Virtual Journal on QCD Matter

Yesterday, the ALICE Collaboration weighed in with a jet quenching result of their own. In their preprint

• Suppression of Charged Particle Production at Large Transverse Momentum in Central Pb-Pb Collisions at √s_NN = 2.76 TeV

the ALICE physicists analyze a quantity much more familiar from the RHIC experiments: the suppression of large-p_T hadrons in nuclear collisions relative to binary collision scaled p+p reactions. Three aspects are remarkable:

1. The maximal suppression factor (1/R_AA ≈ 7) at p_T = 7 GeV is about 40% stronger than that seen at RHIC;
2. R_AA rises strongly with increasing p_T, reaching 0.35 at 20 GeV;
3. The shape of R_AA(p_T) in the intermediate range 1 GeV ≤ p_T ≤ 7 GeV is almost identical to that observed at RHIC.

The lower minimum of R_AA in spite the much flatter slope of the hadron spectrum indicates a much larger energy loss of the leading parton. It will be interesting to see whether the energy loss coefficient scales like the charged particle multiplicity between RHIC and LHC.

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