  The goals of discovering quantum radiation dynamics in high-intensity
laser-plasma interactions and engineering new laser-driven high-energy particle
sources both require accurate and robust predictions. Experiments rely on
particle-in-cell simulations to predict and interpret outcomes, but unknowns in
modeling the interaction limit the simulations to qualitative predictions, too
uncertain to test the quantum theory. To establish a basis for quantitative
prediction, we introduce a `jet' observable that parameterizes the emitted
photon distribution and quantifies a highly directional flux of high-energy
photon emission. Jets are identified by the observable under a variety of
physical conditions and shown to be most prominent when the laser pulse forms a
wavelength-scale channel through the target. The highest energy photons are
generally emitted in the direction of the jet. The observable is compatible
with characteristics of photon emission from quantum theory. This work offers
quantitative guidance for the design of experiments and detectors, offering a
foundation to use photon emission to interpret dynamics during high-intensity
laser-plasma experiments and validate quantum radiation theory in strong
fields.
