The pseudorapidity dependence of elliptic ($v_2$), triangular ($v_3$), and quadrangular ($v_4$) flow coefficients of charged particles measured in Pb-Pb collisions at a centre-of-mass energy per nucleon pair of $\sqrt{s_{\rm NN}}=5.02$ TeV and in Xe-Xe collisions at $\sqrt{s_{\rm NN}}=5.44$ TeV with ALICE at the LHC are presented. The measurements are performed in the pseudorapidity range $-3.5 <~ \eta <~ 5$ for various centrality intervals using two- and multi-particle cumulants with the subevent method. The flow probability density function (p.d.f.) is studied with the ratio of flow coefficient $v_2$ calculated with four- and two-particle cumulant, and suggests that the variance of flow p.d.f. is independent of pseudorapidity. The decorrelation of the flow vector in the longitudinal direction is probed using two-particle correlations. The results measured with respect to different reference regions in pseudorapidity exhibit differences, argued to be a result of saturating decorrelation effect above a certain pseudorapidity separation, in contrast to previous publications which assign this observation to non-flow effects. The results are compared to $3+1$ dimensional hydrodynamic and the AMPT transport model calculations. Neither of the models is able to simultaneously describe the pseudorapidity dependence of measurements of anisotropic flow and its fluctuations. The results presented in this work highlight shortcomings in our current understanding of initial conditions and subsequent system expansion in the longitudinal direction. Therefore, they provide input for its improvement.
Phys. Lett. B 850 (2024) 138477
HEP Data
e-Print: arXiv:2307.11116 | PDF | inSPIRE
CERN-EP-2023-134
Figure group
Figure 1
Illustration of correlator methods showing calculation of $v_{n}\{2\}$ using either a small (a) or large (b) separation in pseudorapidity Darker bands indicate where the differential measurement is performed (i.e. particles of interest) while the other end of the connecting lines indicate reference particles. |  |
Figure 2
Illustration of correlator methods showing calculation of the decorrelation effect (a) as well as $v_{n}\{4\}$ (b). Darker bands indicate where the differential measurement is performed (i.e. particles of interest) while the other end of the connecting lines indicate reference particles. |  |
Figure 3
The differential flow measurements, $v_{n}\{2\}(\eta)$, measured with the 2-particle cumulant and chosing reference particles from the TPC in Pb$-$Pb collisions at $\sNN{} = 5.02\,\TeV{}$. The choice of the reference particles results in $|\Delta \eta | > 0.8$ and $2.6$ in the mid and forward pseudorapidity regions, respectively. At midrapidity, the results are extrapolated to $\pT=0$. AMPT and CLVisc model calculations are compared with the results. |  |
Figure 4
Two-particle cumulant with reference particles chosen from the FMD and 4-particle cumulant with reference particles chosen from the TPC in Pb-Pb collisions at $\sNN{} = 5.02\,\TeV{}$. The pseudorapidity separations are $|\Delta \eta | > 2.0$ and $3.8$ for two-particle cumulants for mid and forward rapidities, respectively. For the $v_{2}\{4\}$ the separations are $|\Delta \eta | > 0$ and $2.0$ for mid and forward rapidities, respectively. At mid rapidity, the results are extrapolated to $\pT=0$. AMPT and CLVisc model calculations are compared with the results. |  |
Figure 5
Ratio of two-particle results with a medium-sized pseudorapidity separation and 4-particle results to the two-particle results employing a large pseudorapidity separation between particles of interest and reference particles in Pb-Pb collisions at $\sNN{} = 5.02\,\TeV{}$. Also shown are the same ratios for the AMPT and CLVisc models. For the latter, the ratios are compatible with unity as the CLVisc model produces negligible non-flow . |  |
Figure 6
The differential flow measurements, $v_{n}\{2\}$, measured with the 2-particle cumulant and choosing reference particles from the TPC in Xe-Xe collisions at $\sNN = 5.44\,\TeV{}$. The choice of the reference particles results in pseudorapidity separations of $|\Delta \eta | > 0.4$ and $2.2$ at mid and forward rapidities, respectively. Also shown are results from the CLVisc model. |  |
Figure 7
The differential flow measurements, $v_{n}\{2\}$, measured with the 2-particle cumulant and choosing reference particles chosen from the FMD in Xe-Xe collisions at $\sNN{} = 5.02\,\TeV{}$, with pseudorapidity separations of $|\Delta \eta | > 2.0$ and $3.8$ at mid and forward rapidities, respectively. Also shown are results from the CLVisc model. |  |
Figure 8
Ratio of two-particle results with a medium-sized separation to the two-particle results employing a large separation between particles of interest and reference particles in Xe-Xe collisions at $\sNN{} = 5.44\,\TeV{}$ Also shown are the same ratios for the AMPT and CLVisc models. For the latter, the ratios are compatible with unity as the CLVisc model produces negligible non-flow . |  |
