Hyperon Polarization Research Articles

Hyperon polarization in relativistic heavy ion collisions and axial U(1) symmetry breaking at high temperature

Experiments at the Relativistic Heavy Ion Collider (RHIC) have measured the net polarization of $\Lambda$ and $\bar{\Lambda}$ hyperons and attributed it to a coupling between their spin and the vorticity of the fluid created in heavy ion collisions, but how the spin comes to equilibrium with vorticity is an open problem. Recently we found that vorticity fluctuations and helicity flip of strange quarks in quark-gluon plasma through perturbative QCD processes resulted in equilibration times far too long to be relevant. Here we consider the Nambu--Jona-Lasinio model with the inclusion of the six-quark Kobayashi--Maskawa--'t Hooft interaction which breaks axial U(1). Using instanton inspired models for the temperature dependence of the axial symmetry breaking, we find that constituent strange quarks can reach spin equilibrium at temperatures below about 170 MeV, just before they hadronize to form hyperons.

Polarization and Vorticity in the Quark–Gluon Plasma

The quark–gluon plasma (QGP) produced by collisions between ultrarelativistic heavy nuclei is well described in the language of hydrodynamics. Noncentral collisions are characterized by very large angular momentum, which in a fluid system manifests as flow vorticity. This rotational structure can lead to a spin polarization of the hadrons that eventually emerge from the plasma, and thus these collisions provide experimental access to flow substructure at unprecedented detail. Recently, the first observations of Λ hyperon polarization along the direction of collisional angular momentum were reported. These measurements are in broad agreement with hydrodynamic and transport-based calculations and reveal that the QGP is the most vortical fluid ever observed. However, there remain important tensions between theory and observation that might be fundamental in nature. In the relatively mature field of heavy-ion physics, the discovery of global hyperon polarization and 3D simulations of the collision have opened an entirely new direction of research. We discuss the current status of this rapidly developing area and directions for future research.

Vorticity and Particle Polarization in Relativistic Heavy-Ion Collisions

We review studies of vortical motion and the resulting global polarization of $\Lambda$ and $\bar{\Lambda}$ hyperons in heavy-ion collisions, in particular, within 3FD model. 3FD predictions for the global midrapidity polarization in the FAIR-NICA energy range are presented. The 3FD simulations indicate that energy dependence of the observed global polarization of hyperons in the midrapidity region is a consequence of the decrease of the vorticity in the central region with the collision energy rise because of pushing out the vorticity field into the fragmentation regions. At high collision energies this pushing-out results in a peculiar vortical structure consisting of two vortex rings: one ring in the target fragmentation region and another one in the projectile fragmentation region with matter rotation being opposite in these two rings.

Is different Λ and [formula omitted] polarization caused by different spatio-temporal freeze-out picture?

Thermal vorticity in non-central Au+Au collisions at energies 7.7≤s≤62.4 GeV is calculated within the UrQMD transport model. Tracing the Λ and Λ¯ hyperons back to their last interaction point we were able to obtain the temperature and the chemical potentials at the time of emission by fitting the extracted bulk characteristics of hot and dense medium to statistical model of ideal hadron gas. Then the polarization of both hyperons was calculated. The polarization of Λ and Λ¯ increases with decreasing energy of nuclear collisions. The stronger polarization of Λ¯ is explained by the different space-time distributions of Λ and Λ¯ and by different freeze-out conditions of both hyperons.

Quantum Field Effects of Acceleration and Rotation: the Chiral Vortical Effect and the Unruh Effect

The effects of vorticity and acceleration can be studied in the framework of a general fundamental approach based on the Zubarev density operator. The calculation of axial current is related to Chiral Vortical Effect (CVE) and hyperon polarization, while the calculation of acceleration corrections to energy density leads to the Unruh effect. The nonperturbative formula for CVE of massive fermions and the possibility of stabilization and suppression of the axial current in the plasma of elementary particles due to the appearance of an effective mass, depending on temperature, is discussed.

Polarization of cascade hyperons and antihyperons

Polarizations of cascade hyperons (Ξ0 and Ξ−) and antihyperons are especially interesting because they are very sensitive to the mechanism of their production. Hyperon polarization for existing data is considered in framework of the model of chromomagnetic polarization of quarks (CPQ). Hyperon and antihyperon polarization predictions are presented for a wide range of c.m.s. energy and Feynman variable xF.

