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 // -*- C++ -*-
 //
 // EtaPiPiFermionsDecayer.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_EtaPiPiFermionsDecayer_H
 #define HERWIG_EtaPiPiFermionsDecayer_H
 // This is the declaration of the EtaPiPiFermionsDecayer class.
 
 #include "Herwig/Utilities/Kinematics.h"
 #include "Herwig/Decay/DecayIntegrator.h"
 #include "Herwig/Decay/PhaseSpaceMode.h"
 #include "Herwig/Decay/FormFactors/OmnesFunction.fh"
 #include "ThePEG/Helicity/LorentzSpinorBar.h"
 
 namespace Herwig {
 using namespace ThePEG;
 
 /** \ingroup Decay
  *
  * The <code>EtaPiPiFermionsDecayer</code> class implements the decay of
  * the \f$\eta\f$ or \f$\eta'\f$ to \f$\pi^+\pi^-\gamma\f$ using either 
  * a VMD type model or a model using either the theoretical or experimental 
  * form of the Omnes function taken from hep-ph/0112150.
  *
  * The matrix element is given by
  * \f[\mathcal{M} = B(s_{+-},s_{+\gamma},s_{-\gamma})\epsilon^{\mu\nu\alpha\beta}
  *      \epsilon^*_{\mu}p_{+\nu}p_{-\alpha}p_{\gamma\beta}\f]
  * where \f$p_{+,-}\f$ are the momenta of the positively and negatively charged pions,
  * \f$p_{\gamma}\f$ is the momentum of the photon and \f$s_{ij} = (p_i+p_j)^2\f$.
  *
  *  The different models take
  *
  *  \f[B(s_{+-},s_{+\gamma},s_{-\gamma}) = 
  *  B_0\left(1+\frac32\frac{s_{+-}}{M^2_\rho-s_{+-}-iM_\rho\Gamma_\rho(s_{+-})}\right)\f]
  *  where \f$M_\rho\f$ and \f$\Gamma_\rho\f$ are the mass and running width 
  *  of the \f$\rho\f$
  *  respectively for the VMD model.
  *
  *  For the Omnes function case we take
  *
  *  \f[B(s_{+-},s_{+\gamma},s_{-\gamma}) = 
  *  B_0\left(1-c+c\frac{1+as_{+-}}{D_1(s_{+-})}\right)\f]
  *  either the experimental or analytic form of the Omnes function \f$D_1(s_{+-})\f$
  *  taken from hep-ph/0112150 can be used.
  *
  *  The coefficient \f$B_0\f$ is given in hep-ph/0112150. We use the values from this
  *  paper and use their default choice \f$c=1\f$, \f$a=\frac1{2M_\rho}\f$.
  *
  * @see DecayIntegrator
  * 
  */
 class EtaPiPiFermionsDecayer: public DecayIntegrator {
 
 public:
 
   /**
    * Default constructor.
    */
   EtaPiPiFermionsDecayer();
   
   /**
    * 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();
   //@}
 
 public:
 
   /**
    *   Set the parameters for a decay mode
    */
   string setUpDecayMode(string arg);
 
 private:
 
   /**
    * Private and non-existent assignment operator.
    */
   EtaPiPiFermionsDecayer & operator=(const EtaPiPiFermionsDecayer &) = delete;
 
 private:
 
   /**
    * the pion decay constant, \f$F_\pi\f$.
    */
   Energy fPi_;
 
   /**
    * the PDG code for the incoming particle
    */
   vector<int> incoming_;
 
   /**
    *  The PDG code of the leptons
    */
   vector<int> lepton_;
 
   /**
    * Coupling for the decay, \f$B_0\f$.
    */
   vector<double> coupling_;
 
   /**
    * The maximum weight
    */
   vector<double> maxWeight_;
 
   /**
    * The option for the energy dependence of the prefactor
    */
   vector<int> option_;
 
   /**
    * The constants for the omnes function form.
    */
   InvEnergy2 aConst_;
 
   /**
    * The constants for the Omnes function form.
    */
   double cConst_;
 
   /**
    * The \f$\rho\f$ mass
    */
   Energy mRho_;
 
   /**
    * The \f$\rho\f$ width
    */
   Energy rhoWidth_;
 
   /**
    * Constant for the running \f$rho\f$ width.
    */
   double rhoConst_;
 
   /**
    * The \f$m_\pi\f$.
    */
   Energy mPi_;
 
   /**
    * Use local values of the parameters.
    */
   bool localParameters_;
 
   /**
    *  Object calculating the Omnes function
    */
   OmnesFunctionPtr omnesFunction_;
 
   /**
    *  Spin densit matrix
    */
   mutable RhoDMatrix rho_;
 
   /**
    *  Spinors for the fermions
    */
   mutable vector<Helicity::LorentzSpinor   <SqrtEnergy> > wave_;
 
   /**
    *  Barred spinors for the fermions 
    */
   mutable vector<Helicity::LorentzSpinorBar<SqrtEnergy> > wavebar_;
 };
 }
 
 #endif /* HERWIG_EtaPiPiFermionsDecayer_H */

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