Speaker
Description
High-energy nuclear collisions can image the intrinsic shape of the colliding nuclei through the hydrodynamic response of the produced medium. Mean transverse momentum $\left[p_{\rm{T}}\right]$ fluctuations probe this geometry primarily through radial flow, since $\left[p_{\mathrm{T}}\right]$ correlates with transverse size of the overlap region [1,2]. Cumulants of $\left[p_{\mathrm{T}}\right]$ carry progressively higher powers of the quadrupole deformation, and odd-order cumulants are sensitive to its sign and to the triaxiality $\gamma$---providing information that is inaccessible through the variance alone and has previously been probed through anisotropic flow [3]. In this poster, we present STAR measurements of the skewness and kurtosis of $\left[p_{\mathrm{T}}\right]$ in ${ }^{238} \mathrm{U}+{ }^{238} \mathrm{U}$ at $\sqrt{s_{\mathrm{NN}}}=193$ GeV and ${ }^{197} \mathrm{Au}+{ }^{197} \mathrm{Au}$ at 200 GeV. Au+Au follows the independent-source scaling expected for a nearly spherical nucleus. $^{238}$U+$^{238}$U departs from this baseline: the skewness is strongly enhanced toward ultra-central collisions, while the kurtosis stays positive to higher multiplicity before turning sharply negative in the most central events. Comparison with hydrodynamic calculations favours a large quadrupole deformation of $^{238}$U, establishing higher-order $\left[p_{\mathrm{T}}\right]$ cumulants as an independent probe for nuclear shape imaging.
[1] STAR Collaboration, Imaging shapes of atomic nuclei in high-energy nuclear collisions, Nature 635, 67-72 (2024)
[2] STAR Collaboration, Imaging nuclear shape through anisotropic and radial flow in high-energy heavy-ion collisions, Rep. Prog. Phys. 88, 108601 (2025)
[3] J. Jia, Probing triaxial deformation of atomic nuclei in high-energy heavy ion collisions, Phys. Rev. C 105, 044905 (2022)
