Collision Geometry and Flow in Uranium+Uranium Collisions

TL;DR

This study uses viscous fluid dynamics to analyze uranium-uranium collisions, revealing a 'knee' in elliptic flow related to collision geometry, which challenges existing models and suggests improved event selection methods.

Contribution

It demonstrates that the two-component MC-Glauber model cannot explain the observed flow structure and proposes enhanced event-shape engineering techniques for better collision geometry selection.

Findings

A 'knee'-like structure in elliptic flow is observed in U+U collisions.

The two-component MC-Glauber model is inconsistent with STAR data.

Event-shape engineering with ZDCs improves tip-tip collision selection.

Abstract

Using event-by-event viscous fluid dynamics to evolve fluctuating initial density profiles from the Monte-Carlo Glauber model for U+U collisions, we report a "knee"-like structure in the elliptic flow as a function of collision centrality, located around the 0.5% most central collisions as measured by the final charged multiplicity. This knee is due to the preferential selection of tip-on-tip collision geometries by a high-multiplicity trigger. Such a knee structure is not seen in the STAR data. This rules out the two-component MC-Glauber model for initial energy and entropy production. Hence an enrichment of tip-tip configurations by triggering solely on high-multiplicity in the U+U collisions does not work. On the other hand, by using the Zero Degree Calorimeters (ZDCs) coupled with event-shape engineering such a selection is possible. We identify the selection purity of body-body and tip-tip events in full-overlap U+U collisions. By additionally constraining the asymmetry of the ZDC signals we can further increase the probability of selecting tip-tip events in U+U collisions.