Knee structure in the elliptic flow in ultracentral U+U collisions

20 May 2014, 16:30
spectrum (darmstadtium)



Board: I-12
Poster Correlations and Fluctuations Poster session


Andy Goldschmidt (The Ohio State University)


Using event-by-event viscous fluid dynamics to evolve fluctuating initial density profiles for U+U collisions, we find a "knee"-like structure in the elliptic flow as a function of collision centrality, located around 0.5% centrality 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. It arises from a similar knee in the ellipticity distribution in the initial states; hydrodynamic evolution translates the knee in the ellipticity vs. centrality into a knee of the elliptic flow in the final state. The conversion efficiency v2/ε2 is found to be independent of centrality in the knee region. A non-monotonic centrality dependence of v2/ε2 as initially reported by the STAR collaboration can be obtained by normalizing the final v2 values with initial eccentricities calculated from a different initial-state model than the one used in the hydrodynamic simulations. – No knee structure is seen in odd flow harmonics. A weaker knee structure in the centrality dependence of the 4th-order eccentricity is washed out by nonlinear hydrodynamic evolution effects and thus no longer visible in the centrality dependence of the final quadrangular flow v4. – We find that the knee structure in v2 is robust against adding Γ-distributed multiplicity fluctuations, consistent with constraints from measured multiplicity distributions in p+p collisions, but may be washed out when allowing for extreme hot spots as implemented by Rybczynski et al. [Phys. Rev. C 87 (2013) 044908]. – A deeper analysis of these structures is facilitated by imposing additionally a tight cut on complete nuclear overlap by using the Zero Degree Calorimeters (ZDCs). We show results exploring the power of such cuts in selecting specific collision geometries and their impact on anisotropic collective flow.

Primary author

Andy Goldschmidt (The Ohio State University)


Chun Shen (Ohio State University) Ulrich Heinz (The Ohio State University)

Presentation Materials