The challenge: “ To prove partonic energy loss onset in high-energy HI collisions”

• “Jet quenching” : Signature of high-density partonic medium formation.

• Estimate of reached gluon density:

• • DEloss ~ s ·CR ·gluon·DL(2)

• How ... ?

• Measuring high transv. mom. hadrons

• (pT>2 GeV/c)  parton fragmentation

• products ("leading particle"):

• • Early production (~1/pT~0.1 fm/c) in parton- parton scatterings with large Q2.
  • Direct probes of partonic phases  Sensitive to hot/dense medium.
  • Comparison to available set of baseline "vacuum" data (pp  AA) via Ncoll (# of binary inelastic collisions) scaling.
  • Elementary processes “ calculable” (in principle) from pQCD.

Notes: