Issue while using mmll cut in Madgraph

Hi,

1) the comment for that cut is:
" min invariant mass of l+l- (same flavour) lepton pair"

So i do not see why it should apply for your process.
To be sure, you can check the run_card, if the mmll cut does not appear in the default card, this means it has no potential impact for your process (need a recent version of the code for that0

2) since you use decay_chain, you need to check the value of the cut_decays, if set on False, a lot of cut will not be applied on the particles comming from the decays.

Cheers,

Olivier

Thanks for the prompt response Olivier. As you correctly said, the mmll cut does not appear as a default cut in the run card for newer versions of Madgraph. But out of curiosity, I added the mmll cut as a hidden parameter to see how it's affecting the cross-section. To my surprise, the cross-section decreased significantly but did not reduce to zero. Can you tell what's going on internally and why the final cross-section is greater than zero.

Hi,

> Can you tell what's going on internally and why the final cross-section
> is greater than zero.

The real question is why the two cross-sections are not the same, the value of that cut is irrelevant for your computation.

I have therefore run the following script:

define wx = w+ w-
define lx = e+ e- mu+ mu-
define vx = ve ve~ vm vm~
generate p p > wx, wx > lx vx @1
add process p p > wx j, wx > lx vx @2
#add process p p > wx j j, wx > lx vx @3
output
launch
launch
set mmll 80

Which keep parameter for a MLM run,
As you can see
1) I bypass the two jet multiplicity to have the code running super fast)
2) I did not run the parton-shower here (so reported cross-section are unphysical for this MLM setup)
3) The run are 100% identical up to the inclusion of the mmll parameter (to 80)

The resulting cross-section are
run 1 (mmll set as default value): 3.686e+04 +- 98
run 2 (mmll set to 80): 3.686e+04 +- 98

So I do not observe any impact of the cut

actually including the 2j sample was fast enought that it finished before me finishing this post.
here is the associated (still unphysical) cross-section

run 1 (mmll set as default value): 3.854e+04 +- 100
run 2 (mmll set to 80): 3.853e+04 +- 84
(again no impact of the cut)

Which is very close to the previous computation (first time I run this one in years, I was expecting bigger difference) looks like the 0j computation is actually very huge here (but likely highly suppresed when the parton-shower will multiply those by the proper sudakov form-factor to have the physical cross-section. Could you confirm me if this is the case or not when you will have the validation plot for your MLM generation?

Cheers,

Olivier

PS: I guess that you change other parameter in the run_card that can explain why your cross-section is not the same.

> On 17 Mar 2021, at 12:31, Mohit sharma <email address hidden> wrote:
>
> Question #696106 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/696106
>
> Mohit sharma posted a new comment:
> Thanks for the prompt response Olivier. As you correctly said, the mmll
> cut does not appear as a default cut in the run card for newer versions
> of Madgraph. But out of curiosity, I added the mmll cut as a hidden
> parameter to see how it's affecting the cross-section. To my surprise,
> the cross-section decreased significantly but did not reduce to zero.
> Can you tell what's going on internally and why the final cross-section
> is greater than zero.
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.