TTbar NLO generation

Asked by Kirill Skovpen


Is it possible to generate ttbar NLO events in 5FS with additional EW contributions by doing:

define p = p b b~
define j = p
import model loop_sm-no_b_mass
generate p p > t t~ QED=2 [QCD]

This fails during collinear tests. Setting decay widths to zero does not seem to help either. Similar process at LO works fine.

Thanks a lot for any insights,


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Best davide.pagani.85 (davide-pagani) said :

Hi Kirill,

at the moment it is not possible to generate Events and shower them including EW corrections.
It is only possible to perform fixed order calculations.

In general, it not possible to calculate QCD corrections on top of subleading tree-level contributions and adding NLO QCD corrections and then shower them.

To my understanding, you are trying to take tree-level contributions purely EW (leading to cross section of order alpha_EW^2) and add NLO QCD corrections on top of them, obtaining the NLO contribution of order alpha_S * alpha_EW^2.
But this can also be seen as the NLO EW corrections on top of order alpha_S * alpha_EW.

Is that what are you aiming?

In the language of you are calculating LO,3+NLO,3 a subclass of the Complete NLO.

As said, it cannot be done with generation of events.
You can do it at Fixed-order, but you need to type

import model loop_qcd_qed_sm_Gmu-no_widths
generate p p > t t~ QED^2=4 [QCD]
output ttbar

as also explained in

the reason why you get the error is that you imported the wrong model, it has nothing to do with generation of events.

If you are instead interested to NLO EW, the story does not change. You simply have to type

import model loop_qcd_qed_sm_Gmu-no_widths
generate p p > t t~ [QED]
output ttbar

for fixed order calculation.

Hope this can help.

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Kirill Skovpen (kirill-skovpen) said :

Thanks davide.pagani.85, that solved my question.