  We show that a forward jet with large transverse momentum in an onium-onium
collision is a hard probe which can be effectively characterized by a colour-
dipole distribution at the time of the interaction. The dipole distribution is
computed, and compared to its counterpart for a virtual photon in the initial
state. We find that while in the photon case, the tail of large sizes is
exponentially cut-off, it contributes sizeably in the forward-jet case, which
signs the sensitivity of observables based on such events to the infrared
region. Moreover, a direct probabilistic interpretation of the dipole
distribution fails since it takes negative values in the large size region.
