Why \alpha_EW=1/132.507?

Hi Mariano,

alpha_EW is indeed running and the exact value of this running depend on the value of alpha_s.
(Please read the PDG for more details on this point).

Note that You can change this value in the param_card.dat:
###################################
## INFORMATION FOR SMINPUTS
###################################
Block sminputs
    1 1.325070e+02 # aEWM1

To be complete, note that we don't make the running of alpha_ew inside MG5 (as we can do for alpha_s)

Cheers,

Olivier

Hello Mariano,

Just an addition to Olivier's reply.

At leading order, all electroweak parameters are given by leading-order expressions with 3 degrees of freedom. In the SLHA parameter input format, the input parameters are chosen to be alpha_EM(M_Z), G_F and M_Z. From these, sin^2\theta_W and M_W are calculated. Therefore, if you change alpha_EM, you need to remember that the W mass is given by a combination of the other paramaters (as is also indicated in the MadGraph param_card):

MW = sqrt(MZ^2/2. + sqrt(MZ^4/4. - (aEW*pi*MZ^2)/(Gf*sqrt(2))))

So if what you care about is alpha_EM, M_Z and M_W, you need to modify G_F accordingly to offset your change in alpha_EM.
This is also the reason why we don't do running of alpha_EM in MadGraph (it anyway changes very slowly with energy, so setting it to a single appropriate value should be good enough for collider processes).

All the best,
Johan