tt+gamma @ NLO

Asked by Ben Nachman

We are generating tt+gamma with NLO+PS. Ignoring photon radiation from the top and its immediate decay products work fine: p p > t t~ [QCD]. However, there is a significant fraction of photons at low photon pT where the photon originates from the top quark or its immediate decay products. At LO, we did e.g.

p p > t t~ > b b~ mu+ mu- vm vm~ a

Is it meaningful to do

p p > t t~ > b b~ mu+ mu- vm vm~ a [QCD]

? Running this naively produces the error

(...)
11:30:10 Soft test 16 PASSED. Fraction of failures: 0.00
11:30:10
11:30:10
11:30:10
11:30:10 Collinear tests done for (Born) config 1
11:30:10 Failures: 100
11:30:10 Collinear test 16 FAILED. Fraction of failures: 1.00
(...)

Thank you for your help!

Sincerely,
Ben

Question information

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

Dear Ben,
at NLO, the two working solutions are:
p p > t t~ a
or
p p > b b~ l+ l- vl vl~ a

note that the second is VERY time and CPU expensive.
Currently there is no way to specify that the photon should be radiated only by the top or its decay

I hope this can help.

Best,

Marco

Revision history for this message
Ben Nachman (bnachman) said :
#2

Dear Marco,

Thank you! I was under the impression that p p > b b~ l+ l- vl vl~ a would call for off-shell top quarks and that this is not currently handled properly. Is this true?

Sincerely,
Ben

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

Dear Ben,
In principle off shell top quarks should be handled correctly now at NLO+PS (it has always been the case for fixed NLO)
In any case,
> p p > b b~ l+ l- vl vl~ a
Is an extremely hard process to compute, probably beyond the capabilities of the code
Best,
Marco

On 11 Oct 2016, at 15:43, Ben Nachman <email address hidden> wrote:

> Question #402827 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/402827
>
> Status: Answered => Open
>
> Ben Nachman is still having a problem:
> Dear Marco,
>
> Thank you! I was under the impression that p p > b b~ l+ l- vl vl~ a
> would call for off-shell top quarks and that this is not currently
> handled properly. Is this true?
>
> Sincerely,
> Ben
>
> --
> You received this question notification because you are subscribed to
> the question.

Revision history for this message
Ben Nachman (bnachman) said :
#4

Dear Marco,

Thank you very much! One more followup question: is it possible to do t > b l+ vl a in MadSpin? If we wanted to get the 'top at NLO' but were okay with the photon at LO (but better than LL) something like this could be great. Thank you again-

Sincerely,
Ben

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

Dear Ben
yes, you can decay the top as a sequence of 2->1 decay
t > b w+
w+ > l+ vl
spin correlations will be kept.
Best,

Marco

On 11 Oct 2016, at 16:47, Ben Nachman <email address hidden> wrote:

> Question #402827 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/402827
>
> Status: Answered => Open
>
> Ben Nachman is still having a problem:
> Dear Marco,
>
> Thank you very much! One more followup question: is it possible to do t
>> b l+ vl a in MadSpin? If we wanted to get the 'top at NLO' but were
> okay with the photon at LO (but better than LL) something like this
> could be great. Thank you again-
>
> Sincerely,
> Ben
>
> --
> You received this question notification because you are subscribed to
> the question.

Revision history for this message
Ben Nachman (bnachman) said :
#6

Dear Marco,

Thanks! But can the photon radiation be included in MadSpin?

Sincerely,
Ben

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

No, unfortunately not
Best
M

On Oct 11, 2016 6:22 PM, "Ben Nachman" <email address hidden>
wrote:

> Question #402827 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/402827
>
> Status: Answered => Open
>
> Ben Nachman is still having a problem:
> Dear Marco,
>
> Thanks! But can the photon radiation be included in MadSpin?
>
> Sincerely,
> Ben
>
> --
> You received this question notification because you are subscribed to
> the question.
>

Revision history for this message
Ben Nachman (bnachman) said :
#8

Thanks marco zaro, that solved my question.