  MSbar-like schemes in QCD have in general the running coupling which contains
Landau singularities, i.e., singularities outside the timelike semi-axis, at
low squared momenta. As a consequence, evaluation of the spacelike quantities,
such as current correlators, in terms of (powers of) such a coupling then
results in quantities which contradict the basic principles of Quantum Field
Theories. On the other hand, in those QCD frameworks where the running coupling
remains finite at low squared momenta (IR freezing), the coupling usually does
not have Landau singularities in the complex plane of the squared momenta. I
argue that in such QCD frameworks the spacelike quantities should not be
evaluated as a power series, but rather as a series in derivatives of the
coupling with respect to the logarithm of the squared momenta. Such series show
considerably better convergence properties. Moreover, Pade-related resummations
of such logarithmic derivative series give convergent series, thus eliminating
the practical problem of series divergence due to renormalons.

High Energy Physics - Phenomenology