  The energy loss mechanism of jets plays a central role in nuclear and high
energy physics. We propose direct measurements of the energy loss of leading
jets and perform a calculation at next-to-leading logarithmic (NLL$'$) accuracy
in the vacuum. The formation of leading jets can be described by jet functions
which constitute probability densities and thus allow for a perturbative
calculation of the average the energy loss. We identify the following three
criteria for a direct measurement of jet energy loss at the cross section
level. $i)$ We measure a well defined object, the leading jet, where the
formation process can be expressed in terms of a probability density. $ii)$ In
addition, we need a measurement of a hard reference scale with respect to which
jet energy loss is defined. $iii)$ At leading logarithmic accuracy, we require
that the jet energy loss can be identified with parton energy loss. We discuss
suitable observables and present numerical results including threshold
corrections by making use of a parton shower Monte Carlo approach.
