With data from the first heavy-ion run at the LHC, the ALICE collaboration has made the first observation of elliptic flow of charged particles in lead-lead collisions at 2.76 TeV per nucleon pair.
Flow is an interesting observable because it provides information on the equation of state and the transport properties of matter created in a heavy-ion collision. The azimuthal anisotropy in particle production is the clearest experimental signature of collective flow; it is caused by multiple interactions between the constituents of the created matter and the initial asymmetries in the spatial geometry of a non-central collision. The second Fourier coefficient of this azimuthal asymmetry is known as elliptic flow.
The magnitude of the elliptic flow depends strongly on the friction in the created matter, which is characterized by the ratio of shear viscosity to entropy ratio: η/s. A good fluid, such as water, has a small value of η/s and supports flow patterns such as waves in the ocean. By contrast, in a poor fluid, such as honey, flow patterns disappear quickly. Measurements of elliptic flow at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven already revealed the fascinating fact that the hot and dense matter created in the collision there flows as a good fluid with almost no friction.
Surprisingly enough, the first theoretical calculation of η/s in heavy-ion collisions did not come from lattice QCD or transport theory – but from string theory. First calculations showed that in a strongly coupled N = 4 supersymmetric Yang Mills theory with a large number of colours, η/s can be calculated using a gauge gravity duality. The famous anti-de Sitter/conformal field theory (AdS/CFT) conjecture yields a ratio of η/s = h/4πkB, which was argued to be a lower bound for any relativistic thermal field theory.
At RHIC, a precise determination of the friction in the partonic fluid is complicated by uncertainties in the initial conditions of the collision, the relative contributions from the hadronic and partonic phase, and the unknown temperature dependence of η/s. Because this temperature dependence is unknown, it was not even clear if the elliptic flow would increase or decrease when going from RHIC to the LHC. A measurement of elliptic flow at the LHC was therefore one of the most anticipated results.
The measurements at 2.76 TeV by ALICE show that the elliptic flow of charged particles increases by about 30% compared with flow measured at the highest RHIC energy of 0.2 TeV. This result indicates that the hot and dense matter created in these collisions still behaves like a fluid with almost zero friction, providing strong constraints on the temperature dependence of η/s.
This first measurement also shows that elliptic flow – and thus the properties of the created matter – can be studied with unprecedented precision at the LHC. This is because of the increase in particle multiplicity compared with RHIC and the increase in the elliptic flow itself.