Measurement of the azimuthal anisotropy in Pb+Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 5.02 TeV with the ATLAS detector at the LHC

Azimuthal anisotropy of particles produced in ultra-relativistic heavy-ion collisions provides unique information about the created hot and dense medium. It is one of the main signatures that, a new state of matter, Quark-Gluon Plasma (QGP) is formed in the nuclear interactions with properties resembling those of perfect fluid, characterised by very low viscosity. The study of azimuthal anisotropy in heavy-ion collisions provides an insight into the initial conditions and collective expansion of QGP. The azimuthal anisotropy originates from the asymmetric shape of the initial volume of the two nuclei interaction. The asymmetry of the collision zone leads to the formation of huge pressure gradients inside the QGP fluid and thereby to intensified particle production along the reaction plane direction. The azimuthal angle distribution of created particles relative to the reaction plane is commonly described by the Fourier series. The main goal of the thesis is to determine the Fourier harmonics amplitudes, $v_{n}$, of azimuthal angle distributions of charged particles produced in Pb+Pb collisions at $\sqrt{s_{\mathrm{NN}}}=$ 5.02 TeV in the ATLAS detector at the LHC. Presented analysis utilises the minimum-bias sample of the integrated luminosity of 22 $\mu\mathrm{b}^{-1}$. Furthermore, the event statistics in the most central collisions are enhanced by dedicated ``ultra-central'' triggers that sampled the total luminosity of 0.49 $n\mathrm{b}^{-1}$. The $v_{n}$ harmonics are measured for $n$ = 2-7 over an extended transverse momentum range ($p_{\mathrm{T}}$ = 0-60 GeV) and wide ranges of pseudorapidity ($|\eta| < $ 2.5) and collision centrality (0-80%). The measurements are based on the event-plane and the scalar-product methods. The results obtained with these methods are compared to each other as well as to complementary results of the two-particle correlation analysis and to the measurements from the CMS experiment. Furthermore, the comparisons with measurements obtained with the lower collision energy of the Pb+Pb system ($\sqrt{s_{\mathrm{NN}}}$ = 2.76 TeV), also with new data from the Xe+Xe collision system and to theoretical predictions are also performed. The results obtained in this thesis are one of the most precise measurements of the flow harmonics and thus allow for strong tests of theoretical models, in particular for testing the ultra-relativistic hydrodynamics that is customarily used to explain the QGP evolution.