How to input the fine structure constant

Asked by Cheng Li on 2019-07-05


I'm now using the whizard with NMSSM model and want to use a loop corrected fine structure alpha as an input. But it seems like the alpha is a dependent parameter and it is derived internally. If i add :

alpha_em_i = 127.92

in the SINDARIN script, then it will give the error message:

*** FATAL ERROR: Model variable 'alpha_em_i' is locked
So I just want to know how to input this fine structure constant, or maybe how to modify the model file to unlock the input of alpha?

Best regards,
Li, Cheng

Question information

English Edit question
WHIZARD Edit question
Pascal Stienemeier Edit question
Last query:
Last reply:
Pascal Stienemeier (pstienem) said : #1

Hi Cheng Li,

thanks for posting the first question here!

By default, WHIZARD uses the GF-mW-mZ scheme, so yes, alpha_em cannot be given directly but instead is derived from GF, mW and mZ.
I think instead of modifying the model, it is easier to compute GF for leading-order calculations from the value of alpha_em you want to use and use GF as input as intended by the model.
In the SINDARIN script you use to steer WHIZARD, you could do something like

model = NMSSM
real my_alpha_ew = 1 / 127.92
mZ = ...
wZ = ...
mW = ...
wW = ...
GF = pi / sqrt(2.0) * my_alpha_ew * 1/(mW^2 * (1 - (mW^2)/(mZ^2) ))


The show(alpha_em_i) tells you then that WHIZARD indeed uses the intended value of alpha_em:
NMSSM.alpha_em_i* => 1.279200000000E+02



Juergen Reuter (j.r.reuter) said : #2

This seems to have solved the issue.