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ALICE measures higher-flow harmonics

12 August 2016
In a collision between two nuclei that exhibits a large impact parameter, the initial spatial anisotropy of the overlap region was conjectured to be smooth and almond shaped. In the past few years, however, experimental measurements and hydrodynamical calculations have changed this paradigm. We now know that the overlap region has an irregular shape originating from the initial random distribution of the gluons and nucleons in the nuclei, which fluctuates from one event to the next. These fluctuations appear as azimuthal correlations between final-state particles relative to the system’s symmetry plane. These correlations are quantified by a Fourier series of the azimuthal distribution of particle production relative to the system’s symmetry plane.

The second harmonic – v2, which is also known as the elliptic-flow coefficient – has recently been the main focus of the heavy-ion community. Indeed, elliptic-flow measurements and hydrodynamical calculations led to the revelation that the quark–gluon plasma (QGP) generated in heavy-ion collisions behaves as an almost perfect liquid. The ratio of shear viscosity to entropy density (η/s) in the QGP, which is a measure of its fluidity, is very close to the lower bound of ħ/4π kB – as conjectured by the anti-de-Sitter/conformal field theory (AdS/CFT) correspondence.

However, there are also higher-order contributions to the particle correlations and these are more sensitive probes of the QGP state than elliptic flow. Indeed, by carrying out such studies for identified particles rather than unidentified ones, it is also possible to probe the effect of the dissipative, late-stage hadronic re-scattering on the flow coefficients.

Profiting from its excellent particle-identification capabilities, the ALICE collaboration has recently used lead–lead collisions recorded in 2011 at a collision energy of 2.76 TeV to measure the elliptic (v2), triangular (v3), quadrangular (v4) and pentagonal (v5) flow coefficients of charged π and K mesons, protons and antiprotons for different centrality intervals (see figure). For ultra-central collisions, in which the evolution of the system is predominantly driven by the initial-state fluctuations, one observes significant nonzero values for all harmonics and particle species. In addition, v3 and v4 become progressively dominant with increasing transverse momentum, while even v5 for pT > 4 GeV/c is comparable to v2. For mid-central collisions, v2 is the dominant flow harmonic and has a significantly larger value. Higher harmonics also have significant nonzero values but do not seem to change significantly with centrality.

These observations confirm that elliptic flow is driven mainly by the anisotropy in the collision geometry, whereas the initial-state fluctuations are the main driving force behind higher harmonics. A mass ordering expected in hydrodynamical calculations is seen that describes the QGP as a nearly perfect liquid. In addition, in the intermediate-pT region, the flow harmonics of different hadrons show a baryon–meson grouping. The analysis of lead–lead collisions collected in 2015 will allow for a higher-precision measurement of such effects and therefore place more stringent limits on η/s and the initial conditions of a heavy-ion collision.

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