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 // -*- C++ -*-
 //
 // TwoPionPhotonCurrent.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_TwoPionPhotonCurrent_H
 #define HERWIG_TwoPionPhotonCurrent_H
 //
 // This is the declaration of the TwoPionPhotonCurrent class.
 //
 #include "WeakCurrent.h"
 
 namespace Herwig {
 using namespace ThePEG;
 
 /** \ingroup Decay
  *
  *  This class implements the decay current for \f$\pi^\pm\pi^0 \gamma\f$ via
  *  an intermediate \f$\omega\f$. It inherits from the <code>WeakCurrent</code>
  *  class and implements the hadronic current.
  *
  *  The model is based on the one used in TAUOLA, Comput.Phys.Commun.76:361-380,1993.
  *  The current is given by
  * \f[J^\mu  = e T \left\{
  *     \epsilon^\mu\left[ m^2_\pi      p_1\cdot p_3
  *                       -p_2\cdot p_3(p_2\cdot p_1-p_1\cdot p_3)\right]
  *     -p_2^\mu\left[p_2\cdot\epsilon p_1\cdot p_3-p_1\cdot\epsilon p_2\cdot p_3\right]
  *     +p_3^\mu\left[\epsilon\cdot p_2-\epsilon\cdot p_1(m^2_\pi+p_2\cdotp_3)\right]
  *\right\}\f]
  *  where
  * - \f$p_1\f$ is the momentum of the charged pion
  * - \f$p_2\f$ is the momentum of the neutral pion
  * - \f$p_3\f$ is the momentum of the photon
  * - \f$\epsilon\f$ is the polarization of the photon
  * - \f$e\f$ is the electric charge of the positron
  * and the normaliztion factor is
  * \f[T = F(q^2)F(0)\frac1{\sqrt{2}B_\omega(s_2)}\f]
  * and
  * \f[F(s) = \sqrt{2}F_\rho g_{\rho\omega\pi}\sum_k\sigma_k B_{\rho_k}(s)\f]
  * where
  * - \f$B_\omega(s)=\frac1{m^2_\omega-s-im_\omega\Gamma_\omega}\f$ is the Breit-Wigner for the \f$\omega\f$.
  * - \f$m_\omega\f$ is the mass of the \f$\omega\f$.
  * - \f$\Gamma_\omega \f$ is the width of the \f$\omega\f$.
  * - \f$F_\rho\f$ is the coupling for the conversion of the \f$\rho\f$ to a photon.
  * - \f$g_{\rho\omega\pi}\f$ is the coupling of \f$\rho\f$, \f$\omega\f$, \f$\pi\f$.
  * - \f$m_{\rho_k}\f$ is the mass of the \f$k\f$th \f$\rho\f$ resonance
  * - \f$B_{\rho_k}(s)=\frac1{m^2_{\rho_k}-s-im_{\rho_k}\Gamma_{\rho_k}}\f$ is the 
  *    Breit-Wigner for  \f${\rho_k}\f$.
  * - \f$m_{\rho_k}\f$ is the mass of the \f${\rho_k}\f$.
  * - \f$\Gamma_{\rho_k} \f$ is the width of the \f${\rho_k}\f$.
  *
  * @see WeakCurrent
  * 
  *  \author Peter Richardson
  *
  */
 class TwoPionPhotonCurrent: public WeakCurrent {
 
 public:
 
   /**
    * Default constructor
    */
   TwoPionPhotonCurrent();
 
   /** @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();
 
 public:
 
   /** @name Methods for the construction of the phase space integrator. */
   //@{ 
   /**
    * Complete the construction of the decay mode for integration.classes inheriting
    * from this one.
    * This method is purely virtual and must be implemented in the classes inheriting
    * from WeakCurrent.
    * @param icharge   The total charge of the outgoing particles in the current.
    * @param resonance If specified only include terms with this particle
    * @param flavour Information on the required flavours of the quarks
    * @param imode     The mode in the current being asked for.
    * @param mode      The phase space mode for the integration
    * @param iloc      The location of the of the first particle from the current in
    *                  the list of outgoing particles.
    * @param ires      The location of the first intermediate for the current.
    * @param phase     The prototype phase space channel for the integration.
    * @param upp       The maximum possible mass the particles in the current are
    *                  allowed to have.
    * @return Whether the current was sucessfully constructed.
    */
   virtual bool createMode(int icharge, tcPDPtr resonance,
               FlavourInfo flavour,
               unsigned int imode,PhaseSpaceModePtr mode,
               unsigned int iloc,int ires,
               PhaseSpaceChannel phase, Energy upp );
 
