3 particle phase space volume with TGenPhaseSpace

I started playing with the phase space generator TGenPhaseSpace inside ROOT and wanted to compute the 3-particle phase space volume for a muon decaying into 3 massless particles with the simple attached macro. The phase space volume should depend on the total CM energy like:


as derived for example in

However, when adding up the weights with the macro one always gets a value around ~0.25 regardless of how big the CM energy really is.
Being the code already used in a number of applications I guess something is missing in how the phase space generator should be used.
Does somebody know what am I doing wrong?
Many thanks in advance!

PSvolume.cc (1.42 KB)