Rotating objects or fluid cells have the capacity to polarize particle spins along the axis of rotation. The Einstein-de Hass effect is one such example, wherein a spinning ferromagnetic block leads to preferential electron spin orientation and therefore a measurable external magnetic field. More recently, this effect has been seen in liquid Mercury flowing in a channel, where the velocity gradient near the edges of the channel lead to rotating fluid cells and polarized electrons. Analogously, relativistic collisions of heavy ions producing the quark-gluon plasma (QGP) have velocity gradients that lead to QGP fluid-cell rotation, or "vorticity", aligned with the angular momentum of the collision. Previous measurements of this polarization across a broad range of collision energies correspond to measurements of the vorticity in the QGP, which is the largest yet observed in nature.

In a letter for the journal Physical Review C, the STAR collaboration has now measured this polarization with Lambda hyperons at the low center-of-mass collision energy of 3 GeV; this is the lowest collision energy at which polarization has yet been measured and the largest polarization of Lambda hyperons yet observed. The measurement was made possible by colliding an ion beam with a fixed target. Operating the experiment in this fixed-target mode introduced explicit asymmetries, for which the measurement procedure was generalized. Hundreds of millions of collisions were recorded in order to achieve a significant result.

This new measurement demonstrates that vorticity within heavy-ion collisions is supported even at very low collision energies, and raises important questions about the spin and thermal equilibration timescales and how they compare to each other. Furthermore, the unique collision energy and detector acceptance windows allowed for a measurement of the dependence of polarization on rapidity, to the limits of Lambda production in rapidity, allowing us to test the numerous model predictions of strong rapidity dependence. Despite the predictions, we observed no statistically significant dependence. Along with upcoming measurements using the STAR forward upgrade, this will continue to provide important information about the distribution of vorticity within heavy-ion collisions.