Offshell interference effects

This page describes step by step how to get the interference between signal and background for the WW and ZZ fully leptonic decay in the SM and some extended model within MG_aMC@NLO. Help can be provided for other decays if needed.

Authors : Benoît Hespel ​email and Eleni Vryonidou ​email

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SM

You can download a modified version of the loop_sm model (loop_sm_modif​) where we made simple changes in the couplings orders to be able to get the pieces (signal, background, interference) separately.

ZZ > ll l'l'

After you imported it inside the MG_2.3.1 loop_induced version, use the following syntax to get the different pieces for the ZZ > e+ e- µ+ µ- for example:

Signal :

generate g g > e+ e- mu+ mu- / e+ e- mu+ mu- [QCD] NP^2==2

Interference :

generate g g > e+ ve mu- vm~ / vm ve e+ mu- [QCD] NP^2==1

Total :

generate g g > e+ ve mu- vm~ / vm ve e+ mu- [QCD] NP=1

Background :

generate g g > e+ ve mu- vm~ / vm ve e+ mu- [QCD] NP=0

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WW > lv l'v'

For the WW > e+ ve mu- vm~ process for example, use:

Signal:

generate g g > e+ ve mu- vm~ [QCD] NP^2==2

Interference:

generate g g > e+ ve mu- vm~ [QCD] NP^2==1

Total:

generate g g > e+ ve mu- vm~ [QCD] NP=1

Background:

generate g g > e+ ve mu- vm~ [QCD] NP=0

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Additional heavy scalar

Then we extended the scalar sector of the SM by adding one additional particle called Y⁰. Via 2 parameters in the param_card, this particle can be set to be either a scalar (gSu33=1 & gPu33=0), a pseudoscalar (gSu33=0 & gPu33=1) or a mixed state (gSu33=x & gPu33=1-x).

The model can be downloaded here Scalar_simp_NLO_UFO​ and allows you to get the different pieces separately via the same commands as before.