NLO ickkw and MadSpin (spin correlations and off shell effects)

Asked by Saurabh Bansal on 2020-02-22

Dear MadGraph5_aMC@NLO team,

I'm generating a single process:

generate p p > t t~ [QCD]

1. First, in the run_card, what should I use for "ickkw"? From earlier discussions, I found that if I'm not generating multiple processes (i.e. add process p p > t t~ j [QCD]), I don't need to turn on ickkw, since there is no double-counting issue. So I'm using ickkw = 0. Can you please confirm this?

# 0: No merging
# 3: FxFx Merging -
# 4: UNLOPS merging (with pythia8 only).
# -1: NNLL+NLO jet-veto computation. See arxiv:1412.8408 [hep-ph].

2. Second, it looks like I cannot decay tops in [QCD] mode. It looks like I have two options, decaying tops either in MadSpin or Pythia8. For MadSpin, I'm using the following syntax in madspin_card.dat, for di-leptonic decays:

define lepton+ = l+ ta+
define lepton- = l- ta-
decay t > w+ b, w+ > lepton+ vl
decay t~ > w- b~, w- > lepton- vl~
launch

Does this take into account a full spin correlation of tops (1405.0301)? Can you please confirm it?

Also does this take into account off-shell effects of tops and W bosons? Can you please confirm it? The syntax I used work at generation-level is "t > b lepton+ vl" which include both on-shell and off-shell effects, but the same syntax didn't work in MadSpin.

Finally, if I decay tops using Pythia8, does this take into account spin correlations and off-shell effects? Is there a difference between using MadSpin and Pythia8?

Thank you in advance!

Sincerely

Saurabh

Question information

Language:
English Edit question
Status:
Solved
For:
MadGraph5_aMC@NLO Edit question
Assignee:
No assignee Edit question
Solved by:
Olivier Mattelaer
Solved:
2020-02-27
Last query:
2020-02-27
Last reply:
2020-02-22

Hi,

1. ckkw:
yes you are correct

> 2. Does this take into account a full spin correlation of tops (1405.0301)? Can you please confirm it?

You have the details in the paper that you quote.
But yes this takes into accout full spin correlation.

> 3. Also does this take into account off-shell effects of tops and W bosons?

Up to some cut-off (in order to keep under control gauge invariance) and without effect of non resonant diagram.

I would add that the precision of the decay is technically pure LO.

> Finally, if I decay tops using Pythia8, does this take into account spin correlations and off-shell effects? Is there a difference between using MadSpin and Pythia8?

Pythia8 does not have (as far as I know) spin-correlation.
For off-shell effects, I think they have it, even if some people reported here that this was not the case.

Cheers,

Olivier

> On 22 Feb 2020, at 03:23, Saurabh Bansal <email address hidden> wrote:
>
> New question #688920 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/688920
>
> Dear MadGraph5_aMC@NLO team,
>
> I'm generating a single process:
>
> generate p p > t t~ [QCD]
>
> 1. First, in the run_card, what should I use for "ickkw"? From earlier discussions, I found that if I'm not generating multiple processes (i.e. add process p p > t t~ j [QCD]), I don't need to turn on ickkw, since there is no double-counting issue. So I'm using ickkw = 0. Can you please confirm this?
>
> # 0: No merging
> # 3: FxFx Merging -
> # 4: UNLOPS merging (with pythia8 only).
> # -1: NNLL+NLO jet-veto computation. See arxiv:1412.8408 [hep-ph].
>
> 2. Second, it looks like I cannot decay tops in [QCD] mode. It looks like I have two options, decaying tops either in MadSpin or Pythia8. For MadSpin, I'm using the following syntax in madspin_card.dat, for di-leptonic decays:
>
> define lepton+ = l+ ta+
> define lepton- = l- ta-
> decay t > w+ b, w+ > lepton+ vl
> decay t~ > w- b~, w- > lepton- vl~
> launch
>
> Does this take into account a full spin correlation of tops (1405.0301)? Can you please confirm it?
>
> Also does this take into account off-shell effects of tops and W bosons? Can you please confirm it? The syntax I used work at generation-level is "t > b lepton+ vl" which include both on-shell and off-shell effects, but the same syntax didn't work in MadSpin.
>
> Finally, if I decay tops using Pythia8, does this take into account spin correlations and off-shell effects? Is there a difference between using MadSpin and Pythia8?
>
> Thank you in advance!
>
> Sincerely
>
> Saurabh
>
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Saurabh Bansal (saurabhbansal) said : #2

Thanks Olivier Mattelaer, that solved my question.