MadGraph5_aMC@NLO

Higgs width at NLO

Asked by Inês Ochoa

Hi,

I am using MadSpin to decay H->bb, in WH events produced at NLO with aMC@NLO.
Does this decay include NLO effects in QCD by default? Therefore, is it appropriate to calculate the Higgs width at LO (http://madgraph.hep.uiuc.edu/Calculators/sm/sm_ufo_calc.html)? I assume not, but in that case can I use an external value (available for example from the LHC Yellow Reports)?

Thank you!
Inês

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Olivier Mattelaer
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Best Olivier Mattelaer (olivier-mattelaer) said : 		#1

Hi Ines,

No the H > b b~ decay is generated at LO not at NLO accuracy (but with full spin-correlation/width effect up to a threshold).
This is not possible for the method to include NLO effect for the decay. The only way to do that would be to neglect spin-correlation effect/width effect.

For the Higgs, the exact value of the width is not important for the distribution (The width is so small that you are not sensitive to it experimentally) but it is off-course important for the total normalization where the use of the most precise one make sense.
Note that since the dependency in the Higgs width is trivial, you can rescale easily after the generation if needed.

Cheers,

Olivier

On Jun 13, 2014, at 2:51 PM, Inês Ochoa <email address hidden> wrote:

> New question #250203 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/250203
>
> Hi,
>
> I am using MadSpin to decay H->bb, in WH events produced at NLO with aMC@NLO.
> Does this decay include NLO effects in QCD by default? Therefore, is it appropriate to calculate the Higgs width at LO (http://madgraph.hep.uiuc.edu/Calculators/sm/sm_ufo_calc.html)? I assume not, but in that case can I use an external value (available for example from the LHC Yellow Reports)?
>
> Thank you!
> Inês
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

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Inês Ochoa (inesochoa) said : 		#2

Hi Olivier,

I see. I do want to get the correct cross-section, so I will use the LO value from the MadGraph calculator.

Thanks!
Inês

Revision history for this message
Inês Ochoa (inesochoa) said : 		#3

Thanks Olivier Mattelaer, that solved my question.