MadGraph5_aMC@NLO

Decay particles when specifying squared orders constraints is not allowed

Asked by Shudong Wang

Dear experts，

I was generating the standalone cpp codes of VH H->gamma gamma process using the SMEFTsim model (SMEFTsim_U35_MwScheme_UFO) to calculate the MEs with the following lines:

import model SMEFTsim_U35_MwScheme_UFO-massless_cHWtil_cHBtil_cHWBtil
set gauge unitary

define v = z w+ w-
define jb = g u c d s b u~ c~ d~ s~ b~

generate p p > v h NP==1, h > a a NP=0
output standalone_cpp XXXXX

In order to have quadratic BSM contributions only, I set the NP==1, and when specifying this squared order constraint, Madgraph throws out an error that says "Decay processes cannot specify squared orders constraints". Since the latest version of MagGraph5 still doesn't support this, I changed the syntax to

...
generate p p > w+ h > jb jb a a NP==1 / a b
...

for the WH process, this will produce all the diagrams I want, but for the ZH process, there are many unwanted diagrams, for example:

d~ d > z > a h, ( h > a z, z > u u~)

Do you know how to exclude those diagrams or is there any way to bypass the restriction and generate the process I want?

Thanks,
Shudong

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MadGraph5_aMC@NLO Edit question
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Solved by:
Olivier Mattelaer
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Awais Saeed (awaishussain867) said : 		#1

It seems like you're encountering issues with MadGraph when trying to generate specific processes with quadratic BSM contributions only. Unfortunately, the error you're encountering ("Decay processes cannot specify squared orders constraints") suggests that MadGraph currently doesn't support specifying squared order constraints for decay processes.

One possible workaround to exclude unwanted diagrams is to use the `output` command in MadGraph to generate the code and then manually edit the generated code to remove the unwanted diagrams. However, this approach can be cumbersome and may not be feasible for complex processes.

Another approach could be to modify the model file (`SMEFTsim_U35_MwScheme_UFO-massless_cHWtil_cHBtil_cHWBtil`) to explicitly remove or suppress the unwanted diagrams. This would require understanding the structure of the model and how it generates Feynman diagrams in MadGraph. for more relevant information, regarding squared orders, please visit Internet Packages https://jazzinternetpackages.pk/internet-packages/

Alternatively, you may consider reaching out to the MadGraph developers or community forums to inquire about potential solutions or workarounds for your specific use case. They may be able to provide insights or guidance on how to achieve your desired process with quadratic BSM contributions only.

If none of these options prove successful, you might need to explore alternative tools or methods for generating the desired processes with quadratic BSM contributions only.

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Best Olivier Mattelaer (olivier-mattelaer) said : 		#2

Did you check the following FAQ?

Cheers,

Olivier (and please ignore the spam-robot)
FAQ #3322: “How to select of subset of diagram for LO computation?”.

Revision history for this message
Shudong Wang (sfsvds) said : 		#3

Hi Olivier,

Thanks for your reply, I didn't notice that there is an FAQ about this.
I'll have a try, thanks again!

Cheers,
Shudong

Revision history for this message
Shudong Wang (sfsvds) said : 		#4

Thanks Olivier Mattelaer, that solved my question.