Global Polarization Research Articles

Anomalous magnetohydrodynamics with temperature-dependent electric conductivity and application to the global polarization

We have derived the solutions of the relativistic anomalous magnetohydrodynamics with longitudinal Bjorken boost invariance and transverse electromagnetic fields in the presence of temperature or energy density dependent electric conductivity. We consider the equations of states in a high temperature limit or in a high chiral chemical potential limit. We obtain both perturbative analytic solutions up to the order of \hbar and numerical solutions in our configurations of initial electromagnetic fields and Bjorken flow velocity. Our results show that the temperature or energy density dependent electric conductivity plays an important role to the decaying of the energy density and electromagnetic fields. We also implement our results to the splitting of global polarization for \Lambda and \bar{\Lambda} hyperons induced by the magnetic fields. Our results for the splitting of global polarization disagree with the experimental data in low energy collisions, which implies that the contribution from gradient of chemical potential may dominate in the low energy collisions.

Switching imidazole reactivity by dynamic control of tautomer state in an allosteric foldamer

Molecular biology achieves control over complex reaction networks by means of molecular systems that translate a chemical input (such as ligand binding) into an orthogonal chemical output (such as acylation or phosphorylation). We present an artificial molecular translation device that converts a chemical input – the presence of chloride ions – into an unrelated chemical output: modulation of the reactivity of an imidazole moiety, both as a Brønsted base and as a nucleophile. The modulation of reactivity operates through the allosteric remote control of imidazole tautomer states. The reversible coordination of chloride to a urea binding site triggers a cascade of conformational changes in a chain of ethylene-bridged hydrogen-bonded ureas, switching the chain’s global polarity, that in turn modulates the tautomeric equilibrium of a distal imidazole, and hence its reactivity. Switching reactivities of active sites by dynamically controlling their tautomer states is an untapped strategy for building functional molecular devices with allosteric enzyme-like properties.

Expanding Avenues for Student Engagement With Diverse Religious, Spiritual, and Secular Worldviews

Distinct in its scope and research design, the Interfaith Diversity Experiences and Attitudes Longitudinal Survey (IDEALS) sheds light on promising practices for fostering the knowledge, skills, and attitudes college students need to navigate a pluralistic society. Importantly, the study culminated at a unique time characterized by a global pandemic, racial reckoning, and political polarization. This article explores how interfaith approaches in the wake of 2020 are both well informed by IDEALS research and responsive to urgent issues of the day. It includes real-world examples of curricular and co-curricular initiatives taking place at institutions of varying types and geographic locations to advance equitable public health, racial justice, and bridgebuilding across political divides.

Solitonic-like interactions of counter-propagating clusters of active particles.

This report considers a set of interacting self-propelled particles immersed in a viscous and noisy environment. The explored particle interaction does not distinguish between alignments and anti-alignments of the self-propulsion forces. More specifically, we considered a set of self-propelled apolar aligning attractive particles. Consequently, there is no genuine flocking transition because the system has no global velocity polarization. Instead, another self-organized motion emerges, where the system forms two counter-propagating flocks. This tendency leads to the formation of two counter-propagating clusters for short-range interaction. Depending on the parameters, these clusters interact, exhibiting two of the four classical behaviors of counter-propagating dissipative solitons (which does not imply that a single cluster must be recognized as a soliton). They interpenetrate and continue their movement after colliding or forming a bound state where the clusters remain together. This phenomenon is analyzed using two mean-field strategies: an all-to-all interaction that predicts the formation of the two counter-propagating flocks and a noiseless approximation for cluster-to-cluster interaction, which explains the solitonic-like behaviors. Furthermore, the last approach shows that the bound states are metastables. Both approaches agree with direct numerical simulations of the active-particle ensemble.

Directed flow and global polarization in Au+Au collisions across energies covered by the beam energy scan at RHIC

We study the directed flow of identified particles in Au+Au collisions at $\sqrt{s_\text{NN}}=7.7$ to 62.4 GeV. The Glauber model is extended to include both a tilted deformation of the QGP fireball with respect to the longitudinal direction and a non-zero longitudinal flow velocity gradient in the initial state. By combining this improved initial condition with a (3+1)-dimensional viscous hydrodynamic model calculation, we obtain a satisfactory description of the transverse momentum spectra and the rapidity dependent directed flow coefficient of different hadron species. Our calculation indicates the sensitivity of the hadron directed flow, especially its splitting between protons and antiprotons, to both the initial geometry and the initial longitudinal flow velocity. Therefore, the combination of directed flow of different hadrons can provide a tight constraint on the initial condition of nuclear matter created in heavy-ion collisions. The initial condition extracted from the directed flow is further tested with the global polarization of $\Lambda$ and $\bar{\Lambda}$ within the same theoretical framework, where we obtain a reasonable description of these hyperon polarization observed at different collision energies at RHIC.

