Evidence for modest octupole deformation in ${}^{238}$U from high-energy heavy-ion collisions

Abstract

We present a novel ``imaging-by-smashing" approach for probing nuclear deformation in high-energy heavy-ion collisions. By analyzing anisotropic-flow ($v_n$) and mean transverse momentum ($\left[p_T\right]$)-based observables in collisions of highly deformed ${}^{238}\mathrm{U}$ nucleus and nearly spherical ${}^{197}\mathrm{Au}$ nucleus, we extract the deformation parameters of ${}^{238}$U. The key observables include the variances $⟨v_n^2⟩,⟨{\left(\delta p_T\right)}^2⟩$, and the covariance $⟨v_n^2\delta p_T⟩$. Ratios of these observables between ${}^{238}$U+${}^{238}$U and ${}^{197}$Au+${}^{197}$Au collisions largely cancel final-state effects, thereby isolating the influence of nuclear deformation. We further report the first experimental indication of octupole deformation in ${}^{238}$U via $v_3$-based observables~\cite{2025rot}. The extracted deformation parameters, comparing with state-of-the-art hydrodynamic model calculations, are consistent with low-energy nuclear structure data. These results establish high-energy collisions as a powerful probe of nuclear shapes on femtosecond timescales.

The 29th International Nuclear Physics Conference (INPC 2025) proceedings, 4 pages, 2 figures