[TGenPhaseSpace] Weight of distributions

Dear all,

I would like to use the TGenPhaseSpace routine in order to implement a generator for the internal pair creation.
For that purpose, I would like firsly to validate the way to apply the weight on the different histograms. I used the beta decay as a test.

By generating events from the TGenPhaseSpace::Generate() function, I could not obtain the expected beta decay kinetic energy and momentum distributions.
In order to reproduce them, I need to apply the weight returned by TGenPhaseSpace::Generate() multiplied by the (Ee x Ev) (with: Ee: Electron total energy, Ev: Antineutrino total energy).

In a previous post,
[url]Weighting in event generator [Lorezo Moneta]
"my calculation is done assuming a matrix element independent of the energy, like is the case in TGenPhaseSpace. If you make the calculation using the full Fermi Weak interaction, after summing on all the spin states you will get a matrix element proportional to E_e*E_nu and you get the distribution that you see in all the text books for beta decay.
That explains the difference in the results between E1 and E2 spectra of your script.
The matrix element calculation is out of the scope of TGenPhaseSpace, you should use for that a dedicated MonteCarlo generator according to the chosen process.

it is mentionned that the energy dependence of the beta decay can be introduced by mutipling by the proportionnalityof the patrix element. I do not see clearly why, and how to implement the energy dependence for any type of distributions produced from the TGenPhaseSpace.

Could you comment on it? or give me more inputs on how the phase space is created or how to reproduce the energy/momentum dependences of the distribution?

Thanks in advance