Measurement of higher harmonic flow coefficients of identified hadrons in Pb-Pb collisions at sNN = 2.76 TeV

The latest ALICE results on the centrality and transverse momentum dependence of v2, v3, v4 and v5 for π±, K± and p(p¯) in Pb–Pb collisions at sNN = 2.76 TeV are presented. The flow coefficients exhibit a clear mass ordering for pT < 2.5 GeV/c for all harmonics. For transverse momentum values larger than about 3 GeV/c, mesons exhibit distinctively lower flow values compared to baryons, suggesting that coalescence might be the relevant particle production mechanism in this region. The experimental data for pT < 2.5 GeV/c are described fairly well by the hydrodynamical model iEBE-VISHNU, which models the hydrodynamical expansion of the fireball using a value of η/s = 0.08, coupled to a hadronic cascade model (UrQMD). Finally, A Multi-Phase Transition Model (AMPT) highlights the significant contribution of the hadronic rescattering phase and of coalescence as the dominant particle production mechanism to the mass ordering at low pT and the baryon-meson grouping at intermediate pT, respectively.


Introduction
The aim of the heavy ion collision experiments is to study the Quark Gluon Plasma (QGP) [1] and constrain its transport properties such as the ratio of shear viscosity to entropy density (η/s) and equation of state. One of the observables sensitive to the properties of the QGP is anisotropic flow which characterises the momentum anisotropy of the final state particles. This momentum anisotropy rises from the spatial anisotropy of the overlapping collision region through interactions between partons and at later stages between the produced particles. The momentum anistropy can be expressed in terms of a Fourier expansion of the particle distribution dN/d(ϕ − Ψ n ) in azimuthal angle ϕ with respect to the n th -order symmetry plane angle Ψ n [2], according to: In this equation, v n is the n th -order flow coefficient and can be calculated according to v n = cos[n(ϕ − Ψ n )] , where the brackets denote an average over all events and particles.
In this article, we report the p T -differential elliptic (v 2 ), triangular (v 3 ), quadrangular (v 4 ) and pentagonal (v 5 ) flow for π ± , K ± and p(p) in two different centrality intervals of Pb-Pb collisions at √ s NN = 2.76 TeV measured with the ALICE detector.  [3]. For Pb-Pb collisions the minimum bias events are triggered by the coincidence between the signals from both sides of the V0 detectors. In addition, an online selection based on the V0 detectors are used in Pb-Pb collisions to increase the number of central (i.e. 0-10% centrality interval) and semi-central (i.e. 10-50%) collisions. For minimum bias pp collisions at least one hit is required in the SPD (the innermost layer of the Inner Tracking System) or V0 detectors. The reconstructed primary vertex position along the z axis (z vtx ) for all analysed events are required to be within 10 cm from the nominal interaction point. In addition, events with multiple reconstructed primary vertices are rejected which leads to a negligible amount of pile-up events for all systems [4]. After applying all the selection critera, a filtered data sample with 25 × 10 6 events are analysed for Pb-Pb and 20 × 10 6 for pp collisions to produce the results presented in this article. The two centrality intervals presented in this article are 0-1% (i.e. smallest impact parameter) and 20-30% (i.e. larger impact parameter) centrality intervals in Pb-Pb collisions.
The track reconstruction is performed using the information from both the Time Projection Chamber (TPC) and the Inner Tracking System (ITS). The charged particle momenta is measured using both ITS and TPC with an azimuthal coverage of |η| < 0.9. The particle identification (PID) for pions (π ± ), kaons (K ± ) and protons (p +p) used in this analysis is based on a procedure reported in [5] which combines the signals from the TPC and Time Of Flight (TOF) detectors. In this analysis, different particle species are identified by requiring a minimum identification probability of 90%. More details on the PID procedure followed can be found in [6].
The flow coefficients are measured with the Scalar Product method [7]. The identified particle of interest (POI), i.e. pions (π ± ), kaons (K ± ) and protons (p +p), and the charged reference particles (RP) are selected from two non-overlapping subevents within the acceptance of the TPC detector (−0.8 < η < 0 and 0 < η < 0.8). Correlations not related to the common symmetry plane, denoted as δ AA,pp n , are estimated using minimum bias pp collisions and are subtracted from the measured flow coefficients in Pb-Pb collisions denoted as v AA n with a procedure described in [6]. In this article, the reported values, v sub n , are extracted according to v sub n = v AA n − δ AA,pp n . Figure 1 presents the p T -differential v sub 2 , v sub 3 , v sub In addition, the magnitude of higher flow harmonics are comparable to that of elliptic flow. However, in mid-central collisions, the magnitude of elliptic flow is significantly larger than in ultra-central collisions and rises to become the dominant flow harmonic. The magnitude of higher harmonics also has increased with respect to the ultra-central collisions but this increase is not as significant as the one of elliptic flow. These observations confirm that elliptic flow is mainly driven by the anisotropy of the collision geometry. However, higher flow harmonics are less dependent on the geometry and more driven by the initial state fluctuations. This can be quantified by taking the ratio of v sub n in ultra-to mid-central collisions. For elliptic flow and for all particle species, this ratio is at most 25%, whereas for v sub 3 the ratio is >60%. Furthermore, a clear mass ordering can be seen in all harmonics and centrality intervals in the low p T region (i.e. p T ≤ 2.5 GeV/c) which rises from the interplay between the anisotropic flow harmonics and radial flow. Radial flow creates a depletion in the low p T region of the particle spectrum which is more pronounced in-than out-of-plane with increasing particle mass. This  (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 √ s NN = 2.76 TeV [6].

Results and discussions
A comparison is performed between v sub n (n = 2 − 4) and recent v n hydrodynamical calculations from [8]. These calculations are based on iEBE-VISHNU, an event-by-event version of the VISHNU hybrid model coupling 2+1 dimensional viscous hydrodynamics (VISH2+1) to a hadronic cascade model (UrQMD). In this model, the ratio of shear viscosity to entropy density is fixed to η/s = 0.08. The model describes the v sub 2 measurements for all particles and centrality intervals within 10-15%. The model predictions show a better agreement (within 5% deviations) with the measured v sub 3 and v sub 4 . In this analysis, three different versions of A Multi-Phase Transport Model (AMPT) are investigated. AMPT can run over two configurations, namely the default and the string melting. In both configurations, the final state hadronic rescattering phase is implemented which also includes resonance decays. In the string melting configuration, final state hadrons are formed via a quark coalescence mechanism. The third version is based also on the string melting configuration with the hadronic rescattering phase switched off. More details on the AMPT configurations used in this analysis can be found in [6]. correlations unrelated to the common symmetry plane, is estimated using pp collisions and then subtracted from the v n measurements. All flow harmonics exhibit an increase in magnitude in more peripheral collisions. This increase is more significant for elliptic flow compared to higher harmonics which indicates that v 2 is more dependent on the anisotropy of the collision geometry. On the other hand, higher harmonics are more driven by the initial state fluctuations. A distinct mass ordering is seen in low p T region which is attributed to the interplay between radial flow and anisotropic flow harmonics. In addition, AMPT measurements highlight the importance of the final state hadronic rescattering phase in the development of the mass ordering of v n in this p T region. In the intermediate p T values, a baryon-meson grouping is seen where baryons show a higher magnitude compared to mesons. This is observed also in the AMPT string melting version which demonstrates the effect of quark coalecence in this p T region.