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 // -*- C++ -*-
 //
 // VectorMeson2MesonDecayer.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_VectorMeson2MesonDecayer_H
 #define HERWIG_VectorMeson2MesonDecayer_H
 //
 // This is the declaration of the VectorMeson2MesonDecayer class.
 //
 #include "Herwig/Decay/DecayIntegrator.h"
 #include "Herwig/Decay/PhaseSpaceMode.h"
 #include "ThePEG/Helicity/LorentzPolarizationVector.h"
 
 namespace Herwig {
 
 using namespace ThePEG;
 
 /** \ingroup Decay
  *
  *  This class is the implementation for the decay of a vector meson to 
  *  two scalar mesons, the classic example is \f$\rho -> \pi\pi\f$, via a current
  *  which is the difference of the momenta of the mesons.
  *  \f[\mathcal{M} = g\epsilon_\mu(p_1-p_2)^\mu,\f]
  *  where \f$p_{1,2}\f$ are the momenta of the decay products,
  *  \f$\epsilon\f$ is the polarization vector of the decaying particle
  *  and \f$g\f$ the coupling.
  *  Obviously the
  *  order of the momenta doesn't matter as it will only effect the sign of the
  *  matrix element.
  *
  *  The incoming vector mesons together with their decay products and the coupling 
  *  \f$g\f$ can be specified using the interfaces for the class. The maximum weights
  *  for the decays can be calculated using the Initialize interface of the
  *  DecayIntegrator class or specified using the interface.
  *
  *  The incoming and outgoing particles, couplings and maximum weights for
  *  many of the common \f$V\to PP\f$ decays are specified in the default
  *  constructor.
  *
  * @see DecayIntegrator
  * @see \ref VectorMeson2MesonDecayerInterfaces "The interfaces"
  * defined for VectorMeson2MesonDecayer.
  * 
  *  \author Peter Richardson
  *
  */
 class VectorMeson2MesonDecayer: public DecayIntegrator {
   
 public:
 
   /**
    * Default constructor.
    */
   VectorMeson2MesonDecayer();
 
   /**
    * 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;
 
   /**
    * Specify the \f$1\to2\f$ matrix element to be used in the running width calculation.
    * @param dm The DecayMode
    * @param mecode The code for the matrix element as described
    *               in the GenericWidthGenerator class, in this case 0.
    * @param coupling The coupling for the matrix element.
    * @return True or False if this mode can be handled.
    */
   bool twoBodyMEcode(const DecayMode & dm, int & mecode, double & coupling) 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.
    */
   VectorMeson2MesonDecayer & operator=(const VectorMeson2MesonDecayer &) = delete;
 
 public:
 
   /**
    *   Set the parameters for a decay mode
    */
   string setUpDecayMode(string arg);
   
 private:
   
 
   /**
    * the PDG codes for the incoming particles
    */
   vector<long> incoming_;
 
   /**
    * The PDG codes for the outgoing mesons
    */
   vector<pair<long,long> > outgoing_;
 
   /**
    * the maximum weight for the integration
    */
   vector<double> maxWeight_;
 
   /**
    * the coupling for the decay
    */
   vector<double> coupling_;
 
   /**
    *  Storage of polarization tensors to try and increase
    *  speed
    */
   mutable vector<Helicity::LorentzPolarizationVector> vectors_;
   
   /**
    *   Storage of the \f$\rho\f$ matrix
    */
   mutable RhoDMatrix rho_;
 };
   
 }
 
 
 #endif /* HERWIG_VectorMeson2MesonDecayer_H */

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