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Register a new userGlobal $Lambda$ hyperon polarization in nuclear collisions: evidence for the most vortical fluid
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STAR Collaboration
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The extreme temperatures and energy densities generated by ultra-relativistic collisions between heavy nuclei produce a state of matter with surprising fluid properties. Non-central collisions have angular momentum on the order of 1000$hbar$, and the resulting fluid may have a strong vortical structure that must be understood to properly describe the fluid. It 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. However, no experimental indications of fluid vorticity in heavy ion collisions have so far been found. Here we present the first measurement of an alignment between the angular momentum of a non-central collision and the spin of emitted particles, revealing that the fluid produced in heavy ion collisions is by far the most vortical system ever observed. We find that $Lambda$ and $overline{Lambda}$ hyperons show a positive polarization of the order of a few percent, consistent with some hydrodynamic predictions. A previous measurement that reported a null result at higher collision energies is seen to be consistent with the trend of our new observations, though with larger statistical uncertainties. These data provide the first experimental access to the vortical structure of the perfect fluid created in a heavy ion collision. They should prove valuable in the development of hydrodynamic models that quantitatively connect observations to the theory of the Strong Force. Our results extend the recent discovery of hydrodynamic spin alignment to the subatomic realm.
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In 2017, STAR Collaboration reported the measurements of hyperon global polarization in heavy ion collisions, suggesting the subatomic fireball fluid created in these collisions as the most vortical fluid. There remains the interesting question: at which beam energy the truly most vortical fluid will be located. In this work we perform a systematic study on the beam energy dependence of hyperon global polarization phenomenon, especially in the interesting $hat{O}(1sim 10) rm GeV$ region. We find a non-monotonic trend, with the global polarization to first increase and then decrease when beam energy is lowered from $27~rm GeV$ down to $3~rm GeV$. The maximum polarization signal has been identified around $sqrt{s_{NN}} = 7.7~rm GeV$, where the heavy ion collisions presumably create the most vortical fluid. Detailed experimental measurements in the $hat{O}(1sim 10) rm GeV$ beam energy region are expected to test the prediction very soon.
Global hyperon polarization, $overline{P}_mathrm{H}$, in Au+Au collisions over a large range of collision energy, $sqrt{s_mathrm{NN}}$, has recently been measured and successfully reproduced by hydrodynamic and transport models with intense fluid vorticity of the Quark-Gluon Plasma (QGP). While na{i}ve extrapolation of data trends suggests a large $overline{P}_mathrm{H}$ as the collision energy is reduced, the behavior of $overline{P}_mathrm{H}$ at small $sqrt{s_mathrm{NN}}<7.7$ GeV is unknown. Operating the STAR experiment in fixed-target mode, we have measured the polarization of $Lambda$ hyperons along the direction of global angular momentum in Au+Au collisions at $sqrt{s_mathrm{NN}}=3$ GeV. The observation of substantial polarization of $4.91pm0.81(rm stat.)pm0.15(rm syst.)$% in these collisions may require a reexamination of the viscosity of any fluid created in the collision, the thermalization timescale of rotational modes, and of hadronic mechanisms to produce global polarization.
Predictions for the global polarization of $Lambda$ hyperons in Au+Au collisions at moderately relativistic collision energies, 2.4 $leqsqrt{s_{NN}}leq$ 11 GeV, are made. These are based on the thermodynamic approach to the global polarization incorporated into the model of the three-fluid dynamics. Centrality dependence of the polarization is studied. It is predicted that the polarization reaches a maximum or a plateau (depending on the equation of state and centrality) at $sqrt{s_{NN}}approx$ 3 GeV. It is found that the global polarization increases with increasing width of the rapidity window around the midrapidity.
Results on $Lambda$ hyperon production are reported for collisions of p(3.5 GeV) + Nb, studied with the High Acceptance Di-Electron Spectrometer (HADES) at SIS18 at GSI Helmholtzzentrum for Heavy-Ion Research, Darmstadt. The transverse mass distributions in rapidity bins are well described by Boltzmann shapes with a maximum inverse slope parameter of about $90,$MeV at a rapidity of $y=1.0$, i.e. slightly below the center-of-mass rapidity for nucleon-nucleon collisions, $y_{cm}=1.12$. The rapidity density decreases monotonically with increasing rapidity within a rapidity window ranging from 0.3 to 1.3. The $Lambda$ phase-space distribution is compared with results of other experiments and with predictions of two transport approaches which are available publicly. None of the prese
With a Yang-Mills flux-tube initial state and a high resolution (3+1)D Particle-in-Cell Relativistic (PICR) hydrodynamics simulation, we calculate the $Lambda$ polarization for different energies. The origination of polarization in high energy collisions is discussed, and we find linear impact parameter dependence of the global $Lambda$ polarization. Furthermore, the global $Lambda$ polarization in our model decreases very fast in the low energy domain, and the decline curve fits well the recent results of Beam Energy Scan (BES) program launched by the STAR collaboration at the Relativistic Heavy Ion Collider (RHIC). The time evolution of polarization is also discussed.
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