The relation between the sigma(cross section) and the aewm1(1/alpha?) in the MadGraph.

HI,

The electroweak scheme used by default does not have a fix value of the W mass when you change aewm1.
and since your coupling also depend on the sintheta and costheta and that those are related to the ratio of mass between the W and Z,
then indeed the dependence in the cross-section in function as a_EW is not as simple as the one that you would get if you use another EW scheme (like the one where neither the W or Z mass does not depend of alpha_EW)

Cheers,

Olivier

> On 21 May 2022, at 15:30, he <email address hidden> wrote:
>
> New question #701900 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/701900
>
> We produce the cross section of e+e- > ZZ SM process in the tree level,
> generate e+ e- > Z Z
> output eeZZ
> launch eeZZ
> and get the following results for the different values of "aewm1" in the param_cards.dat:
> aewm1 = 1.2707E2 sigma = 0.144
> aewm1 = 1.3207E2 sigma = 0.1515
> aewm1 = 1.3707E2 sigma = 0.1605.
> We not change the other parameters.
>
> From theory, the cross section has the relation
> sigma~g**2/(4*pi)~alpha
> , but the results of the Madgraph are not correspond the theory.
>
> Also, we check the model/sm/paramaters.py file, and found the aew=1/aewm1, ee = 2*sqrt(aEW)*sqrt(pi).
>
> So we wonder if “aew” is the alpha (g**2/4*pi), and "aewm1" is the 1/alpha ? Or if we misunderstood elsewhere about MadGraph?
>
>
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