Ultracentral Collisions of Small and Deformed Systems at RHIC: $UU$, $dAu$, ${}^{9}BeAu$, ${}^{9}Be{}^{9}Be$, ${}^{3}He{}^{3}He$, and ${}^{3}HeAu$ Collisions

Abstract

In this paper, we study a range of collision systems involving deformed ions and compare the elliptic and triangular flow harmonics produced in a hydrodynamics scenario versus a color glass condensate (CGC) scenario. For the hydrodynamics scenario, we generate initial conditions using TRENTO and work within a linear response approximation to obtain the final flow harmonics. For the CGC scenario, we use the explicit calculation of two-gluon correlations taken in the high-$p_T$ ``(semi)dilute-(semi)dilute'' regime to express the flow harmonics in terms of the density profile of the collision. We consider ultracentral collisions of deformed ions as a testbed for these comparisons because the difference between tip-on-tip and side-on-side collisions modifies the multiplicity dependence in both scenarios, even at zero impact parameter. We find significant qualitative differences in the multiplicity dependence obtained in the initial conditions+hydrodynamics scenario and the CGC scenario, allowing these collisions of deformed ions to be used as a powerful discriminator between models. We also find that sub-nucleonic fluctuations have a systematic effect on the elliptic and triangular flow harmonics which are most discriminating in $0-1\%$ ultracentral symmetric collisions of small deformed ions and in $0-10\%$ $\mathrm{d}{}^{197}\mathrm{Au}$ collisions. The collision systems we consider are ${}^{238}\mathrm{U}{}^{238}\mathrm{U}$, $\mathrm{d}{}^{197}\mathrm{Au}$, ${}^9\mathrm{Be}{}^{197}\mathrm{Au}$, ${}^9\mathrm{Be}{}^9\mathrm{Be}$, ${}^3\mathrm{He}{}^3\mathrm{He}$, and ${}^3\mathrm{He}{}^{197}\mathrm{Au}$.

19 pages, 26 figures