Behavior of on-shell decay chains
Hi,
I was working on a long decay chain in a BSM model and I noticed a strange behavior.
The point is that if I have something like a > b c with Ma < Mb+Mc I get zero result, of course, because of phase space.
However, if I let, say, b decay further: a > b c, b > d e with Mc + Md + Me < Ma < Mb + Mc the process goes through even though the b cannot be on shell.
I can reproduce it in the sm model UFO file that comes with the MG distribution, so let me use this example.
Try
generate g g > t t~ , t > w+ b , t~ > w- b~
vs.
generate g g > t t~ , ( t > w+ b, w+ > e+ vl) , ( t~ > w- b~, w- > e- vl~ )
with an unphysical bottom mass of say 120 GeV.
The first process fails, as it should, but the second goes through. I am sure you are aware of it, but it makes me uneasy and I cannot find it discussed in previous bugs/questions.
Thanks for your time
Gabriele
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