This is the declaration of the ShowerAlpha class.
This class is the abstract class from which all types of running alphas
used in the Showering (Initial State alphaQCD, alphaQED, alphaEWK,...
and Final State alphaQCD, alphaQED, alphaEWK,...) derived from.
The main purpose of this class, and the ones that derive from, is
to allow systematics evaluation of Initial State Radiation and,
independently, of Final State Radiation effects, by allowing a
multiplicative adimensional factor, which is 1.0 for the "central value",
for the scale argument, mu^2, of the running alpha:
ShowerAlpha( mu^2 ) ---> ShowerAlpha( scaleFactor * mu^2 )
This scale factor is a parameter which is settable by the user.
Although, of course, it is not clear my how much we should scale
in order to get a "one-sigma" systematics error (but factors:
1/2 and 2 are quite common), this method provides a double side error,
and it appears more sensible than the rough and one-side evaluation
via turning off the I.S.R. and/or F.S.R. (possibilities which are,
anyway, provided by Herwig++).
CLASS
SplittingGenerator;