Reported to EvtGen dev mailing list:
Dear EvtGen developers,
I am working with the Belle 2 experiment and I have recently been using
it to generate B->Xs gamma events using the BTOXSGAMMA model, documented
in https://evtgen.hepforge.org/doc/EvtGenGuide.pdf, page 76.
In there you list the default parameters which describe the default
behaviour of the model. If the inputs are not given, a precomputed,
hardcoded set of hadronic masses is used. If the inputs are given this
triggers the calculation of the hadronic masses.
In the default parameters that the documentation states were used to
precompute hadronic masses, λ_1 = 0.3 is listed. However, this cannot
be true, because the lambda parameter is defined as negative, see e.g.
https://arxiv.org/pdf/hep-ph/9805303.pdf, page 24, right after equation
24; What I think was the original default parameter is λ_1 = -0.3. In
fact, when I tried to reproduce the spectrum with λ_1 = 0.3 given
explicitly, this had very long integration times and the integrals for
the hadronic mass computation did not converge. Switching to `λ_1 =
-0.3` I was able to reproduce the "default" behaviour of the BTOXSGAMMA
I think the lambda value should be corrected to avoid any confusion for
those using the model in the future.
Let me know if case you need more information,