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

Abstract Most atomic nuclei exhibit ellipsoidal shapes characterized by quadrupole deformation ₂ and triaxiality, and sometimes even a pear-like octupole deformation ₃. The STAR experiment introduced a new ``imaging-by-smashing'' technique~STAR: 2024eky, Jia: 2025wey 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 vₙ and radial flow via mean transverse momentum pₓ. We present new measurements of the variances of vₙ (n=2, 3, and 4) and pₓ, and the covariance of vₙ² with pₓ, in collisions of highly deformed ^238U and nearly spherical ^197Au. 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 extract ₂ₔ and ₔ values consistent with those deduced from low-energy nuclear structure measurements. Measurements of v₃ and its correlation with pₓ also provide the first experimental suggestion of a possible octupole deformation for ^238U. 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.