Jet Prelude: Why is this hard?

• Cannot look at “true”, traditional calorimetric jets; soft particle energy density dET/dd~ 100 GeV/unit-radian

• Next best thing: leading particles = high-PT hadrons, and also high-PT pairs, either same side (leading and next-leading) or opposite side (leading and opposite leading)

• Ambiguity between hadrons from jet fragmentation source and hadrons from multi-collisional/”thermal” source, even out to several GeV/c (and beyond?)

• Model “thermal” source: hadron gas with temperature, chemical (ie flavor) equilibration, baryon density and overall flow velocity in radial direction