ParticleGun should be able to decay short lived particles, such as taus

[gganis]

Currently the Particle Gun cannot decay a unstable particle. This is inconvenient to study the detector response to a given particle, such as taus.

[vvolkl]

Would be good to have indeed. I think pythia8 can be used for the decay (there is an example in pythia8, main21.cc)