deduce the branching fractions for Z decays

Asked by Sarah Malik on 2021-06-08

I'm trying to deduce the ratio of branching fractions for Z decay to neutrinos and Z decay to charged leptons from the following samples that were centrally generated with MG_aMC@NLO at NLO in CMS:

DY (ll)+ 2j
Z(vv) + 2j

I look at the event yields in my fiducial region (within 20 GeV of Z mass and Z boson pT > 200 GeV) and after correcting the DY sample for the gamma* correction, I obtain a ratio of Z(vv)/Z(ll) which is a few % higher than 6.0. I want to deduce the branching fraction for Z-->ll and for Z-->vv that were used in the MC to see how they compare to the PDG values. Is it possible to do this?

Many thanks!

Sarah

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Olivier Mattelaer (olivier-mattelaer) said :
#1

Hi,

If any cuts are applied on those samples then this will bias the ratio (and the BR can not be used in that case)

Cheers,

Olivier

> On 8 Jun 2021, at 14:20, Sarah Malik <email address hidden> wrote:
>
> New question #697448 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/697448
>
> I'm trying to deduce the ratio of branching fractions for Z decay to neutrinos and Z decay to charged leptons from the following samples that were centrally generated with MG_aMC@NLO at NLO in CMS:
>
> DY (ll)+ 2j
> Z(vv) + 2j
>
>
> I look at the event yields in my fiducial region (within 20 GeV of Z mass and Z boson pT > 200 GeV) and after correcting the DY sample for the gamma* correction, I obtain a ratio of Z(vv)/Z(ll) which is a few % higher than 6.0. I want to deduce the branching fraction for Z-->ll and for Z-->vv that were used in the MC to see how they compare to the PDG values. Is it possible to do this?
>
> Many thanks!
>
> Sarah
>
>
>
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Revision history for this message
Sarah Malik (sarah-alam-malik) said :
#2

Many thanks for your prompt response Olivier.

The only cuts applied are: Z invariant mass within 71-111 GeV, as this is the region in which the gamma* contribution is calculated, and Z boson pT > 200 GeV. If I removed the Z boson pT cut, should I expect the the BR to be consistent with those in the PDG? How can I deduce the BR in the scenario that no cuts are applied?

Thanks

Sarah

Revision history for this message
Olivier Mattelaer (olivier-mattelaer) said :
#3

> The only cuts applied are: Z invariant mass within 71-111 GeV, as this
> is the region in which the gamma* contribution is calculated, and Z
> boson pT > 200 GeV.

As long as you do not have any cut on the lepton, this should be possible.

> should I expect the
> the BR to be consistent with those in the PDG?

Likely not.

> How can I deduce the BR
> in the scenario that no cuts are applied?

You can not have the BR since you likely miss the full width value.
from those two samples you can compute BR_nn/BR_ll

Cheers,

Olivier

> How can I deduce the BR
> in the scenario that no cuts are applied?

> On 8 Jun 2021, at 14:55, Sarah Malik <email address hidden> wrote:
>
> Question #697448 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/697448
>
> Status: Answered => Open
>
> Sarah Malik is still having a problem:
> Many thanks for your prompt response Olivier.
>
> The only cuts applied are: Z invariant mass within 71-111 GeV, as this
> is the region in which the gamma* contribution is calculated, and Z
> boson pT > 200 GeV. If I removed the Z boson pT cut, should I expect the
> the BR to be consistent with those in the PDG? How can I deduce the BR
> in the scenario that no cuts are applied?
>
> Thanks
>
> Sarah
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

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