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 $\left\langle v_n^2\right\rangle,\left\langle\left(\delta p_T\right)^2\right\rangle$, and the covariance $\left\langle v_n^2 \delta p_T\right\rangle$. 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.