An underwater image restoration based on global polarization effects of underwater scene

Currently, most underwater polarization imaging methods assume that the backscattering is polarized and the reflected light of the target is unpolarized. In reality, this assumption is unreliable since both backscattering and reflected light contribute to polarization. Therefore, we propose an underwater image restoration method that combines the polarization effects of reflected light and backscattering. Firstly, Stokes vectors are used to automatically obtain polarization difference images. Based on the enhancement measure evaluation (EME) constraint, the initial estimate of backscattering is obtained. Then, an adaptive local optimization (ALO) method is designed to estimate global backscattering light. A variable factor is introduced for local optimization to obtain global backscattering. Finally, in order to optimize the system, particle swarm optimization (PSO) is introduced, which improves the processing speed of the system. The experimental results show that it is necessary to consider the polarization of the target during underwater polarization descattering, which not only has a good ability to eliminate scattering but also has an excellent effect on restoring target details.

An Improved Core-Corona Model for Λ and Λ Polarization in Relativistic Heavy-Ion Collisions

Due to its sensitivity to the dynamics of strongly interacting matter subject to extreme conditions, hyperon global polarization has become an important observable to study the system created in relativistic heavy-ion collisions. Recently, the STAR and HADES collaborations have measured the global polarization of both Λ and Λ¯ produced in semi-central collisions in a wide range of collision energies. The polarization excitation functions show an increasing trend as the collision energy decreases, with the increase being more pronounced for the Λ¯. In this work, we make a summary of a core-corona model that we have developed to quantify the global polarization contributions from Λ and Λ¯ created in different regions of the fireball. The core-corona model assumes that Λs and Λ¯s are produced in both regions, the high-density core and the lower density corona, with different relative abundances which modulate the polarization excitation function. We have shown that the model works well for the description of experimental results. The global polarization excitation functions computed with the model show a peak at different collision energies in the region sNN≤10 GeV. Finally, we discuss and report on the model global polarization predictions for BES-II, NICA and CBM at FAIR and HADES energies.

Λ and Λ¯ Freeze-Out Distributions and Global Polarizations in Au+Au Collisions

The gold–gold collisions at sNN=7.7 and 11.5 GeV are simulated within the PHSD transport model. In each collision event, the spectator nucleons are separated and the fluidization procedure for the participants is performed. The local velocities are determined in the Landau frame and the kinematic and thermal vorticity fields are evaluated. We analyze the thermodynamic properties of the cells where Λs and Λ¯s were born or had their last interaction. Such cells contribute to the formation of the observed global polarization of hyperons induced by the thermal vorticity of the medium. The Λ¯ polarization signal is found to be mainly determined by hot, dense, and highly vortical cells at the earlier stage of the collision, whereas the Λ polarization signal is accumulated over the longer time and includes cells with lower vorticity. The calculated global polarizations for both Λs and Λ¯s agree well with the experimental finding by the STAR collaboration at energy sNN=11.5 GeV. For collisions at sNN=7.7 GeV, we can reproduce the STAR data for Λ hyperons, but significantly underpredict the observed global polarization of Λ¯. Furthermore, we consider the centrality dependence of the hyperon polarization in collisions at 7.7 GeV. It increases with an increase of centrality, reaches a maximum at 65–75% and then starts decreasing rapidly for peripheral collisions.

Multipolar social systems: Measuring polarization beyond dichotomous contexts

Social polarization is a growing concern worldwide, as it strains social relations, erodes trust in institutions, and thus hurts democratic societies. Polarization has been traditionally studied in binary conflicts where two groups support opposite ideas. However, in many social systems, such as multi-party democracies, political conflicts involve multiple dissenting factions. Despite the prevalence of multipolar systems, there is still a lack of suitable analytical tools to study their polarization patterns. In this work, we introduce new polarization metrics for multidimensional scenarios and develop a methodology that extracts the ideological structure of multipolar contexts from social networks. We propose the trace of the covariance matrix (the total variation) of the multipolar opinion distribution as a measure of global polarization, and its eigendecomposition to identify the directions of maximum polarization. Instead of using a pre-conceived opinion space (conservative vs progressive, liberal vs authoritarian, etc.), our framework reveals the natural ideological axes of the system. We apply our methodology to quadripolar and pentapolar real-world democratic processes, finding non-trivial ideological structures with clear connections to the underlying social context. Our framework opens new avenues for understanding multilateral social tensions and facing the challenges of polarization in a unified way.

