Abstract

In accelerators like RHIC, heavy atomic nuclei are collided at high energies to study emergent properties of the strong-nuclear force. At high-enough energies with heavy-enough nuclei such collisions create a short-lived novel fluid of deconfined quarks and gluons called the “Quark Gluon Plasma”. Because the nuclei themselves are so large, the transverse size of the nuclear overlap is a variable of significance to the field. Collisions with large impact parameters (low degree of overlap) have large angular momentum (~1000 hbar). For a collision which takes a finite amount of time one would expect an excess of particles with spin along the direction of system angular momentum due to spin-orbit coupling.

In 2017 STAR reported the first non-trivial measurement of this alignment, called the global polarization, at the order of a few percent (https://doi.org/10.1038/nature23004). In a thermalized fluid this polarization would come about through a vorticity. Using such a framework it’s possible to extract a vorticity on the order of 10^22 s^-1, which is notably higher than any previously known fluid. This measurement renewed interest in this physics within the heavy-ion physics community and there have been a number of interesting new calculations as well as measurements from STAR, ALICE, and HADES. I plan on discussing this measurement, newer developments, and the future of similar measurements.