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 // -*- C++ -*-
 //
 // TensorMesonVectorVectorDecayer.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_TensorMesonVectorVectorDecayer_H
 #define HERWIG_TensorMesonVectorVectorDecayer_H
 //
 // This is the declaration of the TensorMesonVectorVectorDecayer class.
 //
 #include "Herwig/Decay/DecayIntegrator.h"
 #include "Herwig/Decay/PhaseSpaceMode.h"
 #include "ThePEG/Helicity/LorentzTensor.h"
 
 namespace Herwig {
 using namespace ThePEG; 
 
 /** \ingroup Decay
  *
  *  The TensorMesonVectorVectorDecayer class is designed to simulate
  *  the decay of a tensor meson to two spin-1 particles. In practice, at least
  *  for the light tensor mesons, this is only the decay of a tensor meson to two
  *  photons. In principle for bottom and charm tensors this may be the decay to
  *  two vector mesons an d it is used to simulate three and four body 
  *  \f$a_2\f$, \f$f_2\f$ and \f$K_2\f$ decays. 
  *  
  * The matrix element is
  *  given by \f[\mathcal{M}=\epsilon_0^{\alpha_1\alpha_2} \epsilon_1^{\beta} \epsilon_2^{\gamma} 
  *  \left(g_{\alpha_1\beta}+\frac{p_{1\alpha_1}p_{0\beta}{p_0\cdot p_1-m_0m_1}\right)
  *  \left(g_{\alpha_2\gamma}+\frac{p_{2\alpha_2}p_{0\gamma}{p_0\cdot p_2-m_0m_2}\right)\f]
  *  where \f$p_{0,1,2}\f$ are the momenta of the incoming pseudotensor and outgoing vector mesons, respectively,
  * and \f$\epsilon_{0}\f$ is the polarization tensor of the incoming pseudotensor meson and $\epslion_{1,2}$ are those of the outgoing
  * vector mesons.
  *
  *  The incoming tensor 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.
  *
  * @see DecayIntegrator
  * * @see \ref TensorMesonVectorVectorDecayerInterfaces "The interfaces"
  * defined for TensorMesonVectorVectorDecayer.
  *
  */
 class TensorMesonVectorVectorDecayer: public DecayIntegrator {
 
 public:
 
   /**
    * 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 9.
    * @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.
    */
   TensorMesonVectorVectorDecayer & operator=(const TensorMesonVectorVectorDecayer &) = delete;
 
 public:
 
   /**
    *   Set the parameters for a decay mode
    */
   string setUpDecayMode(string arg);
 
 private:
 
   /**
    * the PDG codes for the incoming particles
    */
   vector<int> incoming_;
 
   /**
    * the PDG codes for the outgoing particles
    */
   vector<pair<int,int> > outgoing_;
 
   /**
    * the coupling for the decay
    */
   vector<Energy> coupling_;
 
   /**
    * the maximum weight for the decay
    */
   vector<double> maxWeight_;
 
   /**
    *  Storage of polarization tensors to try and increase
    *  speed
    */
   mutable vector<Helicity::LorentzTensor<double> > tensors_;
 
   /**
    *  Storage of the polarization vectors 
    */
   mutable vector<Helicity::LorentzPolarizationVector > vectors_[2];
 
   /**
    *   Storage of the \f$\rho\f$ matrix
    */
   mutable RhoDMatrix rho_;
 
 };
 
 }
 
 #endif /* HERWIG_TensorMesonVectorVectorDecayer_H */

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