Numerical instability in top + dilepton

Asked by Nishita


I'm running into a strange problem.

If I ask to generate p p > t a j, I quickly get a normal looking result of 0.0750 ± 0.0001 pb.

However, if i try p p > t a j, a > mu+ mu- I get 6.995 ± 1825.5151662796623 after which MG5 tries to "improve" the result and ends up giving me a random cross section in nanobarns after about 20 minutes of running.

I've put m_ll cuts (both min and max) on the leptons already. What else can I do?

Thanks in advance,

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Nishita (nish-des) said :

Sorry, it seems just doing p p > t j mu+ mu- works. Somehow forcing it only via the photon was breaking something.

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Olivier Mattelaer (olivier-mattelaer) said :

The syntax with the comma is related to the narrow width approximation, and that approximation is never valid for the photon so this is not too surprising.

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