MadGraph5_aMC@NLO

Polarized NLO process (ggZZ) generation

Asked by Mingyi Liu

Dear MadGraph5_aMC@NLO Authors,

We are trying to generate g g > Z Z > 4l process with the polarization state of Z boson specified, like:
g g > z{0} z{0} > 4l
We are testing with MG5 v2.7.3
The definition of our process is:

import model loop_sm
define l+ = e+ mu+
define l- = e- mu-
generate g g > z{0} z{0} [QCD], z > l+ l-

Unfortunately, this definition currently does not work.
The error looks like this:
InvalidCmd : Polarization restriction can not be used for NLO processes

So we would like to check with you: Is there some syntax error with our definition for the process, or just as the error information indicates, polarized NLO processes (or more specifically, ggZZ) are not supported currently?

Thanks in advance!

Sincerely,
Mingyi

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MadGraph5_aMC@NLO Edit question
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Solved by:
Olivier Mattelaer
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Best Olivier Mattelaer (olivier-mattelaer) said : 		#1

Hi,

Yes, I confirm that polarization restriction are not possible for NLO process.
The issue is that NLO process is using the fact that matrix-element are Lorentz invariant within the computation and this is not true anymore for polarised amplitude. The implementation of the polarization at NLO requires therefore some non trivial work (which has been recently funded)

Actually, your process is not NLO but LO (loop-induced) and therefore this is not the real issue.
However, your syntax is also not valid for loop-induced since decay-chain syntax is not allowed for loop-induced processes.
So you can produce
generate g g > z{0} z{0} [noborn=QCD]
but we do not have any "valid" option to decay the z in this case (at least no solution to decay it with full spin-correlation).
The only option, I can see here is to use the old bridge code but that code/method has not been maintained for years (at least since 2014 for the sm only version that was shipped with mg5amc at that time). So you likely need to redevelop the method or to fix the compilation issue of the old code.

Cheers,

Olivier

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Mingyi Liu (myliu) said : 		#2

Thanks Olivier Mattelaer, that solved my question.