MadGraph5_aMC@NLO

value of aEWM1

Asked by leonidas

Hello,
I have a question about the value of aEWM1 on the sm. On PDG I find the Z pole value: 127.9
However MadGraph comes with a default value of 132.50698. Where does this value come from? Is this value recommended for LHC simulations at 13/14 TeV?
Thank you.
Leonidas

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Olivier Mattelaer (olivier-mattelaer) said : 		#1

Hi,

As for the width, you have some freedom in your choice of parameter for the Electroweak sector.

In the EW sector, you have four measured parameters
1) aEWM1
2) Gf
3) MZ
4) MW

But only three free parameters.
In MG5aMC, the choice is that MW is computed (at tree level) from the other three.
Now this creates a bit of freedom on your choice of parameter.
Indeed you have
1) the experimental precision
2) the fact that we use a LO relation between those parameters (needed for the consistency of the computation)

From there you have to decide which experimental constraint you want to really use.

As for the width, I'm not the one who did such choice (this one was done before I started to work on MG5aMC).

Obviously, you are fully free to not use the default. (I actually do not know if CMS/ATLAS are using our default or not)

Cheers,

Olivier

> On 7 Aug 2018, at 21:42, leonidas <email address hidden> wrote:
>
> New question #671420 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/671420
>
> Hello,
> I have a question about the value of aEWM1 on the sm. On PDG I find the Z pole value: 127.9
> However MadGraph comes with a default value of 132.50698. Where does this value come from? Is this value recommended for LHC simulations at 13/14 TeV?
> Thank you.
> Leonidas
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

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leonidas (leonidasprado) said : 		#2

Hello,
Thank you for your answer.
Best,
Leônidas

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Tania Robens (tania-robens) said : 		#3

Hi we have the same question for aEWM1. What are the inputs to get this ? Thanks [and sorry for reopening]

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Olivier Mattelaer (olivier-mattelaer) said : 		#4

Not sure to understand what you want more than the answer above.
Could you be more specific?

Olivier

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Tania Robens (tania-robens) said : 		#5

OK I figure it is the tree level prediction for aewm1 using Gf, mW, and mZ as input [probably for previous values]. For others who are interested.

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Olivier Mattelaer (olivier-mattelaer) said : 		#6

This technically depends of the model , in general the default is that MW is given by the other three parameters.

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Tania Robens (tania-robens) said : 		#7

Hi Olivier

the question was where the number 132.50698 comes from for aEWM1 which is default in the SM model file if I see it correctly. I then tried the tree level definition of al_em with Gf, mZ, mW as input and got 132.168. This was the original question above which to my feeling was not answered completely. Sorry for being unclear.

Best Tania