  We introduce a jet shape observable defined for an ensemble of jets in terms
of two-particle angular correlations and a resolution parameter R. This
quantity is infrared and collinear safe and can be interpreted as a scaling
exponent for the angular distribution of mass inside the jet. For small R it is
close to the value 2 as a consequence of the approximately scale invariant QCD
dynamics. For large R it is sensitive to non-perturbative effects. We describe
the use of this correlation function for tests of QCD, for studying underlying
event and pile-up effects, and for tuning Monte Carlo event generators.
