# How to input the fine structure constant

Asked by Cheng Li on 2019-07-05

Hi!

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

Language:
English Edit question
Status:
Solved
For:
WHIZARD Edit question
Assignee:
Pascal Stienemeier Edit question
Last query:
2019-07-05
2019-09-30
 Pascal Stienemeier (pstienem) said on 2019-07-08: #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) ))

show(alpha_em_i)
show(GF)
...
---------------

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

Cheers,

Pascal

 Juergen Reuter (j.r.reuter) said on 2019-09-30: #2

This seems to have solved the issue.