Empirical Hydrogen Bonding Donor (HBD) Parameters of Organic Solvents Using Solvatochromic Probes - A Critical Evaluation.

The solvatochromicity of established solvatochromic UV/Vis probes, which appear to be sensitive to the so-called hydrogen bond donor (HBD) property of the solvent, is analysed using the hydroxyl group density of alcoholic solvents DHBD as a physical parameter in comparison to the pKa, the chemical benchmark for acidity. Reichardt's dye B30, Kosowers Z-indicator 1-ethyl-4-(methoxycarbonyl) pyridinium iodide (K), Kamlet-Tafts α, Dragos S parameter, Catalans SA scale, the cis-dicyano-bis (1,10-phenanthroline) iron II complex (Schilt's Ferrocyphen dye, Fe) and Gutmann's acceptor number (AN) have been investigated. The observed dependencies of the empirical polarity parameters as a function of DHBD for several alcoholic solvent families requires a ompletely new physicochemical understanding of these established HBD parameters. Only the AN scale (or Fe) is able to bridge the gap between global polarity and acidity, provided the values are interpreted correctly and applied accordingly.

Λ Polarization and Vortex Rings in Heavy-Ion Collisions at NICA Energies

We review recent studies of vortical motion and the resulting polarization of Λ hyperons in heavy-ion collisions at NICA energies, in particular, within the model of three-fluid dynamics (3FD). This includes predictions of the global Λ polarization and ring structures that appear in Au+Au collisions. The global Λ polarization in Au+Au collisions is calculated, including its rapidity and centrality dependence. The contributions of the thermal vorticity and meson-field term (proposed by Csernai, Kapusta, and Welle) to the global polarization are considered. The results are compared with data from recent STAR and HADES experiments. It is predicted that the polarization maximum is reached at sNN≈ 3 GeV if the measurements are performed with the same acceptance. It is demonstrated that a pair of vortex rings are formed, one at forward rapidities and another at backward rapidities, in ultra-central Au+Au collisions at sNN> 4 GeV. The vortex rings carry information about the early stage of the collision, in particular, the stopping of baryons. It is shown that these rings can be detected by measuring the ring observable RΛ, even in the midrapidity region at sNN= 5–20 GeV. At forward/backward rapidities, the RΛ signal is expected to be stronger. The possibility of observing the vortex-ring signal against the background of non-collective transverse polarization is discussed.

Evolution of global polarization in relativistic heavy-ion collisions within a perturbative approach

Extremely large angular orbital momentum can be produced in non-central heavy-ion collisions, leading to a strong transverse polarization of partons that scatter through the quark-gluon plasma (QGP) due to spin-orbital coupling. We develop a perturbative approach to describe the formation and spacetime evolution of quark polarization inside the QGP. Polarization from both the initial hard scatterings and interactions with the QGP have been consistently described using the quark-potential scattering approach, which has been coupled to realistic initial condition calculation and the subsequent (3+1)-dimensional viscous hydrodynamic simulation of the QGP for the first time. Within this improved approach, we have found that different spacetime-rapidity-dependent initial energy density distributions generate different time evolution profiles of the longitudinal flow velocity gradient of the QGP, which further lead to an approximately 15% difference in the final polarization of quarks collected on the hadronization hypersurface of the QGP. Therefore, in addition to the collective flow coefficients, the hyperon polarization may serve as a novel tool to help constrain the initial condition of the hot nuclear matter created in high-energy nuclear collisions.

Spin-orbital coupling in strong interaction and global spin polarization

In non-central relativistic heavy ion collisions, the colliding nuclear system possesses a huge global orbital angular momentum in the direction opposite to the normal of the reaction plane. Due to the spin-orbit coupling in strong interaction, such a huge orbital angular momentum leads to a global spin polarization of the quark matter system produced in the collision process. The global polarization effect in high energy heavy ion collisions was first predicted theoretically and confirmed by STAR experiments at the Relativistic Heavy Ion Collider in Brookhaven National Laboratory. The discovery has attracted much attention to the study of spin effects in heavy ion collision and leads to a new direction in high energy heavy ion physics—Spin Physics in Heavy Ion Collisions. In this paper, we briefly review the original ideas, the calculation methods, the main results and recent theoretical developments in last years. First, we present a short discussion of the spin-orbit coupling which is an intrinsic property for a relativistic fermionic quantum system. Then we review how the global orbital angular momentum can be generated in non-central heavy ion collisions and how the global orbital angular momentum can be transferred to the local orbital angular momentum distribution in two limit model---Landan fireball model and Bjorken scaling model. After that, we review how we can describe the scattering process with initial local orbital angular momentum in the formalism of scattering cross section in impact parameter space and how we calculate the polarization of the quarks and antiquarks in quark gluon plasma produced in non-central heavy ion collisions after single or multiple scattering. We also give a brief review on how the global polarization can be predicted from the formalism of relativistic hydrodynamics with the generalized Cooper-Frye formula with spin. Finally, we discuss how the quark's polarization can be transferred to the final hadron's polarization. We focus on the hyperon's polarization and vector meson's spin alignment produced in heavy-ion collisions.

