# Problem generating SM-BSM interference diagrams

Hi,

I am trying to study the effects of interference diagrams in a W' model but I'm having trouble convincing madgraph to produce the diagrams properly.

If I use the following command:

define bs = b b~
define p = p bs
define j = j bs
define wp = wp+ wp-
define ls = l+ l- vl vl~
define ws = w+ w- wp
define ts = t t~

generate p p > ts bs BSM=2 QED=2

it does as I expect and produces diagrams using both W and W' intermediate particles.

However, if I use:

generate p p > bs ts, (ts > ws bs, ws > ls ls) BSM=4 QED=4

it doesn't produce any QED vertices.

Even when I use:

generate p p > bs ts, (ts > ws bs, ws > ls ls) BSM=0 QED=4

it doesn't produce any QED.

What am I missing here? Is there something wrong with the way I'm entering the generation command for the more complicated diagrams?

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Olivier Mattelaer
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 Revision history for this message Olivier Mattelaer (olivier-mattelaer) said on 2021-06-14: #1

Hi,

Actually I have no idea how the code would react in this case since you have "BSM=0 QED=4" which are not attached to any process.
(So I'm actually surprised that the code does not complains about that.

The way the syntax work is that each piece has it's own coupling definition.
So here you can specify coupling restriction in three valid position (noted by "HERE" )

generate p p > bs ts HERE , (ts > ws bs HERE, ws > ls ls HERE)

It is also possible to add a global restriction on the sum of the processes after the unphysical process id (@X)
which can be
generate p p > bs ts, (ts > ws bs, ws > ls ls) @0 BSM<=4 QED<=4
For such syntax I strongly suggest to use the latest version of the code (or the LTS version-- 2.9.x--) since a lot of bug have been reported on that syntax in previous release.

Cheers,

Olivier

 Revision history for this message David Overton (doverton) said on 2021-06-14: #2

Thanks Olivier Mattelaer, that solved my question.