Particle Polarization and Structure of Vortical Field in Relativistic Heavy-Ion Collisions

We review studies of vortical motion and the resulting global polarization of Λ and hyperons in heavy-ion collisions within 3FD model. The 3FD simulations indicate that energy dependence of the observed global polarization of hyperons in the midrapidity region is a consequence of the decrease of the vorticity in the central region with the collision energy rise because of pushing out the vorticity field into the fragmentation regions. At high collision energies this pushing-out results in a peculiar vortical structure consisting of two vortex rings: one ring in the target fragmentation region and another one in the projectile fragmentation region with matter rotation being opposite in these two rings.

Vorticity structure and polarization of Λ hyperons in heavy-ion collisions

Simulations of peripheral Au+Au collisions at NICA energies are performed in the PHSD transport model. The properties of vorticity field are studied at different impact parameters. Quadrupole structures of the vorticity field in all planes are obtained. A thin layer of large vorticity is found at the boundary of the fireball, the so-called vortex sheet. The effect of helicity separation is detected. Calculation of hyperon polarization in thermodynamic and anomalous models is performed.

Λ hyperon reconstruction at the BM@N experiment and prospects for polarization studies

The BM@N detector at Nuclotron has collected data with different beams (C, Ar, Kr) on different targets. Results of the data analysis on Λ hyperon production in interactions of the 4 GeV kinetic energy carbon beam with C, Al, Cu targets are presented. Plans for future running of the experiment and physics analyses are described including expectations for the Λ hyperon polarization studies on the basis of the Monte Carlo simulated event sample.

STAR Highlights and Future

We present recent results from the STAR experiment at Relativistic Heavy Ion Collider (RHIC): beam-energy dependence of directed flow of identified particles (including open charm hadrons), flow decorrelation, global and local hyperon polarization, femtoscopic measurements, heavy flavour production, jets, and plan for the future upgrades.

Local spin polarization in high energy heavy ion collisions

We revisit the azimuthal angle dependence of the local spin polarization of hyperons in heavy-ion collisions at 200 GeV in the framework of the (3+1)D viscous hydrodynamic model CLVisc. Two different initial conditions are considered in our simulation: the optical Glauber initial condition without initial orbital angular momentum and the AMPT initial condition with an initial orbital angular momentum. We find that the azimuthal angle dependence of the hyperon polarization strongly depends on the choice of the so-called "spin chemical potential" $\Omega_{\mu\nu}$. With $\Omega_{\mu\nu}$ chosen to be proportional to the temperature vorticity, our simulation shows qualitatively coincidental results with the recent measurements at RHIC for both the longitudinal and transverse polarization. We argue that such a coincidence may be related to the fact that the temperature vorticity is approximately conserved in the hot quark-gluon matter.

Polarization of Λ (Λ[over ¯]) Hyperons along the Beam Direction in Au+Au Collisions at sqrt[s_{NN}]=200 GeV.

The Λ (Λ[over ¯]) hyperon polarization along the beam direction has been measured in Au+Au collisions at sqrt[s_{NN}]=200 GeV, for the first time in heavy-ion collisions. The polarization dependence on the hyperons' emission angle relative to the elliptic flow plane exhibits a second harmonic sine modulation, indicating a quadrupole pattern of the vorticity component along the beam direction, expected due to elliptic flow. The polarization is found to increase in more peripheral collisions, and shows no strong transverse momentum (p_{T}) dependence at p_{T} greater than 1 GeV/c. The magnitude of the signal is about 5 times smaller than those predicted by hydrodynamic and multiphase transport models; the observed phase of the emission angle dependence is also opposite to these model predictions. In contrast, the kinematic vorticity calculations in the blast-wave model tuned to reproduce particle spectra, elliptic flow, and the azimuthal dependence of the Gaussian source radii measured with the Hanbury Brown-Twiss intensity interferometry technique reproduce well the modulation phase measured in the data and capture the centrality and transverse momentum dependence of the polarization signal.

Recent STAR Results from Heavy-Ion and Polarized Proton Programs

We present recent physics results from the STAR experiment at Relativistic Heavy Ion Collider (RHIC). The proceedings cover studies of azimuthal anisotropy in small and large systems, global and local hyperon polarization, correlation femtoscopy, antideuteron and J/ψ production from heavyion program as well as the measurements of longitudinal spin asymmetry from polarized proton program.

Searching for the subatomic swirls in the CuCu and CuAu collisions

Recently the STAR Collaboration reported measurement of the “subatomic swirls”, that is, the most vortical fluid flow structures in the quark–gluon plasma produced via the AuAu collisions at the Relativistic Heavy Ion Collider (RHIC). This result has attracted significant interest and generated wide enthusiasm. For such an important finding, it is crucial to look for additional evidences of confirmation and to critically test the current interpretation of the global polarization measurement. We suggest that the CuCu and CuAu colliding systems at RHIC provide such opportunity. Interestingly, our calculations reveal that the fluid vorticity in the CuCu or CuAu collision is comparable to that in the AuAu collision. We find the computed Λ hyperon polarization effect is stronger in the CuCu and CuAu systems than the AuAu system at the same collisional beam energy and centrality class, with an interesting hierarchy CuCu > CuAu > AuAu due to interplay between Λ production timing and the time evolution of the vorticity. These predictions can be readily tested by experimental data.

