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 // -*- C++ -*-
 //
 // PScalar4FermionsDecayer.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_PScalar4FermionsDecayer_H
 #define HERWIG_PScalar4FermionsDecayer_H
 // This is the declaration of the PScalar4FermionsDecayer class.
 
 #include "Herwig/Decay/DecayIntegrator.h"
 #include "Herwig/Decay/PhaseSpaceMode.h"
 #include "ThePEG/Helicity/LorentzSpinorBar.h"
 
 namespace Herwig {
 using namespace ThePEG;
 
 /** \ingroup Decay
  *
  *  The <code>PScalar4FermionsDecayer</code> class is designed for the
  *  decay of the neutral pion to four leptons, in this case electrons.
  *  The propagator for the off-shell boson is taken to be \f$\frac1{m^2}\f$. There
  *  is also the option of including a vector meson dominance  form-factor.
  *
  *  In this case the matrix element is
  *  \f[\mathcal{M} = \frac{g}{m^2_{f_1\bar{f_1}}m^2_{f_2\bar{f_2}}}
  *                   \epsilon^{\mu\nu\alpha\beta}    
  *                   \bar{u}(p_{f_1})\gamma_\mu    v(p_{\bar{f_1}})p_{f_1\bar{f_1}\nu}
  *                   \bar{u}(p_{f_2})\gamma_\alpha v(p_{\bar{f_2}}) p_{f_2\bar{f_2}\beta}
  *  \f]
  *  It includes the option of a vector meson dominance (VMD) type form factor  
  *  \f$\frac{-M^2+i\Gamma M}{(m^2_{f\bar{f}}-M^2+i\Gamma M)}\f$. In the case of identical
  *  fermions in also includes the exchange diagram.
  *  
  * @see DecayIntegrator
  * @see PScalarVectorVectorDecayer
  * @see PScalarVectorFermionsDecayer
  * 
  *  \author Peter Richardson
  *
  */
 class PScalar4FermionsDecayer: public DecayIntegrator {
 
 public:
 
   /**
    * Default constructor.
    */
   PScalar4FermionsDecayer();
   
   /**
    * Which of the possible decays is required
    * @param cc Is this mode the charge conjugate
    * @param parent The decaying particle
    * @param children The decay products
    */
   virtual int modeNumber(bool & cc, tcPDPtr parent, 
              const tPDVector & children) const;
 
   /**
    * Return the matrix element squared for a given mode and phase-space channel.
    * @param ichan The channel we are calculating the matrix element for. 
    * @param part The decaying Particle.
    * @param outgoing The particles produced in the decay
    * @param momenta  The momenta of the particles produced in the decay
    * @param meopt Option for the calculation of the matrix element
    * @return The matrix element squared for the phase-space configuration.
    */
   double me2(const int ichan,const Particle & part,
          const tPDVector & outgoing,
          const vector<Lorentz5Momentum> & momenta,
          MEOption meopt) const;
 
   /**
    *   Construct the SpinInfos for the particles produced in the decay
    */
   virtual void constructSpinInfo(const Particle & part,
                  ParticleVector outgoing) const;
 
   /**
    * Output the setup information for the particle database
    * @param os The stream to output the information to
    * @param header Whether or not to output the information for MySQL
    */
   virtual void dataBaseOutput(ofstream & os,bool header) const;
 
 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);
   //@}
 
   /**
    * Standard Init function used to initialize the interfaces.
    */
   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 {return new_ptr(*this);}
 
   /** 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 {return new_ptr(*this);}
   //@}
 
 protected:
 
   /** @name Standard Interfaced functions. */
   //@{
 
   /**
    * Initialize this object after the setup phase before saving and
    * EventGenerator to disk.
    * @throws InitException if object could not be initialized properly.
    */
   virtual void doinit();
 
   /**
    * Initialize this object to the begining of the run phase.
    */
   virtual void doinitrun();
   //@}
 
 private:
 
   /**
    * Private and non-existent assignment operator.
    */
   PScalar4FermionsDecayer & operator=(const PScalar4FermionsDecayer &) = delete;
 
 public:
 
   /**
    *   Set the parameters for a decay mode
    */
   string setUpDecayMode(string arg);
 
 private:
 
   /**
    * coupling for a decay
    */
   vector<InvEnergy> coupling_;
 
   /**
    * the PDG codes for the incoming particle
    */
   vector<int> incoming_;
 
   /**
    * the PDG codes for the first outgoing fermion
    */
   vector<pair<long,long> > outgoing_;
 
   /**
    * maximum weight for a decay
    */
   vector<double> maxweight_;
 
   /**
    * Include the VMD factor
    */
   vector<int> includeVMD_;
 
   /**
    * PDG code of the particle for the VMD factor
    */
   vector<int> VMDid_;
 
   /**
    * Mass of the particle for the VMD factor
    */
   vector<Energy> VMDmass_;
 
   /**
    * Width of the particle for the VMD factor
    */
   vector<Energy> VMDwidth_;
 
   /**
    *  Spin density matrix
    */
   mutable RhoDMatrix rho_;
 
   /**
    *  Spinors for the decay products
    */
   mutable vector<Helicity::LorentzSpinor   <SqrtEnergy> > wave_[2];
 
   /**
    *  Barred spinors for the decay products
    */
   mutable vector<Helicity::LorentzSpinorBar<SqrtEnergy> > wavebar_[2];
 };
 
 }
 
 
 #endif /* HERWIG_PScalar4FermionsDecayer_H */

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