Global polarization of hyperons and spin alignment of vector mesons in quark matters

<sec>Relativistic heavy ion collider (RHIC) as a dedicated nuclear facility has made a few major discoveries in physics. This year marks the 30th year STAR Collaboration formation and the 23th year of STAR detector operation and data collection at RHIC. In the last two decades, STAR has collected many datasets, exhibiting scientific versatility and flexibility of the RHIC facility. The total dataset in the first year is less than 1 million good events, and currently there are about 1 billion events per dataset.</sec><sec>The Global Hyperon Polarization was proposed in 2004. This immediately prompted the STAR Collaboration to search for this phenomenon from the early datasets. The null results were presented at Quark Matter Conference in Shanghai in 2006 and subsequently published. Although there were peripheral and continuous efforts in the following decade, no positive result has been observed experimentally. This situation changed in the following decade with the upgrade of high data rate and time-of-flight (TOF) detector and the progress of the Beam Energy Scan Phase I (BES-I). The experimental discoveries of the global polarization of hyperons in 2017 and the spin alignment of vector mesons in 2023 at RHIC-STAR confirm the theory which was established nearly twenty years ago. The theory and these measurements open the way to studying the properties of the hot and dense nuclear matter created in high-energy heavy ion collisions from a new degree of freedom, spin.</sec><sec>We briefly review these discoveries from the proposals of theory to the experimental measurements, and summarize the related measurements at the existing facilities and the theoretical explanations to the original proposal. The basic understanding and the original proposal are still valid and fundamental, that is, the angular momentum of system can transform into a spin effect observable in experiment. However, it appears that in each case a new model is needed to explain the new experimental observation. We need a more basic theory to help us unify all these spin related phenomena.</sec><sec>Over the past five years, STAR has successfully installed 3 new detectors and we have begun to see the physical analysis results from datasets with those new functions. What makes the STAR detector viable after 20 years of operation is its continuous evolution through successful upgrades, with new scientific programs added year by year. The next big thing is to forward upgrade a tracking system (3 layers of silicon strips and 4 layers of sTGC chambers) and a calorimetry system (electromagnetic and hadronic calorimeters). In addition to studying the spin structure of protons by using the polarized proton beams at RHIC, the upgrades also provide a unique ability to investigate the origin of Λ Global Polarization as a function of rapidity and rapidity (de-)correlations in Au+Au collisions.</sec>

Measurements of global polarization of QCD matter in heavy-ion collisions

The experimental data of the global polarization of Λ hyperon, ϕ and K<sup>*0</sup> vector mesons in high-energy heavy ion collision confirm the new phenomenon of global polarization of hot-dense QCD matter, which has attracted extensive attention from researchers and has become a new hot research direction in the frontier of high-energy nuclear physics. This paper reviews the recent global polarization measurements. We focus on the global polarization measurements of Λ hyperon and ϕ, K<sup>*0</sup> mesons, carried out by the solenoidal tracker detector (STAR) collaboration group at the Relativistic Heavy Ion Collider (RHIC) at its Phase I of Beam Energy Scan program, and extend to the global polarization measurements containing multiple strange quark particles, such as Ξ, Ω and the local polarization studies of Λ along the beam direction. In the paper, we also briefly comment on the measurements at higher energy from the large hadron collider (LHC) and at very low energy in HADES experiment. In the end of the paper, the physical information given by these experimental results is also briefly discussed.

Measurements of global and local polarization of hyperons in isobar collisions at 200 GeV from STAR

In heavy-ion collisions, the observation of the global and local polarization of hyperons has revealed the existence of large vorticities perpendicular to reaction plane due to system’s orbital angular momentum and along beam direction due to collective velocity field, respectively. With the high-statistics data from isobar collisions of Ru+Ru and Zr+Zr at √SNN = 200 GeV collected by the STAR experiment, we present differential measurements of global polarization for Λ/Λ¯ as a function of centrality. These measurements allow us to study the possible magnetic field driven effects through the polarization difference between Ru+Ru and Zr+Zr, owing to a larger magnetic field in the former. Furthermore, the first measurements of Λ hyperon local polarization along beam direction relative to the third-order event plane as well as the second-order event plane are presented. A comparison of results from isobar and Au+Au collisions provides important new insights into the collision system size dependence of the vorticities in heavy-ion collisions.

