Virtual Journal on QCD Matter

Today the ALICE Collaboration posted the first analyses of data from Pb+Pb collisions at the LHC:

• Elliptic flow of charged particles in Pb-Pb collisions at 2.76 TeV
• Charged-particle multiplicity density at mid-rapidity in central Pb-Pb collisions at sqrt(sNN) = 2.76 TeV

Not only is the speed of publication of these results – only 10 days after the start of the heavy ion run – unprecedented, the results also render a large number of theoretical predictions, including many of those published in

• Heavy Ion Collisions at the LHC – Last Call for Predictions

obsolete. Following is a first quick assessment of the data reported in these two manuscripts.

The charged particle multiplicity per unit pseudorapidity of dN_ch/dη = 1584 measured in the 5% most central collisions is about a factor 2.2 higher than that measured in Au+Au collisions with the same number of participants at the highest RHIC energy. Assuming that the multiplicity is proportional to the entropy of the final state, this corresponds to a 30% increase in the temperature of the quark-gluon plasma produced in these collisions (at the same time after the onset of the reaction) compared with RHIC, or an increase in the energy density by a factor 2.85.

The result implies that the multiplicity grows as a power of the c.m. energy: dN_ch/dη ∼ s^0.15, considerably faster than extrapolated from RHIC and lower energy data. Most likely, the increase is an indication of a growing contribution of semi-hard QCD processes (minijets) to the total multiplicity.

Even more remarkable are the results from the second ALICE letter, on elliptic flow: When measured as a function of the transverse momentum, the elliptic flow v₂(p_T) is nearly identical with the elliptic flow measured at the highest RHIC energy, independent of the centrality and over the whole momentum range up to 4 GeV/c ! When integrated over momentum, v₂ is approximately 30% higher than at RHIC, because the average transverse momentum of the emitted hadrons is larger than at RHIC. In view of the 30% higher initial temperature, we can estimate the increase in the mean p_T to be of about the same size. Since v₂ grows linearly with the momentum for small momenta, this explains the increase in the integrated elliptic flow.

The practically unchanged differential elliptic flow suggests that viscous properties of the QGP fluid produced at the LHC are essentially identical with those of the QGP produced at RHIC: it is still a nearly “perfect” fluid! It will be interesting to see more detailed analyses of these properties, which will be forthcoming over the next few months. It will also be interesting whether the stopping power of the QGP produced at the LHC for hard partons are the same (in proportion to its increased density) as those observed at RHIC.

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