MadGraph5_aMC@NLO

generate command (forbidding particles) syntax

Asked by Steven Jeremies

Dear MG5 team,

I am having trouble with the generate command syntax. In the process p p > j t, I want to exclude the b~ quark.
I did the following commands:

>> import model sm-no_b_mass
>> generate p p > j t /b~
>> display diagrams

Still the s-channel diagrams were included. Have I done something wrong with the syntax of the particle excluding?
Thanks in advance!

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Best mattelaer_bot (mattelaer-bot) said : 		#1

Hi,

The "/" will forbid any intermediate particle to be present. This will not forbid the initial/final state to be present.
so the following syntax
g g > g g / g
will produce one diagram (the four point interaction) -- this obviously breaks gauge invariance/qcd and you can/should trash any prediction done by such syntax , but this is the result of the diagram filtering--
If you want to remove b~ from the initial/final state, you have to change the definition of the multi-particle.
i.e.
define p = p / b~
define j = j / b~
(obviously such definition will prevent to have physical run of the parton-shower, but I guess/hope that you are not going to run those)

Now, the "/" syntax does not make distinction between particle and anti-particle, the reason is the for T-channel, such distinction does not make any sense (it is not lorentz invariant).
So in your case the two following syntax are identical:
generate p p > j t / b
generate p p > j t / b~

Cheers,

Olivier

PS: Remember that when using diagram filtering (like the "/" is doing) that you need to verify that your matrix-element is gauge invariant and lorentz invariant, you can use the check command for that.

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Steven Jeremies (minits) said : 		#2

Thanks mattelaer_bot, that solved my question.