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Global Λ hyperon polarization in nuclear collisions

Abstract

The extreme energy densities generated by ultra-relativistic collisions between heavy atomic nuclei produce a state of matter that behaves surprisingly like a fluid, with exceptionally high temperature and low viscosity1. Non-central collisions have angular momenta of the order of 1,000ћ, and the resulting fluid may have a strong vortical structure2,3,4 that must be understood to describe the fluid properly. The vortical structure is also of particular interest because the restoration of fundamental symmetries of quantum chromodynamics is expected to produce novel physical effects in the presence of strong vorticity5. However, no experimental indications of fluid vorticity in heavy ion collisions have yet been found. Since vorticity represents a local rotational structure of the fluid, spin–orbit coupling can lead to preferential orientation of particle spins along the direction of rotation. Here we present measurements of an alignment between the global angular momentum of a non-central collision and the spin of emitted particles (in this case the collision occurs between gold nuclei and produces Λ baryons), revealing that the fluid produced in heavy ion collisions is the most vortical system so far observed. (At high energies, this fluid is a quark–gluon plasma.) We find that Λ and hyperons show a positive polarization of the order of a few per cent, consistent with some hydrodynamic predictions6. (A hyperon is a particle composed of three quarks, at least one of which is a strange quark; the remainder are up and down quarks, found in protons and neutrons.) A previous measurement7 that reported a null result, that is, zero polarization, at higher collision energies is seen to be consistent with the trend of our observations, though with larger statistical uncertainties. These data provide experimental access to the vortical structure of the nearly ideal liquid8 created in a heavy ion collision and should prove valuable in the development of hydrodynamic models that quantitatively connect observations to the theory of the strong force.

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Figure 1: The STAR detector system.
Figure 2: A single Au + Au collision in the STAR TPC.
Figure 3: A sketch of a Au + Au collision in the STAR detector system.
Figure 4: The hyperon average polarization in Au + Au collisions.

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Acknowledgements

We thank the RHIC Operations Group and RCF at Brookhaven National Laboratory, the NERSC Center at Lawrence Berkeley National Laboratory, and the Open Science Grid consortium for providing resources and support. This work was supported in part by the Office of Nuclear Physics within the US Department of Energy Office of Science, the US National Science Foundation, the Ministry of Education and Science of the Russian Federation, the National Natural Science Foundation of China, the Chinese Academy of Science, the Ministry of Science and Technology of China and the Chinese Ministry of Education, the National Research Foundation of Korea, the GA and MSMT of the Czech Republic, Department of Atomic Energy and Department of Science and Technology of the Government of India, the National Science Centre of Poland, the National Research Foundation, the Ministry of Science, Education and Sports of Croatia, and RosAtom of Russia.

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Reviewer Information Nature thanks H. Petersen, K. Schweda and X.-N. Wang for their contribution to the peer review of this work.

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Extended data figures and tables

Extended Data Figure 1 The uncorrected average polarization in Au + Au collisions.

The polarization signal plotted in Fig. 4 is plotted without applying the reaction-plane resolution correction. As in Fig. 4, statistical uncertainties are indicated by error bars, while boxes indicate systematic uncertainty. Although the number of particles used to estimate increases with the energy of a collision, the resolution () with which is estimated actually decreases with increasing . This is because the strength of the momentum–space anisotropy (called ‘directed flow’) generated in the collision is greater22 at low . Therefore, the polarization signal falls more steeply with if the resolution correction is not applied. The uncorrected signal is shown in Extended Data Fig. 1, which may be compared to Fig. 4. Since for a given , varies slightly with collision multiplicity, the raw signal is measured separately for three bins in centrality (20–30%, 30–40% and 40–50%). These are each corrected with the corresponding resolution factor, and a weighted sum is reported in Fig. 4.

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The STAR Collaboration. Global Λ hyperon polarization in nuclear collisions. Nature 548, 62–65 (2017). https://doi.org/10.1038/nature23004

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