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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.
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