## 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 doublecounting, 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.