Hard-Scattering Results in Heavy-Ion Collisions at the LHC

We present the first LHC results on hard-probe observables in heavy-ion collisions. During the first 3 years of operation, the ALICE, ATLAS, and CMS experiments each collected Pb + Pb interaction data, corresponding to an integrated luminosity above 100 μb−1 at TeV. Fresh insights into the properties of QCD matter under extreme conditions are emerging. Although the basic picture established at RHIC—that of a hot, dense medium that (a) flows with a viscosity-to-entropy ratio close to the conjectured lower bound and (b) quenches the energy of hard probes, such as jets, heavy flavor, and quarkonia—has been confirmed at the LHC, the higher-energy data indicate new ways to extract its properties in more detail. These include the first observations of a different strength for the suppression of charm and beauty quarks, reduced low-pT suppression for J/ψ, and detailed measurements of high-ET jet suppression and jet-fragmentation modifications.