Perturbative and non-perturbative aspects of moments of the thrust distribution in e+e−

Resummation and power-corrections play a crucial role in the phenomenology ofevent-shape variables like the thrust T. Previous investigations showed thatthe perturbative contribution to the average thrust is dominated by gluons ofsmall invariant mass, of the order of 10% of Q, where Q is the center-of-massenergy. The effect of soft gluons is also important, leading to anon-perturbative 1/Q correction. These conclusions are based on renormalonanalysis in the single dressed gluon (SDG) approximation. Here we analyzehigher moments of the thrust distribution using a similar technique. We findthat the characteristic gluon invariant mass contributing to <(1-T)^m>increases with m. Yet, for m=2 this scale is quite low, around 27% of Q, andtherefore renormalon resummation is still very important. On the other hand,the power-correction to <(1-T)^2> from a single soft gluon emission is found tobe highly suppressed: it scales as 1/Q^3. In practice, <(1-T)^2> and highermoments depend also on soft gluon emission from configurations of three hardpartons, which may lead to alpha_s(Q^2)/Q power-corrections. This issue is yetto be investigated.Comment: 27 pages including 3 postscript figures; final version - to appear in JHEP; note the extended discussion in the conclusion sectio