MadGraph5_aMC@NLO

Difference in the top quark decays between MG5 and Pythia8

Asked by Mykhailo Dalchenko

During the gen-level study of the signal samples for the new physics search we found some discrepancy between the results from MadGraph 5 (v 1_5_14) and Pythia 8.2.
We generate the process pp > X > t, n, where p is for protons, X is for new colored scalar, t is for top quark and n is for the DM candidate.
Since we are interested in the leptonic decays of the top-quark, we tried to generate only particular final state with the t > b W > b l v with the Mad Graph.
We observed that the transverse momentum spectrum of the generated b-quark is softer than we expected.
In fact, Mad Graph gives softer spectrum of the b-quark transverse momentum than Pythia 8.
Do you have any thoughts about what can cause such a difference?

Best regards,
Mykhailo Dalchenko
on behalf of CMS Monotop analysis team

Question information

Language:
English Edit question
Status:
Answered
For:
MadGraph5_aMC@NLO Edit question
Assignee:
No assignee Edit question
Last query:
Last reply:
Revision history for this message
Olivier Mattelaer (olivier-mattelaer) said : 		#1

Hi,

First guess is spin correlation which is not cover at all by Pythia8 and fully cover by MG5.

Cheers,

Olivier

On 27 Feb 2015, at 19:21, Mykhailo Dalchenko <email address hidden> wrote:

> New question #262931 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/262931
>
> During the gen-level study of the signal samples for the new physics search we found some discrepancy between the results from MadGraph 5 (v 1_5_14) and Pythia 8.2.
> We generate the process pp > X > t, n, where p is for protons, X is for new colored scalar, t is for top quark and n is for the DM candidate.
> Since we are interested in the leptonic decays of the top-quark, we tried to generate only particular final state with the t > b W > b l v with the Mad Graph.
> We observed that the transverse momentum spectrum of the generated b-quark is softer than we expected.
> In fact, Mad Graph gives softer spectrum of the b-quark transverse momentum than Pythia 8.
> Do you have any thoughts about what can cause such a difference?
>
> Best regards,
> Mykhailo Dalchenko
> on behalf of CMS Monotop analysis team
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Can you help with this problem?

Provide an answer of your own, or ask Mykhailo Dalchenko for more information if necessary.

To post a message you must log in.