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

STAR Collaboration

doi:10.1088/1361-6633/ae0fc3

paper

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

arXiv:2506.17785 · doi:10.1088/1361-6633/ae0fc3

Abstract

Most atomic nuclei exhibit ellipsoidal shapes characterized by quadrupole deformation $β_{2}$ and triaxiality $γ$ , and sometimes even a pear-like octupole deformation $β_{3}$ . The STAR experiment introduced a new "imaging-by-smashing" technique [arXiv:2401.06625, arXiv:2501.16071] 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 $v_{n}$ and radial flow via mean transverse momentum $[p_{T}]$ . We present new measurements of the variances of $v_{n}$ ( $n = 2$ , 3, and 4) and $[p_{T}]$ , and the covariance of $v_{n}^{2}$ with $[p_{T}]$ , in collisions of highly deformed $^{238}$ U and nearly spherical $^{197}$ Au. 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 $β_{2 U}$ and $γ_{U}$ values consistent with those deduced from low-energy nuclear structure measurements. Measurements of $v_{3}$ and its correlation with $[p_{T}]$ also provide the first experimental suggestion of a possible octupole deformation for $^{238}$ U. 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.

arXiv comment20 pages, 29 figures + an appendix

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

Abstract

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