Study of nuclear deformation effects in Au+Au and U+U collisions from STAR experiment

<!--HTML-->Nuclear deformation is a ubiqutous phenomenon for most atomic nuclei, reflecting collective motion induced by interaction between valance nucleons and shell structure. In most cases, the deformation has a quadrupole shape $\beta$ that is axially and refection symmetric, either prolate $\beta>0$ or oblate $\beta<0$. Collisions of deformed nucleus lead to large shape and size fluctuations in the initial state geometry, which after collective expansion, lead to enhance fluctuation of harmonic flow $v_n$ and event-by-event mean transverse momentum $[p_T]$. Therefore detailed study of the $v_n$, and $[p_T]$ and correlations between them could probe the deformation parameter. In this talk, we present results of $[p_T]$ fluctuations and $v_n^2-[p_T]$ correlation for $n=2,3,4$ in near-spherical $^{197}$Au+$^{197}$Au collisions at $\sqrt{s_{NN}}=200$ GeV and highly-deformed $^{238}$U+$^{238}$U collisions at $\sqrt{s_{NN}}=193$ GeV. Significant differences for mean, variance $c_2$ and skewness $c_3$ of $[p_T]$ fluctuations are observed between the two systems as a function of centrality. The recently proposed intensive skewness of $c_3\langle[p_T]\rangle/c_2^2$, sensitive to the initial size fluctuation, is found to differ significantly between the two systems, particular in the ultra-central collisions. The $v_2^2-[p_T]$ results remain positive over the full centrality in Au+Au collisions, while they change sign in 0-5\% central U+U collisions. In contrast, the $v_3^2-[p_T]$ and $v_4^2-[p_T]$ results are nearly identical between these two systems. The sign-change of $v_2^2-[p_T]$ is used to provide novel ways to constrain $\beta$ for Uranium nuclei in heavy ion collisions. Comparison with state-of-art model calculations is discussed.