Session HE: Novel Probes of the Quark Gluon Plasma

Subatomic fluid spintronics - Global hyperon polarization in heavy ion collisions measured by STAR

In 1915, Barnett et al found that rotation of a metal cylinder can induce a magnetization in the object.  This remains a rare example of a coupling between macroscopic mechanical rotation and quantum spin (though this was not the paradigm of the day).  Just last year (2016), Takahashi et al discovered the first polarization of electrons induced by mechanical vorticity induced by viscous effects in a fluid; they thus heralded the new field of ``fluid spintronics.'' In 2000, first collisions at Brookhaven National Lab's Relativistic Heavy Ion Collider (RHIC) led to the surprising discovery that the deconfined quark-gluon plasma (QGP) is best described as a "nearly perfect fluid."  These fluid properties remain the focus of intense study, and are providing insights into the Strong force in the non-perturbative regime.  However, fundamental features of the fluid-- including its vorticity-- are largely unexplored. I will discuss recent measurements by the STAR Collaboration at RHIC, on the spin alignment, or polarization, of Lambda hyperons with the angular momentum of the collision. I will argue that a RHIC collision generates the subatomic analog of Takahashi's observation, the vorticity generated by initial viscous forces and maintained by subsequent low viscosity. These measurements allow an estimate of both the vorticity of the QGP and the magnetic field in which it evolves.  Both of these quantities far surpass any known system in the universe.  Furthermore, knowledge of both is crucial to recent studies that may reveal the onset of chiral symmetry restoration in QCD.   

Jet Physics in Heavy Ion Collisions

Jet studies in heavy ion collisions have been rapidly evolving since the first observations of medium interactions at RHIC through back to back hadron correlations and at LHC via reconstructed jets. In order to completely characterize the final state via jet-medium interactions and distinguish between competing energy loss mechanisms, complementary and robust jet observables are investigated. In this talk, with an emphasis on experimental results from LHC, we will discuss the latest developments of jet finding techniques and their applications on new jet structure observables in heavy ion environments. These new measurements could be used to differentiate whether the medium affects the jet formation process from the hard process through hadronization, or whether the parton loses energy to the medium with the showers only affected at much later stages.   

Hydrodynamic modelling of heavy-ion collisions

The Quark Gluon Plasma (QGP), nature's first and most perfect liquid, has been successfully reproduced in heavy-ion collisions at RHIC and the LHC. The dynamics of the QGP can be well described by relativistic viscous hydrodynamics, allowing for precise comparisons to experimental data in order to extract the properties of the QGP. While a small shear viscosity is well-established, questions still remain regarding the precise initial state, the magnitude of bulk viscosity effects, the smallest system that displays QGP-like properties, and the equation of state at large densities. In this talk, the most recent advances in hydrodynamic modeling, current hot topics, and open-ended questions will be explored.