Vorticity and hyperon polarization at energies available at JINR Nuclotron-based Ion Collider fAcility

We study the formation of fluid vorticity and the hyperon polarization in heavy-ion collisions at NICA energies in the framework of the Parton-Hadron-String Dynamic Model, taking into account both hadronic and quark-gluonic (partonic) degrees of freedom. The vorticity properties in peripheral Au+Au collisions at $\sqrt{s_{NN}}=$7.7\,GeV are demonstrated and confronted with other models. The obtained result for the $\Lambda$ polarization is in agreement with the experimental data by the STAR collaboration, whereas the model is not able to explain the observed high values of the anti-hyperon $\bar\Lambda$ polarization.

Hyperon polarization in heavy-ion collisions and holographic gravitational anomaly

We study the energy dependence of global polarization of $\Lambda$ hyperons in peripheral $Au-Au$ collisions. We combine the calculation of vorticity and strange chemical potential in the framework of kinetic Quark-Gluon String Model with the anomalous mechanism related to axial vortical effect. The earlier found effect of helicity separation implies the quadrupole structure of longitudinal vorticity. We pay special attention to the temperature dependent contribution related to gravitational anomaly and found that the preliminary RHIC data are compatible with its suppression discovered earlier in lattice calculations. The antihyperons polarization is excessing that of hyperons and the difference is more pronounced at smaller energies.

A Monte Carlo Study of Lambda Hyperon Polarization at BM@N

Heavy strange objects (hyperons) can provide essential signatures of the excited and compressed baryonic matter. At NICA, it is planned to study hyperons both in the collider mode (MPD detector) and the fixed-target one (BM@N setup). Measurements of strange hyperon polarization can give additional information on the strong interaction mechanisms. In heavy-ion collisions, such measurements are even more valuable since the polarization is expected to be sensitive to characteristics of the QCD medium (vorticity, hydrodynamic helicity) and to QCD anomalous transport. In this analysis, the possibility to measure at BM@N the polarization of the lightest strange hyperon Λ is studied in Monte Carlo event samples of Au + Au collisions produced with the DCM-QGSM generator. It is shown that the detector will allow to measure polarization with a precision required to check the model predictions.

Theoretical perspective on strangeness production

A brief review of some highlights and puzzles on strangeness production in heavy ion collisions is given. These include strangeness production and the nuclear equation of state; deeply subthreshold strangeness production; mean-field potentials on strange hadrons; phi meson in dense matter; anomalous strange hadron to pion ratios; density fluctuations on particle production; A hyperon polarization and the vorticity field, and exotic hadrons.

Measurements of spin alignment of vector mesons and global polarization of hyperons with ALICE at the LHC

We present the measurements related to global polarization of Λ hyperons and spin alignment of K*0 vector mesons at mid-rapidity for Pb-Pb collisions at [see formula in PDF] = 2.76 TeV using the ALICE detector at the LHC. The global polarization measurements are carried out with respect to the first order event plane while the spin alignment measurements are carried out with respect to the production plane. No global polarization signal for Λ is observed for 5-15% and 15-50% central Pb-Pb collisions. The spin density matrix element ρ00 is found to have values slightly below ⅓ at low transverse momentum (pT) for K*0 mesons, while it is consistent with ⅓ (no spin alignment) at higher pT. No spin alignment is observed for K*0 in pp collisions at √s = 13 TeV and for the spin zero hadron K0S in 20-40% Pb-Pb collisions at [see formula in PDF] = 2.76 TeV.

Quark-hadron duality in hydrodynamics: an example

We consider the problem of transferring overall rotation of quark-gluon plasma to polarization of hyperons along the rotation axis. As a toy theoretical model, we exploit that of pionic superfluidity induced by chemical potentials violating isotopic symmetry. Apparently, the model accounts only for the light degrees of freedom, that is pions. The rotation, however, results in vortices which are infinitely thin in the hydrodynamic approximation. Field theory resolves the singularity and predicts that the core of the vortices is build up on spins of baryons. We review consequences from the quark-hadron duality in this case. First, an anomalous triangle graph in effective field theory turns to be dual to the vorticity term in the standard hydrodynamic expansion. And, then, the overall coefficient determining the polarization of baryons is fixed by duality with the triangle graph in the fundamental field theory.