Non-linear flow modes of identified particles in Pb-Pb collisions at $$ \sqrt{s_{\mathrm{NN}}} $$ = 5.02 TeV

S. Acharya, The ALICE collaboration, D. Adamová, A. Adler, J. Adolfsson, M. M. Aggarwal, G. Aglieri Rinella, M. Agnello, N. Agrawal, Z. Ahammed, S. Ahmad, S. U. Ahn (+998 others)
2020 Journal of High Energy Physics  
The p T -differential non-linear flow modes, v 4,22 , v 5,32 , v 6,33 and v 6,222 for π ± , K ± , K 0 S , p + p, Λ + Λ and φ-meson have been measured for the first time at √ s NN = 5.02 TeV in Pb-Pb collisions with the ALICE detector at the Large Hadron Collider. The results were obtained with a multi-particle technique, correlating the identified hadrons with reference charged particles from a different pseudorapidity region. These non-linear observables probe the contribution from the second
more » ... on from the second and third order initial spatial anisotropy coefficients to higher flow harmonics. All the characteristic features observed in previous p T -differential anisotropic flow measurements for various particle species are also present in the non-linear flow modes, i.e. increase of magnitude with increasing centrality percentile, mass ordering at low p T and particle type grouping in the intermediate p T range. Hydrodynamical calculations (iEBE-VISHNU) that use different initial conditions and values of shear and bulk viscosity to entropy density ratios are confronted with the data at low transverse momenta. These calculations exhibit a better agreement with the anisotropic flow coefficients than the non-linear flow modes. These observations indicate that non-linear flow modes can provide additional discriminatory power in the study of initial conditions as well as new stringent constraints to hydrodynamical calculations. Open Access, Copyright CERN, for the benefit of the ALICE Collaboration. Article funded by SCOAP 3 . JHEP06(2020)147 constituents. This leads to the so-called number of constituent quarks (NCQ) scaling, observed to hold at an approximate level of ±20% for p T > 3 GeV/c [18, 39, 40, 61] . The measurements of non-linear flow modes in different collision centralities could pose a challenge to hydrodynamic models and have the potential to further constrain both the initial conditions of the collision system and its transport properties, i.e. η/s and ζ/s (the ratio between bulk viscosity and entropy density) [54, 62] . The p T -dependent non-linear flow modes of identified particles, in particular, allow the effect of late-stage interactions in the hadronic rescattering phase, as well as the effect of particle production to be tested via the coalescence mechanism to the development of the mass ordering at low p T and particle type grouping in the intermediate p T region, respectively [33, 42] . In this article, we report the first results of the p T -differential non-linear flow modes, i.e. v 4,22 , v 5,32 , v 6,33 and v 6,222 for π ± , K ± , K 0 S , p + p, Λ + Λ and φ measured in Pb-Pb collisions at a centre of mass energy per nucleon pair √ s NN = 5.02 TeV, recorded by the ALICE experiment [63] at the LHC. The detectors and the selection criteria used in this analysis are described in section 2 and 3, respectively. The analysis methodology and technique are presented in section 4. In this article, the identified hadron under study and the charged reference particles are obtained from different, non-overlapping pseudorapidity regions. The azimuthal correlations not related to the common symmetry plane (known as non-flow), including the effects arising from jets, resonance decays and quantum statistics correlations, are suppressed by using multi-particle correlations as explained in section 4 and the residual effect is taken into account in the systematic uncertainty as described in section 5. All coefficients for charged particles were measured separately for particles and anti-particles and were found to be compatible within statistical uncertainties. The measurements reported in section 6 are therefore an average of the results for both charges. The results are reported within the pseudorapidity range |η| < 0.8 for different collision centralities between 0-60% range of Pb-Pb collisions. 2 Experimental setup ALICE [63, 64] is one of the four large experiments at the LHC, particularly designed to cope with the large charged-particle densities present in central Pb-Pb collisions [65] . By convention, the z-axis is parallel to the beam direction, the x-axis is horizontal and points towards the centre of the LHC, and the y-axis is vertical and points upwards. The apparatus consists of a set of detectors located in the central barrel, positioned inside a solenoidal magnet which generates a maximum of 0.5 T field parallel to the beam direction, and a set of forward detectors. The Inner Tracking System (ITS) [63] and the Time Projection Chamber (TPC) [66] are the main tracking detectors of the central barrel. The ITS consists of six layers of silicon detectors employing three different technologies. The two innermost layers, positioned at r = 3.9 cm and 7.6 cm, are Silicon Pixel Detectors (SPD), followed by two layers of Silicon Drift Detectors (SDD) (r = 15 cm and 23.9 cm). Finally, the two outermost layers are double-sided Silicon Strip Detectors (SSD) at r = 38 cm and 43 cm. The TPC has a cylindrical shape with an inner radius of about 85 cm, an outer radius of about 250 cm, -4 -i Deceased
doi:10.1007/jhep06(2020)147 fatcat:2bvmbe2aureqnbep74gxo3kjj4