The smallest of the Little Bangs: thermalization and collective flow in pp and pA collisions (??!)
- Prof. Ulrich HEINZ
- Prof. Ulrich HEINZ (The Ohio State University)
Recent experimental data from p+Pb and high-multiplicity p+p collisions at the LHC, as well as p+Au, d+Au and He3+Au collisions at RHIC, suggest strongly that the same collective phenomena that have been attributed to hydrodynamic flow in collisions between heavy nuclei also manifest in qualitatively identical ways in collisions where one or both collision partners are as small as a proton. How can this be??! To resolve this surprise, I will discuss the conditions under which hydrodynamic behavior can be expected, and show that modern (resummed) versions of viscous fluid dynamics that account more efficiently for unavoidable strong local momentum anisotropies in rapidly and strongly anisotropically expanding systems can quite accurately describe the evolution of fireballs whose local phase-space distribution is far from thermal equilibrium. I will argue that the largest stumbling block on the way towards a quantitative dynamical understanding of the evolution of small collision systems is not a likely breakdown of the hydrodynamic framework, but our insufficient knowledge of the internal spatial structure of the proton and its quantum fluctuations from collision event to event. Some recent suggestions for improved modelling of the initial-state fluctuations in small collision systems will be reported.