Speaker
Ryan Weller
(MIT)
Description
In recent years, suggestive signatures of collective flow-like behavior have been observed in $p$+$p$ collisions at the LHC and also in light+heavy-ion collisions. We review hydrodynamic model calculations that reasonably describe the experimentally measured $dN_\mathrm{ch}/d\eta$ and $v_2,v_3,v_4$ at $\eta=0$ in collisions from Pb+Pb down to $p$+$p$. Nevertheless, it is still uncertain whether the flow-like correlations in small collisions should be ascribed the same hydrodynamic origin as in heavy+heavy-ion collisions. Resolving this problem requires knowing (1) how a proton should impart its fluctuating shape on hydrodynamic initial data (e.g. $\varepsilon_2$, $\varepsilon_3$), and (2) in what situations hydrodynamics is justified. It turns out the entire non-hydrodynamic behavior of a system is encoded at large orders in the hydrodynamic gradient expansion, whose resummation yields a subset of microscopic system trajectories known as a hydrodynamic attractor. The behavior of trajectories near this attractor define an "off-equilibrium" version of hydrodynamics, whose applicability for small collisions is justified. This provides an answer to (2), but leaves (1), the choice of hydro initial data, as an open issue.
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| Funding source | U.S. Department of Energy, under grant Contract Number DE-SC0011090 |
Primary author
Ryan Weller
(MIT)
