Z total width at NLO

Asked by Sarah Malik


I wanted to know if it is possible to get the total Z width at NLO. I see that the LO width is in the input cards . Is there any way I can ascertain the total Z decay width at NLO. I've checked that the BR of Z-->vv and Z-->ll are consistent with PDG values (20% and 10.1% respectively) but would like to use these values and the total width to extract the partial widths for Z(vv) and Z(ll).

Many thanks


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Hua-Sheng Shao Edit question
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Hua-Sheng Shao (erdissshaw) said :

Hi Sarah, yes, there is a dedicated fast total width computation tool SMWidth for the SM at NLO presented in the appendix E.3 of arXiv:1804.10017. The receipt of how to run it in MG5_aMC can be found in the appendix E.3.2, e.g. via

> import mode loop_qcd_qed_sm_Gmu
> compute_widths z --nlo

On there other hand, you can also certainly get NLO (partial) widths of Z boson in the normal NLO computation, and summing all decay channels you obtain the total width.



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Sarah Malik (sarah-alam-malik) said :

Many thanks for your prompt response - this worked!

I did have one follow-up question.

I'm seeing a small (< 1%) shift in the Zvv/Zll ratio when I ask for the Z boson pT (calculated from the neutrinos/leptons) to be > 200 GeV. I'm trying to investigate this and I wonder if it is possible to cut directly on the Z boson pT (i.e on it's PDG ID) rather than calculate the Z pT from the decay products?



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Hua-Sheng Shao (erdissshaw) said :

Yes, you can certainly cut on Z boson pT at the generation level in run_card.dat via, e.g.

{23: <a number>} = pt_min_pdg

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Sarah Malik (sarah-alam-malik) said :

Many thanks for this - I did indeed discover this feature, although I think it only works if I'm just producing the Z in MG and not decaying it. If I allow it to decay then this cut has no effect and I need to cut on the Z boson pT via it's decay products.

I'm basically trying to answer the following question and haven't managed to figure out the optimal way to do it.

 I want to know what the Z total width and the Z partial decay widths for Z-->vv and Z-->ll are if I apply a Z boson pT cut of 200 GeV (and ask for the invariant mass to be between 71 and 111 GeV). I'm doing this at NLO (in version 2.6.7) and for Z+0,1j.
We would (perhaps naively) expect the width/partial widths to be the same with or without the Z boson pT cut but I'm not sure this is the case and I'd like to check this.

Do you have any suggestions for what would be the best way to check this?


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