SPINUP information for Ztautau at NLO

Asked by Josh McFayden on 2021-05-14

Dear MG5_aMC team,

We are currently developing a Z+jet FxFx setup with up to 3 jets in the final state and where the Z decays to tau pairs. Our process is defined with the standard commands:
generate p p > ta+ ta- [QCD] @0
add process p p > ta+ ta- j [QCD] @1
add process p p > ta+ ta- j j [QCD] @2
add process p p > ta+ ta- j j j [QCD] @3

We intend to shower the events using Pythia8. We want to include all tau decay modes and would prefer
(if possible) not use the narrow width approximation, but we find that the SPINUP information is not present in
 the LHE file. It seems that this is intentional from:
https://answers.launchpad.net/mg5amcnlo/+question/246759
https://answers.launchpad.net/mg5amcnlo/+question/249080

But we wanted to ask if there is any way to generate these events so that spin correlations are preserved? Is there a fundamental reason why the SPINUP information cannot be added to the LHE for NLO processes?

Best,

Josh

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Olivier Mattelaer (olivier-mattelaer) said :
#1

Hi,

The issue is that some counter-event does not have the same helicity as the event and therefore they are ambuiguity on the associated meaning. Additionally not all contribution are computed in the same lorentz frame which is problematic for massive particles since this is not the same quantity in all the frame. Finally the compuation of the loops are done for the sum of helicity directly since otherwise the computation of that term will be even slower.

Now MadSpin has a mode for assigning a SPINUP information to a event file (without performing any decay).
The assignment is using a naive mapping (S-event will use born matrix-element for the determination of SPINUP) while H-event will use the real matrix-element.

Cheers,

Olivier

> On 14 May 2021, at 17:11, Josh McFayden <email address hidden> wrote:
>
> New question #697059 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/697059
>
> Dear MG5_aMC team,
>
> We are currently developing a Z+jet FxFx setup with up to 3 jets in the final state and where the Z decays to tau pairs. Our process is defined with the standard commands:
> generate p p > ta+ ta- [QCD] @0
> add process p p > ta+ ta- j [QCD] @1
> add process p p > ta+ ta- j j [QCD] @2
> add process p p > ta+ ta- j j j [QCD] @3
>
> We intend to shower the events using Pythia8. We want to include all tau decay modes and would prefer
> (if possible) not use the narrow width approximation, but we find that the SPINUP information is not present in
> the LHE file. It seems that this is intentional from:
> https://answers.launchpad.net/mg5amcnlo/+question/246759
> https://answers.launchpad.net/mg5amcnlo/+question/249080
>
> But we wanted to ask if there is any way to generate these events so that spin correlations are preserved? Is there a fundamental reason why the SPINUP information cannot be added to the LHE for NLO processes?
>
> Best,
>
> Josh
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

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