31 Chapter 2: Motivation be due to final state energy loss. Now IF energy loss is under theoretical con- trol and IF the combined theoretical and experimental system is not fragile, then jet tomography is possible and the observed suppression pattern can be inverted to gain knowledge about the medium. Final state suppression calculations began with Bjorken’s estimate of the energy lost by a high- p$_{T}$ parton through elastic, 2 → 2, processes [147]. He considered the simple t -channel diagram associated with a parton trav- eling through a thermalized, deconfined QGP of temperature T . In this case |M| 2 ∼ 1 /t 2 ∼ 1 / ( q $^{2}$+ M $^{2}$)$^{2}$, where M ∼ µ$_{D}$ ∼ gT is the infrared cutoff given by the Debye mass. With some simplifying assumptions, weighed with the appropriate flux and kinematic factors this gives an austere analytic formula for the energy lost per unit distance, where C$_{R}$ is the jet Casimir, 4/3 for a quark or 3 for a gluon, N$_{f}$ is the number of active flavors in the medium, and 〈 k 〉 ≈ 3 T is the average momentum of the plasma particles. This estimate has been improved upon by a number of subsequent papers. Thoma and Gyulassy [224] calculated the linear response to a jet in classical Abelianized QCD, incorporating the hard thermal loop (HTL) work of Braaten dE dx = πC$_{R}$α 2 $_{s}$T 2 ( 1 + N$_{f}$ 6 ) log 2 〈 k 〉 T M 2 , (2.6) 2.2.3 Elastic Energy Lost History