Abstract
Ultracentral collisions of heavy nuclei, in which the impact parameter is nearly zero, are especially sensitive to the details of the initial state model and the microscopic mechanism for collective flow. In a hydrodynamic “flow” picture, the final state momentum correlations are a direct response to the fluctuating initial geometry, although models of the initial geometry differ widely. Alternatively, dynamical mechanisms based in the color glass condensate (CGC) formalism can naturally lead to many-body correlations with very different systematics. Here we present a calculation of event-by-event elliptic flow in both the hydrodynamic and CGC paradigms and show that they can be qualitatively distinguished in ultracentral collisions of deformed nuclei. Specifically, the multiplicity dependence in such collisions is qualitatively opposite, with the CGC correlations increasing with multiplicity while the hydrodynamic correlations decrease. The consistency of the latter with experimental data on UU collisions appears to rule out a CGC-mediated explanation. We find that these qualitative features also persist in small deformed systems and can therefore be a valuable test of the microscopic physics in that regime. The authors acknowledge support from the US-DOE Nuclear Science Grant No. DE-SC0019175, and the Alfred P. Sloan Foundation, and the Zuckerman STEM Leadership Program.
Highlights
This Proceedings is a brief summary of the important results derived in Ref. [1]; we refer the interested reader there for additional information and details.While “centrality” in heavy-ion collisions is usually considered to be synonymous with “impact parameter,” the physics which drives multiplicity production arises from many factors
In ultracentral collisions, where b ≈ 0 already, the remaining variation in multiplicity production is sensitive to the presence of deformed nuclear geometry
This kind of geometry engineering in ultracentral collisions has been used to rule out the twocomponent Glauber model of initial conditions [5,6,7] and extended to other deformed nuclei such as xenon [8,9,10,11]
Summary
This Proceedings is a brief summary of the important results derived in Ref. [1]; we refer the interested reader there for additional information and details. We find that two competing models for the microscopic origin of collective flow – hydrodynamics and the color glass condensate – lead to qualitatively opposite predictions for the multiplicity dependence in ultracentral collisions of deformed systems. This is especially important for understanding the role which non-hydrodynamic response may play in small deformed systems [12, 13] and how the transition from large systems with hydrodynamic response may occur [14]. Non-flow effects such as jet correlations are suppressed through the use of rapidity cuts, non-flow correlations which are long-range in rapidity can survive such a procedure
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