Physical Coupling Schemes and Qcd Exclusive Processes

where nf is the effective number of quark flavors. The finite-order expression depends on both μ and the choice of scheme used to define the coupling. In fact, Eq. (1) can be made to take on essentially any value by varying μ and the renormalization scheme, which are a priori completely arbitrary. The scale/scheme problem is that of choosing μ and the scheme s in an “optimal” way, so that an unambiguous theoretical prediction, ideally including some plausible estimate of theoretical uncertainties, can be made.()