External Decays

DecayHandler is a base class for the external handling of decays. It is intended for normal particle decays, primarily B mesons and tau, and cannot be used to redirect decays of heavy resonances like t or Z^0. The user-written derived class is called if a pointer to it has been given with the pythia.decayPtr() method, where it also is specified which particles it will be called for. This particle information is accessible with the doExternalDecay() method.

There is only one pure virtual method in DecayHandler, to do the decay:

virtual bool DecayHandler::decay(vector<int>& idProd, vector<double>& mProd, vector<Vec4>& pProd, int iDec, const Event& event)  
where
argument idProd : is a list of particle PDG identity codes,
argument mProd : is a list of their respective masses (in GeV), and
argument pProd : is a list of their respective four-momenta.

At input, these vectors each have size one, so that idProd[0], mProd[0] and pProd[0] contain information on the particle that is to be decayed. At output, the vectors should have increased by the addition of all the decay products. Even if initially defined in the rest frame of the mother, the products should have been boosted so that their four-momenta add up to the pProd[0] of the decaying particle.

Should it be of interest to know the prehistory of the decaying particle, e.g. to set some helicity information affecting the decay angular distribution, the full event record is available read-only, with info in which slot iDec the decaying particle is stored.

The routine should return true if it managed the decay and false otherwise, in which case Pythia will try to do the decay itself. This e.g. means you can choose to do some decay channels yourself, and leave others to Pythia. To avoid double-counting, the channels you want to handle should be switched off in the Pythia particle database. In the beginning of the external decay method you should then return false with a probability given by the sum of the branching ratios for those channels you do not want to handle yourself.

Note that the decay vertex is always set by Pythia, and that B-Bbar oscillations have already been taken into account, if they were switched on. Thus idProd[0] may be the opposite of event[iDec].id(), where the latter provides the code at production.

A sample test program is available in main17.cc, providing a simple example of how to use this facility.