MadGraph5_aMC@NLO

Slow running in a peculiar kinematical region

Asked by Alessandro Strumia

we are running a process, and after one day the result is a cross section with a huge error, like 1 ± 7000.
Does it mean that this cannot be computed?
I don't need the cross section, I just need the events.

The process running here

p p > t1 t1~ , t1 > n1 b ve e+ , t1~ > n1 b~ ve~ e-
http://<email address hidden>/PROC71/

is p p --> stop stop
followed by stop decay
within the MSSM with just 2 sparticles: a stop at 400 GeV and a neutralino at 370 GeV.

So each stop decay is a very slow 4-body process stop --> Neutralino b ell nu

I don't know if there is a better way of implementing the same process in Madgraph

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Johan Alwall (johan-alwall) said : 		#1

Hello Alessandro,

You are in a very tiny sliver of allowed phase space in this run, with a 25 GeV difference between the stop and neutralino+b masses.

However, it might still work if you make sure to turn off the cuts on the electron - right now, you are running with a 10 GeV pt cut and 2.5 GeV eta cut on the lepton, which reduces the available phase space even more. Try turning this off (either by setting ptl=0, etal=1d2 or by setting cut_decays to F), then it might work.

All the best,
Johan

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