%0 Generic
%A Ciupek, Michael Rudolf
%C Heidelberg
%D 2024
%F heidok:34656
%R 10.11588/heidok.00034656
%T Using spectator neutrons to determine the fluctuating shapes of the QGP droplets created in heavy-ion collisions at the LHC
%U https://archiv.ub.uni-heidelberg.de/volltextserver/34656/
%X The strong force is one of the four fundamental forces that describes the interaction of quarks and  gluons. Its theory is called quantum chromodynamics (QCD), and one of its features is the running  of the coupling constant, which depends on the momentum transfer between quarks and gluons.  This gives rise to the notion that nuclear matter can be heated up to the temperatures when a new  state of matter, governed by deconfined quarks and gluons, will be created. This state is called  the quark-gluon plasma (QGP). The different phases and transitions of the QCD matter can be  studied via relativistic heavy-ion collisions, in which extreme temperature and baryon density can  be achieved. Under conditions of small baryon chemical potential, μb ≈ 0 MeV, the lattice QCD  predicts a crossover transition at temperature about 156 MeV. These conditions can be achieved in  relativistic heavy-ion collisions at LHC energies.    The initial energy density in the overlap region of two colliding nuclei is asymmetric, and its shape  fluctuates due to the inner structure of the nuclei. This spatial anisotropy is converted during the  hydrodynamic expansion of the QGP into momentum anisotropies in the distribution of produced  particles, known as anisotropic flow. The anisotropic flow develops a particle-type dependence due  to the hydrodynamic expansion since the particles of different masses are affected differently by  the radial fluid velocities.    A detailed understanding of the initial state of a heavy-ion collision is important in order  to extract the transport properties of the QGP, e.g., the shear and bulk viscosities, from the  comparison with the hydrodynamic model calculations. A unique role in this context is played  by the spectator nucleons, which are the remnants of the collision. Due to the strong Lorentz  contraction of the nuclei, their passing time is much shorter than the expansion time of the QGP.  Consequently, spectator nucleons are sensitive to the early times of the collision evolution and using  them in flow measurements has a unique potential to improve understanding of the initial conditions.    The results presented in this thesis provide new insights for the understanding of the initial state and  the hydrodynamic evolution of the QGP. The thesis presents novel measurements of the anisotropic  flow relative to the neutron spectator plane, v2 {ΨSP}, in Pb–Pb collisions at √sNN = 2.76TeV  recorded by the ALICE experiment at the LHC. The measurements are performed for charged pions,  kaons, and (anti)protons at mid-rapidity as a function of transverse momentum, pT = 0.2-6 GeV/c,  and collision centrality. The v2 {ΨSP} is compared to anisotropic flow relative to the participant  plane, estimated by the two- and four-particle cumulants. A significant difference between charged  pions and (anti)protons (kaons) of 3.6% (1.6%) was found, suggesting a coupling of the effects of  the hydrodynamic expansion of the QGP and the subsequent hadronization with the initial state  fluctuations. These novel measurements open new opportunities to control contributions from initial  state fluctuations in flow measurements, which is crucial for precise determination of the QGP  transport properties from the systematic comparison of the experimental data to the hydrodynamic  model calculations.