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Issue in BTOXSGAMMA model documentation
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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
model.

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,

Kind Regards,
Henrikas

Event Timeline

tlatham created this task.May 9 2022, 10:57 AM
tlatham created this object with visibility "Public (No Login Required)".
tlatham created this object with edit policy "Restricted Project (Project)".
jback triaged this task as Low priority.May 9 2022, 2:49 PM
jback removed a subscriber: tlatham.
jback added a subscriber: jback.May 9 2022, 3:08 PM

Yes, the default lambda1 parameter quoted in the documentation should be -0.3, as given by Eq 39 (not Eq 24) on page 24 of https://arxiv.org/pdf/hep-ph/9805303.pdf . The BTOXSGAMMA model description in the guide has various other grammar problems that also need correcting. We could update the latex for the guide in the repository, or put the correct info in the paper draft (or do both).

Code logic:

  1. Kagan model is first selected then initialised, calling EvtBtoXsgammaKagan::init()
  2. Providing just 1 decay file parameter sets the masses using default arrays
  3. This is probably equivalent to providing the default list of parameters given in the guide, except with lambda = -0.3.
tlatham added a comment.May 11 2022, 4:15 PM

Hi @jback, many thanks for checking this.
Certainly we should make sure that this is correct in the paper draft.
Once the paper is out we can take down the old guide and replace it with a copy of the paper or a link to the paper on arXiv.
But in the meantime, I think it is probably best also to correct the latex for the guide and perhaps put a new pdf on the webpage.