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star focus: Global Λ hyperon polarization in nuclear collisions

STAR has recently reported the first observation of global polarization of Lambda hyperons in heavy ion collisions. The discovery has been published in Nature 548, 62 (2017) as a cover story.



Due to the parity-violating nature of their weak decay, Lambdas reveal the direction of their spin by preferentially emitting the daughter proton along that direction. The average spin direction of a population of Lambdas is the polarization. Lambdas at midrapidity were topologically reconstructed in the STAR TPC, and the Beam-Beam Counters (BBC) at forward and backward rapidity were used to estimate the direction of the total angular momentum of the collision. We discovered that the polarization direction of the Lambdas was correlated at the level of several percent with the direction of the system angular momentum in non-central collisions at √sNN=7.7-32 GeV.

It has been well-established that the hot system created at midrapidity in the system may be considered a fluid, and hydrodynamic calculations relate the polarization of emitted particles is directly related to the vorticity - the curl of the flow field - of the fluid. Using this relation, we estimate that the curl of the fluid created at RHIC is about 9×1021 s-1, 14 orders of magnitude higher than any fluid ever observed. Previous results have established the system at RHIC to be the hottest and the least viscous (relative to entropy density) fluid ever created. Our new result adds another record - collisions at RHIC produce the most vortical fluid.

The coupling between mechanical rotation of a system and the quantum spin of a particle has been observed in only a few systems. The Barnett effect - the magnetic polarization of a metallic cylinder due to rapid rotation - was discovered a century ago. And in 2016, the first observation of fluid vorticity in liquid mercury generating polarized electrons was reported in Nature. This latter result has been called the dawning of the field of fluid spintronics. Thus, our result at RHIC opens the field of subatomic fluid spintronics.

Left Figure: A schematic cartoon of a non-central collision at RHIC. The fluid at midrapidity has a whirling substructure oriented (on average in the direction of the total angular momentum, J. The direction of J in each event is estimated by the slight deflection (directed flow) of particles emitted in the forward and backward directions detected in the BBCs. Right Figure: The polarization of Lambdas (blue stars) and AntiLambdas (red circles) are shown as a function of collision energy. The new results, at BES energies, are shown as filled symbols. Open symbols represent lower-statistics measurements published by STAR in 2007 [PRC 76, 024915 (2007); (E) PRC 95, 039906 (2017)]. While statistical uncertanties were too large to measure a finite polarization in the previous analysis, these data are seen to be consistent with the new results. A higher-statistics study of the higher energy data is underway.

This first view of the rotational substructure of the fluid at RHIC represents an entirely new direction in hot QCD research. It has generated considerable theoretical activity in the field, and may have important connections with the Chiral Magnetic and Chiral Vortical Effects (CME and CVE). With increased statistics, there may even be the opportunity to probe the magnetic field produced in heavy ion collisions by measuring the difference in polarization of Lambda and AntiLambda hyperons. Such studies are planned for the future.

BNL News

Posted Aug. 16, 2017


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