Dalitz Plot by FOWL and TGenPhaseSpace

Hi, I am trying to generate a Dalitz plot for the 3-body final state: J/\psi\pi^+n. I use GENEV in FOWL and TGenPhaseSpace to make a cross check. However, with the same final particles, center-of-mass energy and number of the monte carlo events, the J/\psi\pi^+ invariant mass spectrums are different. As shown in the attachments, the result from FOWL is larger than that from root TGenPhaseSpace. Could anyone tell me what the problem is? I also wonder if the definitions of the phase space factor are different in these two frames.


You used some weight when filling the histogram in your FORTRAN code since clearly the integral of taht histogram is much larger than the number of entries (1kevt). The histogram from TGenPhaseSpace is more consistent there.

Hi honk, I am also aware of this. However, there are two options in using TGenPhase Space, with constant cross-section (default) or with Fermi energy dependence (opt=“Fermi”). When I choose the default one ,the result seems consistent with the entries. However, if the opt=“Fermi” is adopted, a same result as to that from the fortran will be obtained

If that was all clear you need to explain better what kind of difference you are worried about.

Yes, now I wonder what the difference is between these two options. I am a new user for root, maybe this is a naive question. However, this should be a crucial point for my understanding for the program.
I also find a post years ago with the meaning of the return weight.
[url]TGenPhaseSpace::Generate() - meaning of the weighting factor
Could someone explain me more details about my puzzles?