External Decays
DecayHandler
is a base class for the external handling of
decays. 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.
There is only one pure virtual method in DecayHandler
,
to do the decay:
method name="virtual bool decay(vector<int>& idProd,
vector<double>& mProd, vector<Vec4>& pProd, int iDec,
const Event& event)"
where
argument name="idProd": is a list of particle PDG identity codes,
argument name="mProd": is a list of their respective masses (in GeV), and
argument name="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]
contains 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.
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 main04.cc
, providing
a simple example of how to use thing facility.