The VII-th International Conference on the Initial Stages of High-Energy Nuclear Collisions : Initial Stages 2023

Imaging the shape of atomic nuclei in high-energy collisions from STAR

Jun 20, 2023, 2:20 PM

20m

Aud. 2 (H.C. Ørsted)

Oral The initial stages and nuclear structure in heavy-ion collisions Parallel Session 1

Speaker

Jiangyong Jia (Stony Brook University (US))

Description

Atomic nuclei across the nuclide chart exhibit a wide range of collective degrees of freedom, such as quadruple, triaxial, and octupole deformations. Nuclear deformations enhance the fluctuation of harmonic flow and radial flow, increasing $v_2$, $v_3$, and mean transverse momentum $[p_{\rm T}]$. As demonstrated in recent model studies, the shape parameters can be constrained precisely from ratios of observables between collisions of nuclei with similar mass numbers, such as between $^{96}$Ru+$^{96}$Ru and $^{96}$Zr+$^{96}$Zr or between $^{197}$Au+$^{197}$Au and $^{238}$U+$^{238}$U collisions.

We present measurements of $v_n$, $[p_{\rm T}]$ fluctuations as well as $v_n$-$p_{\rm T}$ correlations in these collision systems from the STAR experiment. Significant differences are observed for mean, variance, and skewness of $[p_{\rm T}]$ fluctuations between $^{197}$Au+$^{197}$Au and $^{238}$U+$^{238}$U collisions, which can be quantitatively explained by the large prolate deformation of $^{238}$U, $\beta_{2,\rm U}\sim0.28$. Striking differences are also observed in isobar collisions of $^{96}$Ru and $^{96}$Zr, where ratios of many observables show significant deviations from unity and exhibit rich patterns as a function of centrality. A comparison with hydrodynamic model simulations suggests a large quadruple deformation in Ru nucleus $\beta_{2,\rm Ru}\sim0.16$ and a large octupole deformation in Zr nucleus $\beta_{3,\rm Zr}\sim0.2$. The non-monotonic dependence of ratios of multiplicity distribution, $v_2$, and $p_{\rm T}$ fluctuations in the mid-central collisions also require a difference in the surface diffuseness between Ru and Zr. By combining all these observables, we can simultaneously constrain the various nuclear deformations in isobar nuclei. Our results provide the first observation and quantitative extraction of the quadruple and octupole deformation in Ru and Zr nuclei using heavy ion collisions.

Presentation materials

JiangyongJia_is2023parallelv5.pdf