  We present a strategy to quantify the performance of jet definitions in
kinematic reconstruction tasks. It is designed to make use exclusively of
physical observables, in contrast to previous techniques which often used
unphysical Monte Carlo partons as a reference. It is furthermore independent of
the detailed shape of the kinematic distributions. We analyse the performance
of 5 jet algorithms over a broad range of jet-radii, for sources of quark jets
and gluon jets, spanning the energy scales of interest at the LHC, both with
and without pileup. The results allow one to identify optimal jet definitions
for the various scenarios. They confirm that the use of a small jet radius
(R\simeq 0.5) for quark-induced jets at moderate energy scales, O(100 GeV), is
a good choice. However, for gluon jets and in general for TeV scales, there are
significant benefits to be had from using larger radii, up to R\gtrsim 1. This
has implications for the span of jet-definitions that the LHC experiments
should provide as defaults for searches and other physics analyses.
