Universality of 1/Q corrections to jet-shape observables rescued

We address the problem of potential non-universality of the leading 1/Q power corrections to jet shapes emerging from the non-inclusive character of these observables. We consider the thrust distribution as an example and analyse the non-inclusive contributions which emerge at the two-loop level, Although formally subleading in alpha(s), they modify the existing naive one-loop result for the expected magnitude of the power term by a factor of order unity. Such a promotion of a subleading correction into a numerical factor is natural since the non-perturbative power terms are explicitly proportional to powers of the QCD scale Lambda which can be fixed precisely only at the two-loop level, The "jet-shape scaling factor" depends on the observable but remains perturbatively calculable, Therefore it does not undermine the universal nature of 1/Q power corrections, which remain expressible in terms of the universal running coupling and universal soft-gluon emission. (C) 1998 Elsevier Science B.V.