First results from the LHC heavy ion program

Abstract

The Large Hadron Collider (LHC) at CERN outside Geneva, Switzerland provides Pb + Pb beams at a nucleon–nucleon center-of-mass energy of 2.76 TeV, which is nearly 14 times higher than the energy available at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven (200 GeV). The first LHC heavy ion run ended in December 2010, and already several results are available which give some indications of future directions in heavy ion physics. Results have been released both for bulk observables, the charged particle multiplicity and elliptic flow, as well as for ‘hard probes’ such as jets and J/ψ. The charged particle multiplicity near mid-rapidity shows no anomalous rise relative to lower energies, although an extrapolation to full phase space using extended longitudinal scaling may be revealing the first known violation of the Landau–Fermi multiplicity formula. Elliptic flow is found to agree surprisingly well with lower-energy data when measured as a function of transverse momentum, agreeing with viscous hydrodynamic calculations that treat the matter at the LHC similarly to that found at RHIC. Despite the similar features found at lower energies, the higher center-of-mass energies provide much higher rates of high-pT ‘hard’ probes, to study the medium microscopically. It makes it possible for the first time to study ultrahigh-energy jets, which are found to show dramatic event-by-event asymmetries in their energies, possibly reflecting strong energy loss in the hot, dense medium. Measurements of the spectra of charged tracks within jets are reported, to indicate a softening of fragmentation of the lower-energy jet. J/ψ rates have also been measured, and were found to be suppressed at a similar level as lower-energy results. In total, the medium formed at the LHC appears to be qualitatively similar to that measured at lower energies. However, future measurements will certainly take even more advantage of the particular strengths of the LHC program, in particular the higher multiplicities and higher rates of large-momentum processes, so it is too early to draw strong conclusions.