  We study power corrections to the spin-averaged structure functions F_2 and
F_L in the semi-inclusive region. The dramatic breakdown of the operator
product expansion at large Bjorken x due to the formation of a narrow jet with
invariant mass W (W^2=Q^2(1-x)/x) calls for an alternative approach. Our main
conjecture is that the dominant contribution at each twist is the one which
mixes under renormalization with the leading twist (ultraviolet dominance). At
twist two the dominant perturbative corrections at large x due to soft and
collinear radiation can be factorized into a jet function. From the ultraviolet
dominance conjecture it follows that the twist two parton distribution as well
as the jet function actually factorize to any order in the twist expansion.
This factorization suggests that non-perturbative corrections ~1/W^{2n}
exponentiate together with the leading twist. We verify explicitly, at the
level of a single dressed gluon, the cancellation between ambiguities due to
infrared renormalons in the twist-two coefficient functions and those due to
the ultraviolet divergence of twist-four matrix elements. Independently of the
renormalon analysis, we show that the dominant contribution to twist four at
large x is associated with a twist-two like configuration: the final states are
identical to those of twist two, whereas the initial states differ just by
additional partons carrying small momentum fractions. This picture is
consistent with the ultraviolet dominance conjecture.
