MadGraph5_aMC@NLO

problems with factorization and bwcutoff

Asked by Gennaro Corcella

Hello,

I have been running a process like p p > a b, a > c d, b > e f, where a and b are particles with a very small width (accounted for
in the parameter card) with Madgraph 2.6.1.
Likewise, I have been running p p > a b (inclusive final states).

Naively, in the narrow width approximation, I would expect the following factorization:
sigma(p p > a b, a > c d, b > e f)=sigma(p p >a b) * BR (a > c d) * BR (b > e f).
No cuts on decays have been set.

However, I don't find such a result, as the chain cross section looks much larger than expected. Also, the results for the full decay chain exhibit a strong dependence on bwcutoff, although this should not be the case if the widths are much smaller than the masses. Have I misunderstood anything? Shall I set bwcutoff to some very large/small number to obtain factorization?
I played around with it, but got no meaningful result.

Thanks, Gennaro

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Olivier Mattelaer (olivier-mattelaer) said : 		#1

Did you check the above FAQ?

Otherwise, we introduce in 2.6.4 a security for cases where the width where too small which was creating numerical unstability of the computation. As this might be your issue. Testing with our Long Term Stable version might be worth the test. Especially since 2.6.1 is so old that I have no clue on what is the status of such code (and that code does not work anymore on my machine).

Cheers,

Olivier

FAQ #2442: “why production and decay cross-section didn't agree.”.

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