 
  unsigned int _maxflavour;
 
  unsigned int _process;
  
  ProductionMatrixElement _me;
 
  mutable double _diagwgt[2];
 
  double scalePreFactor_;
};
 
}
 
#endif /* HERWIG_MEPP2GammaGamma_H */