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One important challenge in our field is to understand the initial condition of the QGP and constrain it using sensitive experimental observables. Recent studies show that the Pearson Correlation Coefficient (PCC) between flow coefficients v_n, and event-wise mean transverse momentum [p_T], ρ(v_n^2,[p_T]) and its centrality dependence can probe several ingredients of the initial state, such as number and size of sources, nuclear deformation, volume fluctuation, and initial momentum anisotropy.
This talk presents comprehensive and precision measurements of v_n-[ p_T]correlation in 129Xe+129Xe and 208Pb+208Pb collisions. In central collisions, where models generally show good agreement, the v_2-[ p_T] correlations are sensitive to the triaxiality of the quadruple deformation. Comparison of the model with the Pb+Pb and Xe+Xe data confirms that the 129Xe nucleus is a highly deformed triaxial ellipsoid that has neither a prolate nor oblate shape. This provides strong evidence for a triaxial deformation of the 129Xe nucleus from high-energy heavy-ion collisions.