   /**
    * The particles produced by the current. This just returns the pseudoscalar
    * meson.
    * @param icharge The total charge of the particles in the current.
    * @param imode The mode for which the particles are being requested
    * @param iq The PDG code for the quark
    * @param ia The PDG code for the antiquark
    * @return The external particles for the current.
    */
   virtual tPDVector particles(int icharge, unsigned int imode, int iq, int ia);
   //@}
 
   /**
    * Hadronic current. This method is purely virtual and must be implemented in
    * all classes inheriting from this one.
    * @param resonance If specified only include terms with this particle
    * @param flavour Information on the required flavours of the quarks
    * @param imode The mode
    * @param ichan The phase-space channel the current is needed for.
    * @param scale The invariant mass of the particles in the current.
    * @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 current. 
    */
   virtual vector<LorentzPolarizationVectorE> 
   current(tcPDPtr resonance,
       FlavourInfo flavour,
       const int imode, const int ichan,Energy & scale,
       const tPDVector & outgoing,
       const vector<Lorentz5Momentum> & momenta,
       DecayIntegrator::MEOption meopt) const;
 
   /**
    *   Construct the SpinInfo for the decay products
    */
   virtual void constructSpinInfo(ParticleVector decay) const;
 
   /**
    * Accept the decay. Checks the meson against the list
    * @param id The id's of the particles in the current.
    * @return Can this current have the external particles specified.
    */
   virtual bool accept(vector<int> id);
 
   /**
    * Return the decay mode number for a given set of particles in the current. 
    * Checks the meson against the list
    * @param id The id's of the particles in the current.
    * @return The number of the mode
    */
   virtual unsigned int decayMode(vector<int> id);
 
   /**
    * 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
    * @param create Whether or not to add a statement creating the object
    */
   virtual void dataBaseOutput(ofstream & os,bool header,bool create) const;
 
 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);}
   //@}
 
 private:
 
   /**
    * Private and non-existent assignment operator.
    */
   TwoPionPhotonCurrent & operator=(const TwoPionPhotonCurrent &) = delete;
 
 private:
   
   /**
    * Calculate the \f$F(q^2)\f$ function at a given scale
    * @param q2 The scale \f$q^2\f$.
    * @return The value of the function. 
    */
   complex<InvEnergy> FFunction(Energy2 q2) const {
     complex<InvEnergy2> output(ZERO);
     for(unsigned int ix=0; ix<_resweights.size();++ix) {
       output -= _resweights[ix]*BreitWigner(q2,ix);
     }
     return output*_grho*_grhoomegapi*sqrt(2.);
   }
 
   /**
    * Fixed width Breit wigner
    * @param q2 The scame \f$q^2\f$
    * @param ires The resonance required (0,1,3) are the \f$\rho\f$'s and 10 is the 
    * \f$\omega\f$.
    * @return The breit wigner
    */
   complex<InvEnergy2> BreitWigner(Energy2 q2,unsigned int ires) const {
     static const Complex ii(0.,1.);
     complex<Energy2> denom;
     if(ires<_rhomasses.size()) {
       denom = q2-_rhomasses[ires]*_rhomasses[ires]+ii*_rhomasses[ires]*_rhowidths[ires];
     }
     else if(ires==10) {
       denom = q2-_omegamass*_omegamass+ii*_omegamass*_omegawidth;
     }
     else assert(false);
     return 1./denom;
   }
   
 private:
   
   /**
    * Coupling of the rho to the photon, \f$F_\rho\f$.
    */
   Energy2 _grho;
   
   /**
    * Coupling of the rho to the omega and a pion, \f$g_{\rho\omega\pi}\f$.
    */
   InvEnergy _grhoomegapi;
   
   /**
    * Weights of the different rho resonances in the current
    */
   vector<double> _resweights;
 
   /**
    * Masses of the \f$\rho\f$ resonances
    */
   vector<Energy> _rhomasses; 
 
   /**
    * Widths of the \f$\rho\f$ resonances
    */
   vector<Energy> _rhowidths;
 
   /**
    * The \f$\omega\f$ mass.
    */
   Energy _omegamass;
 
   /**
    * The \f$\omega\f$ width.
    */
   Energy _omegawidth;
 
   /**
    * Mass for the intermediate in the phase-space, this is a technical parameter to
    * improve the phase-space integration efficiency.
    */
   Energy _intmass;
 
   /**
    * Width for the intermediate in the phase-space, this is a technical parameter to
    * improve the phase-space integration efficiency.
    */
   Energy _intwidth; 
 
 };
 
 }
 
 
 #endif /* HERWIG_TwoPionPhotonCurrent_H */

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