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 // -*- 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;
 
 /**
  * The MEPP2GammaGamma class implements the production of photon pairs in 
  * hadron hadron collisions.
  *
  * @see \ref MEPP2GammaGammaInterfaces "The interfaces"
  * defined for MEPP2GammaGamma.
  */
 class MEPP2GammaGamma: public HwMEBase {
 
 public:
 
   /**
    * The default constructor.
    */
   MEPP2GammaGamma() : _maxflavour(5),_process(0), scalePreFactor_(1.) {
     massOption(vector<unsigned int>(2,0));
   }
   
   /** @name Virtual functions required by the MEBase class. */
   //@{
   /**
    * Return the order in \f$\alpha_S\f$ in which this matrix
    * element is given.
    */
   virtual unsigned int orderInAlphaS() const;
 
   /**
    * Return the order in \f$\alpha_{EW}\f$ in which this matrix
    * element is given.
    */
   virtual unsigned int orderInAlphaEW() const;
 
   /**
    * The matrix element for the kinematical configuration
    * previously provided by the last call to setKinematics(), suitably
    * scaled by sHat() to give a dimension-less number.
    * @return the matrix element scaled with sHat() to give a
    * dimensionless number.
    */
   virtual double me2() const;
 
   /**
    * Return the scale associated with the last set phase space point.
    */
   virtual Energy2 scale() const;
 
   /**
    * Add all possible diagrams with the add() function.
    */
   virtual void getDiagrams() const;
 
   /**
    * Get diagram selector. With the information previously supplied with the
    * setKinematics method, a derived class may optionally
    * override this method to weight the given diagrams with their
    * (although certainly not physical) relative probabilities.
    * @param dv the diagrams to be weighted.
    * @return a Selector relating the given diagrams to their weights.
    */
   virtual Selector<DiagramIndex> diagrams(const DiagramVector & dv) const;
 
   /**
    * Return a Selector with possible colour geometries for the selected
    * diagram weighted by their relative probabilities.
    * @param diag the diagram chosen.
    * @return the possible colour geometries weighted by their
    * relative probabilities.
    */
   virtual Selector<const ColourLines *>
   colourGeometries(tcDiagPtr diag) const;
 
   /**
    *  Construct the vertex of spin correlations.
    */
   virtual void constructVertex(tSubProPtr);
   //@}
 
 
 public:
 
   /** @name Functions used by the persistent I/O system. */
   //@{
   /**
    * Function used to write out object persistently.
    * @param os the persistent output stream written to.
    */
   void persistentOutput(PersistentOStream & os) const;
 
   /**
    * Function used to read in object persistently.
    * @param is the persistent input stream read from.
    * @param version the version number of the object when written.
    */
   void persistentInput(PersistentIStream & is, int version);
   //@}
 
   /**
    * The standard Init function used to initialize the interfaces.
    * Called exactly once for each class by the class description system
    * before the main function starts or
    * when this class is dynamically loaded.
    */
   static void Init();
 
 protected:
 
   /** @name Clone Methods. */
   //@{
   /**
    * Make a simple clone of this object.
    * @return a pointer to the new object.
    */
   virtual IBPtr clone() const;
 
   /** Make a clone of this object, possibly modifying the cloned object
    * to make it sane.
    * @return a pointer to the new object.
    */
   virtual IBPtr fullclone() const;
   //@}
 
 protected:
 
   /** @name Standard Interfaced functions. */
   //@{
   /**
    * Initialize this object after the setup phase before saving an
    * EventGenerator to disk.
    * @throws InitException if object could not be initialized properly.
    */
   virtual void doinit();
   //@}
 
 private:
 
   /**
    *  Members to return the matrix elements for the different subprocesses
    */
   //@{
   /**
    * Matrix element for \f$q\bar{q}\to \gamma\gamma\f$.
    * @param fin Spinors for incoming quark
    * @param ain Spinors for incoming antiquark
    * @param p1  Polarization vectors for the first  outgoing photon
    * @param p2  Polarization vectors for the second outgoing photon
    * @param me  Whether or not to calculate the matrix element for spin correlations
    */
   double qqbarME(vector<SpinorWaveFunction> & fin, vector<SpinorBarWaveFunction> & ain,
          vector<VectorWaveFunction> & p1 , vector<VectorWaveFunction>    & p2 ,
          bool me) const;
 
   /**
    * Matrix element for \f$gg \to \gamma\gamma\f$.
    * @param g1  Polarization vectors for the first  incoming gluon
    * @param g2  Polarization vectors for the second incoming gluon
    * @param p1  Polarization vectors for the first  outgoing photon
    * @param p2  Polarization vectors for the second outgoing photon
    * @param me  Whether or not to calculate the matrix element for spin correlations
    */
   double ggME(vector<VectorWaveFunction> & g1 , vector<VectorWaveFunction>    & g2 ,
           vector<VectorWaveFunction> & p1 , vector<VectorWaveFunction>    & p2 ,
           bool me) const;
   //@}
 
   /**
    *  \f$gg\to\gamma\gamma\f$ matrix element for the \f$++++\f$ helicity configuration.
    * @param s The \f$s\f$ invariant
    * @param t The \f$t\f$ invariant
    * @param u The \f$u\f$ invariant
    */
   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:
 
   /**
    * The assignment operator is private and must never be called.
    * In fact, it should not even be implemented.
    */
   MEPP2GammaGamma & operator=(const MEPP2GammaGamma &) = delete;
 
 private:
 
   /**
    *  Pointer to the quark-antiquark-photon vertex
    */
   AbstractFFVVertexPtr _photonvertex;
 
   /**
    *  Maximum PDG code of the quarks allowed
    */
   unsigned int _maxflavour;
 
   /**
    *  Option for which processes to include
    */
   unsigned int _process;
   
   /**
    * Matrix element for spin correlations
    */
   ProductionMatrixElement _me;
 
   /**
    *  weights for the different quark annhilation diagrams
    */
   mutable double _diagwgt[2];
 
   /**
    *  Scale prefactor
    */
   double scalePreFactor_;
 };
 
 }
 
 #endif /* HERWIG_MEPP2GammaGamma_H */

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