Imaging nuclear shape through anisotropic and radial flow in high-energy heavy-ion collisions

Most atomic nuclei exhibit ellipsoidal shapes characterized by quadrupole deformation<i>β</i>₂and triaxiality<i>γ</i>, and sometimes even a pear-like octupole deformation<i>β</i>₃. The STAR experiment introduced a new 'imaging-by-smashing' technique ((STAR Collaboration) 2024<i>Nature</i><b>635</b>67; Jia 2025<i>Rep. Prog. Phys.</i><b>88</b>092301) to image the nuclear global shape by colliding nuclei at ultra-relativistic speeds and analyzing outgoing debris. Features of nuclear shape manifest in collective observables like anisotropic flow<i>v_n</i>and radial flow via mean transverse momentum[pT]. We present new measurements of the variances of<i>v_n</i>(<i>n</i> = 2, 3, and 4) and[pT], and the covariance ofvn2with[pT], in collisions of highly deformed²³⁸U and nearly spherical¹⁹⁷Au. Ratios of these observables between the two systems effectively suppress common final-state effects, isolating the strong impact of uranium's deformation. By comparing results with state-of-the-art hydrodynamic model calculations, we extractβ2UandγUvalues consistent with those deduced from low-energy nuclear structure measurements. Measurements of<i>v</i>₃and its correlation with[pT]also provide the first experimental suggestion of a possible octupole deformation for²³⁸U. These findings provide significant support for using high-energy collisions to explore nuclear shapes on femtosecond timescales, with implications for both nuclear structure and quark-gluon plasma studies.