CERN Accelerating science

The primary goal of ultra-relativistic heavy-ion physics is the study of the created strongly-interacting quark-gluon plasma (sQGP). Measurements of the azimuthal anisotropy of created particles, quantified by anisotropic flow $v_{n}$ coefficients, compared to hydrodynamic calculations, can be used to investigate this unique state of nuclear matter and its properties. Due to the absence of the QGP-like medium, small collision systems have been considered as a baseline for the heavy-ion measurements. However, high-multiplicity pp and p-Pb collisions have revealed similar collective behaviour traditionally associated with the expanding medium. The origin of such features is not yet understood and is a focus point of the present work. This work presents the analysis of azimuthal correlations of inclusive charged hadrons and identified $\pi^{\pm}$, ${\rm K^{\pm}}$, ${\rm K_{S}^{0}}$, ${\rm p/\bar{p}}$, $\Lambda/\bar{\Lambda}$, and $\phi$ meson in Pb-Pb and p-Pb collisions at $\sqrt{s_{\rm NN}}=5.02~{\rm TeV}$ recorded by ALICE detector at the Large Hadron Collider. Besides the $v_{n}$ measurement obtained for the first time using the 4-particle cumulants, more advanced observables such as flow fluctuations and non-linear flow response are studied in Pb-Pb collisions. The presented results exhibit an explicit mass ordering consistent with the radial expansion with universal velocity, as described by hydrodynamical calculations based on the iEBE-VISHNU model for low $p_{\rm T}<3~{\rm GeV}/c$, and baryon/meson grouping at intermediate $p_{\rm T}$ region. Non-flow correlations, arising from jet fragmentation and resonance decays, present a challenge as they generally dominate in small collision systems. In order to reduce such contamination, a pseudo-rapidity separation between correlated particles is applied as well as subtraction of remaining non-flow estimate based on a measurement of minimum-bias pp collisions at $\sqrt{s}=13~{\rm TeV}$. Motivated by Pb-Pb measurements, an initial attempt for extracting $v_{2}$ using the 4-particle cumulant is made. The mass-related features observed in Pb-Pb collisions are also apparent in non-flow subtracted p-Pb measurement. Overall, these differential measurements of $v_{n}$ coefficients present challenges for theoretical calculations further constraining the medium properties as well as initial conditions of the collision. In addition to the inclusive particles, the measurements using identified hadrons provide unique insights into particle production mechanisms. The results presented here mark a starting point for potential future development in investigating the obscure origin of the collectivity by analysing high-multiplicity pp collisions.