Spin-vorticity Coupling Research Articles

Lambda polarization in heavy ion collisions: from RHIC BES to LHC energies

STAR collaboration at RHIC has recently measured the polarization of Λ hyperons in non-central heavy ion collisions in the RHIC Beam Energy Scan (BES) program. The magnitude of the polarization was found to decrease from few percents at the lowest BES energies to ≈ 0.2% at the top RHIC energy. The polarization signal has been reproduced in different hydrodynamic calculations assuming a thermodynamic spin-vorticity coupling mechanism at the Cooper-Frye hypersurface [1].In this work an extension of our existing calculations [2] of the Λ polarization in the RHIC BES program to the top RHIC and 2.76 TeV LHC energies is presented. The longitudinal component of the Λ polarization, which is the dominant component of the polarization at the LHC energies, is discussed. Finally we show that the global polarization of Λ originates dominantly from the relativistic analogue of the classical vorticity, whereas the quadrupole longitudinal component originates from the gradients of temperature and acceleration of the medium when the Λs are produced out of the fluid.

Spin–vorticity coupling in viscous electron fluids

We consider spin–vorticity coupling—the generation of spin polarization by vorticity—in viscous two-dimensional electron systems with spin–orbit coupling. We first derive hydrodynamic equations for spin and momentum densities in which their mutual coupling is determined by the rotational viscosity. We then calculate the rotational viscosity microscopically in the limits of weak and strong spin–orbit coupling. We provide estimates that show that the spin–orbit coupling achieved in recent experiments is strong enough for the spin–vorticity coupling to be observed. On the one hand, this coupling provides a way to image viscous electron flows by imaging spin densities. On the other hand, we show that the spin polarization generated by spin–vorticity coupling in the hydrodynamic regime can, in principle, be much larger than that generated, e.g. by the spin Hall effect, in the diffusive regime.

Generation of Spin Current from Lattice Distortion Dynamics: Spin–Orbit Routes

Generation of spin current from lattice distortion dynamics in metals is studied with special attention on the effect of spin-orbit coupling. Treating the lattice distortion by local coordinate transformation, we calculate spin current and spin accumulation with the linear response theory. It is found that there are two routes to the spin-current generation, one via the spin Hall effect and the other via the spin accumulation. The present effect due to spin-orbit coupling can be comparable to, or even larger than, the one based on the spin-vorticity coupling in systems with strong spin-orbit coupling.

Vortical Fluid and Λ Spin Correlations in High-Energy Heavy-Ion Collisions.

Fermions become polarized in a vortical fluid due to spin-vorticity coupling, and the polarization density is proportional to the local fluid vorticity. The radial expansion converts spatial vortical structures in the transverse plane to spin correlations in the azimuthal angle of final Λ hyperons' transverse momentum in high-energy heavy-ion collisions. Using a (3+1)D viscous hydrodynamic model with fluctuating initial conditions from a multiphase transport (AMPT) model, we reveal two vortical structures that are common in many fluid dynamic systems: a right-handed toroidal structure around each beam direction for transverse vorticity and pairing of longitudinal vortices with opposite signs in the transverse plane. The calculated azimuthal correlation of the transverse spin is shown to have a cosine form plus an offset due to the toroidal structure of the transverse vorticity around the beam direction and the global spin polarization. The longitudinal spin correlation in the azimuthal angle shows an oscillatory structure due to multiple vorticity pairs in the transverse plane. Mechanisms of these vortical structures, physical implications of hyperon spin correlations, dependence on colliding energy, rapidity, centrality, and sensitivity to the shear viscosity are also investigated.

Charge Redistribution from Anomalous Magnetovorticity Coupling

We investigate novel transport phenomena in a chiral fluid originated from an interplay between a vorticity and strong magnetic field, which induces a redistribution of vector charges in the system and an axial current along the magnetic field. The corresponding transport coefficients are obtained from an energy-shift argument for the chiral fermions in the lowest Landau level due to a spin-vorticity coupling and also from diagrammatic computations on the basis of the linear response theory. Based on consistent results from both methods, we observe that the transport coefficients are proportional to the anomaly coefficient and are independent of temperature and chemical potential. We therefore speculate that these transport phenomena are connected to quantum anomaly.

Polarization of massive fermions in a vortical fluid

Fermions become polarized in a vorticular fluid due to spin-vorticity coupling. Such a polarization can be calculated from the Wigner function in a quantum kinetic approach. Extending previous results for chiral fermions, we derive the Wigner function for massive fermions up to the next-to-leading order in spatial gradient expansion. The polarization density of fermions can be calculated from the axial vector component of the Wigner function and is found to be proportional to the local vorticity $\omega$. The polarizations per particle for fermions and anti-fermions decrease with the chemical potential and increase with energy (mass). Both quantities approach the asymptotic value $\hbar\omega/4$ in the large energy (mass) limit. The polarization per particle for fermions is always smaller than that for anti-fermions, whose ratio of fermions to anti-fermions also decreases with the chemical potential. The polarization per particle on the Cooper-Frye freeze-out hyper-surface can also be formulated and is consistent with the previous result of Becattini et al..

Magnetic moment, vorticity-spin coupling and parity-odd conductivity of chiral fermions in 4-dimensional Wigner functions

We demonstrate the emergence of the magnetic moment and spin-vorticity coupling of chiral fermions in 4-dimensional Wigner functions. In linear response theory with space–time varying electromagnetic fields, the parity-odd part of the electric conductivity can also be derived which reproduces results of the one-loop and the hard-thermal or hard-dense loop. All these properties show that the 4-dimensional Wigner functions capture comprehensive aspects of physics for chiral fermions in electromagnetic fields.

Existence and causality of cosmological models with spinning fluids and torsion

Cosmological models with time dependent scale factors for Lewis type cylindrically symmetric geometries and Ray-Smalley rotating spin fluids cannot be expanding in Einstein Cartan theory. However, these fluids can generate anisotropic and inhomogeneous stationary stably causal rotating spacetimes with strong (and weak) spin-vorticity coupling and reasonable physical properties.

Stably causal Gödel-type models

The problem of finding stationary Gödel-type cosmological space-times endowed with stable causality is solved in general relativity. These models are generated by spin fluids with strong spin-vorticity coupling, and present interesting physical properties. Their intrinsic characterization is made by means of an invariant algebraic classification. A simple technique to test the stable causality is applied to new and old distinct stationary space-times.

Existence and causality of cylindrically symmetric cosmological models with rotating spin fluids

Some recent articles have proposed expanding and rotating fluids with the Krechet-Godel metric as cosmological models. The authors show that, allowing a time-dependent scale factor for cylindrical Lewis-type geometries, the rotating spin fluids of Ray-Smalley (1982) and of Ray-Smalley-Krisch (1987) cannot be expanding in the framework of general relativity. They also found that for a subclass of these metrics, both spin fluids yield three kinds of solutions: stationary, Friedman and de Sitter. With respect to Lewis-type stationary metrics, they have found that the Ray-Smalley spin fluid admits a stably causal inhomogeneous model with strong spin-vorticity coupling. The simplest solution given by the Ray-Smalley-Krisch spin fluid presents causality violation.