Relativistic spin transport theory for spin-1/2 fermions

Global polarization effect is an important physical phenomenon reflecting spin-orbit couplings in heavy ion collisions. Since STAR’s observation of the global polarization of <inline-formula><tex-math id="M2">\begin{document}$\Lambda$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="11-20222470_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="11-20222470_M2.png"/></alternatives></inline-formula> hyperons in Au+Au collisions in 2017, this effect has attracted a lot of interests in the field. In the hot and dense matter produced in heavy ion collisions, the spin-orbit couplings come from non-local collisions between particles, in which the orbital angular momentum involves the space and momentum information of the colliding particles, so it is necessary to describe the particle collisions with spin-orbit couplings in phase space. In addition, the spin-orbit coupling is a quantum effect, which requires quantum theory. In combination of two aspects, the quantum kinetic theory based on covariant Wigner functions has become a powerful tool to describe the global polarization effect. In this paper, we introduce the quantum kinetic theory for spin-1/2 Fermion system based on Wigner functions as well as the spin transport theory developed on this basis. The recent research progress of spin transport theory provides a solid theoretical foundation for simulating the space-time evolution of spin polarization effects in heavy ion collisions.

The direct flow of charged particles and the global polarization of hyperons in 200 AGeV Au+Au collisions at RHIC

In non-central relativistic heavy-ion collisions, the non-colliding nucleons drag the colliding nucleons along the longitudinal direction asymmetrically, producing a longitudinally tilted quark-gluon plasma (QGP) fireball. Meanwhile, these colliding nuclei deposit a huge initial orbital angular momentum into the system, leading to the polarization of partons inside the QGP along the direction of the total angular momentum. Based on the optical Glauber model, we develop a 3-dimensional initial condition of the tilted QGP. By combining it with the (3+1)-dimensional viscous hydrodynamic model CLVisc, we investigate the directed flow of charged hadrons and the global polarization of <inline-formula><tex-math id="M2">\begin{document}$ \Lambda/\bar{\Lambda} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="7-20222391_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="7-20222391_M2.png"/></alternatives></inline-formula> hyperons in heavy-ion collisions. Our calculation indicates that the combination of a tilted initial condition of the QGP and the hydrodynamic model can provide a satisfactory description of the directed flow and global polarization observed at RHIC-STAR. This offers a theoretical baseline for using these observables to further constrain the initial geometry and kinematic properties of the nuclear matter created in heavy-ion collisions.

Spin polarization and alignment in heavy-ion collisions

We report the measurements of global spin polarization of hyperons (PΛ) from FAIR, RHIC and LHC energies. The PΛ continues to follow an increasing trend at √SNN < 7.7 GeV indicating that enormous vorticity prevails even in hadronic dominant region. New measurements of local spin polarization (Pz,2) from isobar collisions follow the same pattern observed in previous RHIC and LHC energies. An observable motivated by predicted baryonic Spin Hall Effect, the net local polarization (Pz,2net), show a negative value in Au+Au collisions at √SNN = 19.6 and 27 GeV. The first Pz measurement with respect to third order harmonic in 200 GeV RHIC isobar collisions shows a non-zero sine modulation, indicating the presence of complex vortical structure in the medium. Surprising new patterns of spin alignment (ρ00) for various vector mesons (K*0, K*±, ϕ and J/ψ emerged at both RHIC and LHC energies. The large deviation in ρ00 of several species poses challenge to the theory.

Rescattering effect on the measurement of K*0 spin alignment in heavy-ion collisions

Spin alignment of vector mesons in noncentral relativistic heavy-ion collisions provides a novel probe of the global quark polarization and hadronization mechanism. In this paper, we discuss the spin alignment of a short-lived vector meson, namely ${K}^{*0}$, arising from the hadronic rescattering process using the UrQMD model. This spin alignment is not related to global quark polarization but cannot be distinguished from those arising from global quark polarization in experiment. The spin alignment parameter ${\ensuremath{\rho}}_{00}$ is found to deviate from 1/3 by up to $\ensuremath{-}0.008$ (0.03) with respect to the reaction (production) plane. These deviations are much larger than the expected signal from all the theoretical models implementing the conventional global quark polarization mechanism as well as the current experimental precision, and should be considered seriously when comparing measurements with theoretical calculations.