  We investigate the exclusive jet mass distribution in e+e- events, defined
with a veto on the out-of-jet radiation, at two-loop order. In particular, we
calculate the two-loop soft function, which is required to describe this
distribution in the threshold region. When combined with other ingredients
using soft-collinear effective theory, this generates the complete singular
distribution for jet thrust, the sum of the jet masses, at two-loop order. The
result is in excellent agreement with full QCD. The integrated jet thrust
distribution is found to depend in an intricate way on both the finite jet cone
size, R, and the jet veto scale. The result clarifies the structure of the
potentially large logarithms (both global and non- global) which arise in jet
observables for the first time at this order. Somewhat surprisingly, we find
that, in the small R limit, there is a precise and simple correspondence
between the non-global contribution to the integrated jet thrust distribution
and the previously calculated non-global contribution to the integrated
hemisphere soft function, including subleading terms. This suggests that the
small R limit may provide a useful expansion for studying other exclusive jet
substructure observables.
