期刊
NATURE
卷 548, 期 7665, 页码 62-+出版社
NATURE PORTFOLIO
DOI: 10.1038/nature23004
关键词
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资金
- RHIC Operations Group
- RCF at Brookhaven National Laboratory
- NERSC Center at Lawrence Berkeley National Laboratory
- Open Science Grid consortium
- Office of Nuclear Physics within the US Department of Energy Office of Science
- US National Science Foundation
- Ministry of Education and Science of the Russian Federation
- National Natural Science Foundation of China
- Chinese Academy of Science
- Ministry of Science and Technology of China
- Chinese Ministry of Education
- National Research Foundation of Korea
- GA of the Czech Republic
- MSMT of the Czech Republic
- Department of Atomic Energy
- Department of Science and Technology of the Government of India
- National Science Centre of Poland
- National Research Foundation
- Ministry of Science, Education and Sports of Croatia
- RosAtom of Russia
- Direct For Mathematical & Physical Scien
- Division Of Physics [1613939, 1614835] Funding Source: National Science Foundation
- Direct For Mathematical & Physical Scien
- Division Of Physics [1307188] Funding Source: National Science Foundation
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 viscosity(1). Non-central collisions have angular momenta of the order of 1,000., and the resulting fluid may have a strong vortical structure(2-4) 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 vorticity(5). 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 Lambda 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 Lambda and (Lambda) over bar hyperons show a positive polarization of the order of a few per cent, consistent with some hydrodynamic predictions(6). (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 measurement(7) 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 liquid(8) 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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