MadGraph5_aMC@NLO

O(alpha_ew^6) W+W- production at NLO in QCD

Asked by lenzip

Dear Experts,
I would like to generate purely electroweak opposite sign WW at NLO in QCD.
The proc_card I have been using to do this at LO is the following.

set group_subprocesses Auto
set ignore_six_quark_processes False
set loop_optimized_output True
set complex_mass_scheme False
import model sm-ckm_no_b_mass
define p = g u c d s b u~ c~ d~ s~ b~
define j = p
generate p p > w+ w- j j $ t t~ QED=4 QCD=0, w+ > e+ ve, w- > mu- vm~
add process p p > w+ w- j j $ t t~ QED=4 QCD=0, w+ > mu+ vm, w- > e- ve~
output VBS_OS

Can I simply add the [QCD] tag to the two process lines to get NLO in QCD?

Many thanks in advance
Giulio

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Olivier Mattelaer (olivier-mattelaer) said : 		#1

Hi,

No. Decay-chain are not supported for NLO processes. You are forced to use MadSpin to handle the decay.
so you can do

generate p p > w+ w- j j $ t t~ QED<=4 QCD=0
and then ask madspin for the decay
decay w+ > l+ vl
decay w- > l- vl~

When you generate this process, you will see a lot of warning:
WARNING: Some loop diagrams contributing to this process are discarded because they are not pure (QCD)-perturbation.
Make sure you did not want to include them.
Those are related to the pentagon diagram which can not be included (since they are a mix QCD/QED loop).

If I'm correct, those are typically negligeable. They can however spoil the cancelation of the pole:
You can see here on how to fix this:
https://answers.launchpad.net/mg5amcnlo/+faq/2720

Cheers,

Olivier

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