MadGraph5_aMC@NLO

Including BSM NLO contributions to ttbar

Asked by andlessa

Hi,

I'm trying study BSM loop contributions to p p > t t~. The new physics is parametrized by a single coupling order (NP).

The model file I'm using (which I can provide) contains the UV and R2 terms with contributions from both QCD and BSM loops, so I'm able to run:

p p > t t~ [NP]

and I get the expected born diagrams and the loop diagrams from new physics. In this case there is no real emission, since all the loop particles are heavy. However, the total cross-section I get is the same as the pure SM (LO) one.
Also, it does not depend on the value of the NP couplings (if it is tiny or order ~1, I get exactly the same total cross-section)
.
Finally, if I try to run at fixed order I get the error: "ERROR IN SBORN_SF, both color and charged links are needed".

Could you point to what I could be doing wrong?

Cheers,

-Andre

Question information

Language:
English Edit question
Status:
Answered
For:
MadGraph5_aMC@NLO Edit question
Assignee:
marco zaro Edit question
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Last reply:
Revision history for this message
marco zaro (marco-zaro) said : 		#1

Dear Andre,
can you provide me (by email or linking it here) the model file?
I will have a look

marco

Revision history for this message
andlessa (lessa-a-p) said : 		#2

Dear Marco,

Thanks for the quick reply. The model files can be downloaded here:

https://cernbox.cern.ch/s/ay1pyScOuW3hLE5

Best,

-Andre

Revision history for this message
marco zaro (marco-zaro) said : 		#3

Dear Andre,
the process you are generating has only virtual NLO corrections (not reals), as far as I can see...
this feature needs some ad-hoc tweaking of the code
I will try to have a deeper look on what needs to be done and let you know
Thanks

Best,

Marco

Revision history for this message
andlessa (lessa-a-p) said : 		#4

Thanks!
It would be very helpful to know how to generate this process.

Revision history for this message
marco zaro (marco-zaro) said : 		#5

Hi Andre`,
as I told you, I don't think there is a "standard" way to do what you need.
One way you might consider, is to generate the standalone virtual amplitude (interfered with the born), and use it to reweight events generated in the SM; otherwise, I don't know what to suggest here.

I tried to hack fks_singular by adding line 6622 (this skips the error about charge/color links)

6620 if (abrv.eq.'born') goto 549
6621 if (abrv.eq.'virt') goto 547
6622 if (abrv.eq.'bovi') goto 548

but then it is stuck in the function which computes the virtual (BinothLHA) like forever...

So I don't know...

Best ,

marco

Revision history for this message
andlessa (lessa-a-p) said : 		#6

Hi Marco,

Thanks for the prompt reply.

I believe I have already tried what you suggested, but the resulting
cross-section (including only the Born plus BSM loops) seems to be
independent of the BSM contributions. I checked it increasing the BSM
coupling, which should increase the contribution from the BSM loops, but
the cross-section does not change.

One quick question: assume I generate the diagrams for the Born plus the
BSM virtual corrections.
The MG5 events in this case will correspond to |born + virtual|^2 or to
|born|^2 + born*virtual ?

Best,

Andre

Em sex., 23 de jun. de 2023 15:05, marco zaro <
<email address hidden>> escreveu:

> Your question #706710 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/706710
>
> marco zaro posted a new comment:
> Hi Andre`,
> as I told you, I don't think there is a "standard" way to do what you need.
> One way you might consider, is to generate the standalone virtual
> amplitude (interfered with the born), and use it to reweight events
> generated in the SM; otherwise, I don't know what to suggest here.
>
> I tried to hack fks_singular by adding line 6622 (this skips the error
> about charge/color links)
>
> 6620 if (abrv.eq.'born') goto 549
> 6621 if (abrv.eq.'virt') goto 547
> 6622 if (abrv.eq.'bovi') goto 548
>
> but then it is stuck in the function which computes the virtual
> (BinothLHA) like forever...
>
> So I don't know...
>
> Best ,
>
> marco
>
> --
> You received this question notification because you asked the question.
>

Revision history for this message
marco zaro (marco-zaro) said : 		#7

Hi,

> On 23 Jun 2023, at 15:55, andlessa <email address hidden> wrote:
>
> Question #706710 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/706710
>
> andlessa gave more information on the question:
> Hi Marco,
>
> Thanks for the prompt reply.
>
> I believe I have already tried what you suggested, but the resulting
> cross-section (including only the Born plus BSM loops) seems to be
> independent of the BSM contributions. I checked it increasing the BSM
> coupling, which should increase the contribution from the BSM loops, but
> the cross-section does not change.
>
>
> One quick question: assume I generate the diagrams for the Born plus the
> BSM virtual corrections.
> The MG5 events in this case will correspond to |born + virtual|^2 or to
> |born|^2 + born*virtual ?

the second one (ie no virtual squared);

Cheers,

Marco
>
> Best,
>
> Andre
>
> Em sex., 23 de jun. de 2023 15:05, marco zaro <
> <email address hidden>> escreveu:
>
>> Your question #706710 on MadGraph5_aMC@NLO changed:
>> https://answers.launchpad.net/mg5amcnlo/+question/706710
>>
>> marco zaro posted a new comment:
>> Hi Andre`,
>> as I told you, I don't think there is a "standard" way to do what you need.
>> One way you might consider, is to generate the standalone virtual
>> amplitude (interfered with the born), and use it to reweight events
>> generated in the SM; otherwise, I don't know what to suggest here.
>>
>> I tried to hack fks_singular by adding line 6622 (this skips the error
>> about charge/color links)
>>
>> 6620 if (abrv.eq.'born') goto 549
>> 6621 if (abrv.eq.'virt') goto 547
>> 6622 if (abrv.eq.'bovi') goto 548
>>
>> but then it is stuck in the function which computes the virtual
>> (BinothLHA) like forever...
>>
>> So I don't know...
>>
>> Best ,
>>
>> marco
>>
>> --
>> You received this question notification because you asked the question.
>>
>
> --
> You received this question notification because you are assigned to this
> question.

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