renormalization and factorization scale

Asked by Yingjie Wei

Dear Madgraph experts,

          I am sorry to ask about this technique question but I couldn't find the answer anywhere: why do we set the renormalization and factorization scale as 1/2 and 2 to get the +/- "1 sigma" (correct me if I was wrong) uncertainties from the scale, instead of 1/4 and 4, or 1/3 and 3? Is there some reference on this?

Many thanks,
Yingjie

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Olivier Mattelaer (olivier-mattelaer) said :
#1

Hi,

I do not think that they are any good reference on that point.
Mainly because they are no good reason to use a factor 2 instead of a factor of 3 or 4 (or even more).
Scaling up and down the scale gives SOME contribution of higher order term and therefore provides an indication of the sensitivity but such scaling does not provide the full picture.

Actually, no one should call that a 1-sigma error since the range is not related to a probability that the true value his in that given range.

Cheers,

Olivier

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Yingjie Wei (yingjiewei) said :
#2

Thanks a lot for your prompt answer, especially outside office hours.

Cheers,
Yingjie