unrealisitically high cross section with strange drastic reduction of events

Hi,

Did you look at the following FAQ?
https://answers.launchpad.net/madgraph5/+faq/1987

In short a particle with a zero width shouldn't be allow to decay (since then by definition the width is not zero)

Cheers,

Olivier
On Nov 11, 2013, at 12:16 AM, Peter Winslow <email address hidden> wrote:

> New question #239002 on MadGraph5:
> https://answers.launchpad.net/madgraph5/+question/239002
>
> Hi,
>
> I've implemented a very simple model for lepton number violation at the LHC with 2 new states: a scalar (NS) with Q=1 and a majorana fermion (F0). Both have masses of 1 TeV but the scalar has a finite width (120 GeV) while the majorana fermion has zero width. There are only 2 interactions, i.e., ubar d NS and ebar F0 NS^*, both with couplings equal to 1 (F0 can decay through the second interaction when the mass of NS is less than F0 but not when they're degenerate). I generate the process p p > j j e- e- which proceeds through 2 kinds of graphs: 1) t-channel exchange of both NS and F0 and 2) s-channel production of NS to F0 and e- followed by off-shell decay of F0 to e- NS --> e- u dbar. The resulting cross section is unrealistically high (~10^10 pb) and had a strange drastic reduction of events (I specified 10000 but only ended up getting 22). This is all happening at the parton level.
>
> I'm not sure why this is happening, can you please help me understand what is causing the problem? I thought at one point that it may have something to do with the zero width of F0 when the masses are degenerate but is there no way to handle off-shell decays in the limit of degeneracy?
>
> Thanks very much in advance for any/all advice you may have.
>
> Best,
> Peter
>
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