by Naghmeh Mohammadi. Published: 03 February 2017

In a recent CERN seminar the ALICE Collaboration presented a review of the latest results from measurements of elliptic (v2), triangular (v3), quadrangular (v4) and pentagonal (v5) flow of identified particles at the LHC measured with ALICE. These results allow us to gain insight into the transport properties of the QGP and the initial conditions of a heavy-ion collision. In addition, they reveal the role of different hadronisation mechanisms as well as the highly dissipative hadronic rescattering phase to the development of vn.

Here the main author of the research, Naghmeh Mohammadi, of Nikhef (The Netherlands), provides a summary of the talk.

Figure 1: The pT-differential v2 (first column), v3 (second column), v4 (third column), and v5 (fourth column), for different particle species for the 0-1% (upper row) and the 20-30% (lower row) centrality intervals of Pb-Pb collisions at √SNN = 2.76 TeV. Data extracted from [1].


It was recently realized that in a non-central heavy ion collision the overlap region between the colliding nuclei should not be described as a smooth almond since it exhibits an irregular shape. These irregularities originate from the event-by-event fluctuations in the position of the participating nucleons. Throughout the evolution of the system, these fluctuations survive due to the very small value of shear viscosity to entropy ratio (η/s) of the quark-gluon plasma (QGP) that reflects its small mean free path, and appear as final state correlations between the produced particles. The final state correlations are quantified by the vn coefficients of the Fourier expansion of the azimuthal distribution of produced particles relative to the symmetry planes Ψn. The second Fourier coefficient, namely the elliptic flow, has been the focus of many studies at both RHIC and LHC. These studies and the subsequently good description of experimental data from hydrodynamical calculations, established the picture of a QGP that behaves as a nearly perfect liquid with a value of η/s very close to the lower bound of  ħ/4πkB conjectured within the anti-de Sitter/conformal field theory correspondence (AdS/CFT).

Recently, the focus has turned to higher flow harmonics (vn with n > 2) that probe modulations in smaller spatial scales. As a result, they are more sensitive probes of the value of η/s as well as the initial state fluctuations. The measurements of flow harmonics contributed in limiting the range of η/s ranging between  ħ/4πkB <η/s< 2.5ħ/4πkB  with the range mainly coming from systematic uncertainties due to the incomplete knowledge of the initial state and its fluctuations.

In addition, if these studies are performed for identified particles they can probe the effect of the dissipative, late-stage hadronic rescattering to the development of flow coefficients. In this study, the elliptic (v2), triangular (v3), quadrangular (v4) and pentagonal (v5) flow coefficients of charged π and K, p and p-bar were obtained using Pb-Pb collisions at √SNN = 2.76 TeV with ALICE experiment for different centrality intervals. Different particle species were identified with the combination of the response of the TPC and the TOF detectors that provide unique identification capabilities among all LHC experiments.

The main results of these measurements are illustrated in Fig. 1, which presents the development v2, v3, v4 and v5 as a function of transverse momentum, pT, for different particle species within the same centrality interval in ultra-central (0-1%) and mid-central (20-30%) Pb-Pb collisions [1]. All flow harmonics show significant non-zero values in ultra-central collisions, where the spatial anisotropy of the system is solely driven by the initial state fluctuations. Furthermore, v3 and v4 become dominant with increasing pT compared to v2 and for pT > 4 GeV/c even v5 reaches to the magnitude of v2. For mid-central collisions, where the spatial anisotropy is mainly driven by the geometry of the collision, v2 has a significantly larger value compared to higher harmonics. Contrary to v2, the values of higher harmonics do not have significant centrality dependence.

The measurements of flow harmonics confirm that the origin of elliptic flow is mainly the anisotropy originating from the collision geometry, whereas higher harmonics are mainly driven by the initial state fluctuations. For both centralities and for all flow harmonics, a clear mass ordering is seen in the low pT region (i.e. pT < 2 GeV/c) where heavier particles have lower vn values with respect to lighter particles. This observation demonstrates the interplay between radial flow with not only elliptic flow [2] but also higher harmonic flow coefficients. Hydrodynamic calculations performed in this pT region, favour a value of η/s close to the lower bound for the QGP created at the LHC. Furthermore, the values of flow harmonics in the intermediate pT show a particle-type grouping with the vn of baryons being larger than the one of mesons. This feature has been explained by models, which incorporate quark coalescence as the dominant particle production mechanism.

The Pb-Pb collision data sample at higher collision energies collected in the second run of the LHC can lead to a more precise measurement of flow harmonics. Comparison of these measurements to models will in turn provide us with an even better constraint of the initial conditions of a heavy-ion collision. In addition, it will open the window to probe the temperature dependence of η/s for the first time.



[1] J. Adam et al. [ALICE Collaboration], arXiv:1606.06057 [nucl-ex]. 

[2] B. B. Abelev et al. [ALICE Collaboration], JHEP 1506 (2015) 190 
 doi:10.1007/JHEP06(2015)190 [arXiv:1405.4632 [nucl-ex]].