Spin and polarization: a new direction in relativistic heavy ion physics

Abstract

Since the first evidence of a global polarization of Λ hyperons in relativistic nuclear collisions in 2017, spin has opened a new window in the field, both at experimental and theoretical level, and an exciting perspective. The current state of the field is reviewed with regard to the theoretical understanding of the data, reporting on the most recent achievements and envisioning possible developments. The intriguing connections of spin physics in relativistic matter with fundamental questions in quantum field theory and applications in the non-relativistic domain are discussed.

Gyrohydrodynamics: Relativistic spinful fluid with strong vorticity

“…Such a framework may pave the way to quantitative understanding of spin transports in quark-gluon plasma, supernova explosions, and so on. Specifically, it may be used to resolve the so-called sign problem of the local spin polarization, that is, the discrepancy between the experimental data for azimuthal-angle dependence of hyperon polarization and the theoretical calculations based on thermal vorticity (see below for definition); see reviews [22][23][24][25][26] and references therein for more details.…”

Tensor Polarization and Spectral Properties of Vector Meson in QCD Medium

“…While substantial spin alignment has been observed for vector mesons mentioned above, the theoretical descriptions for φ, K are much smaller than the data and are difficult to accommodates the sign changes of δρ 00 at different transverse momenta [11,[38][39][40], since the predicted spin alignment is proportional to the square of the vorticity. Efforts have been made from the other perspectives [41][42][43][44] to reveal the mechanism of the spin alignment of the vector mesons as well.…”

“…[9,[22][23][24][25][26][27][28][29][30][31][32][33] and see Refs. [34][35][36][37] for reviews), deepening our understandings on the relativistic spin physics and making spin related observables as new tools to study the properties of the hot and dense matter created in heavy-ion collisions.…”

“…Efforts have been made from the other perspectives [41][42][43][44] to reveal the mechanism of the spin alignment of the vector mesons as well. But the origin of the spin alignment is still considered as an open ques-tion [40]. Besides the theoretical descriptions mentioned above, a systematic calculation under the framework of thermal field theory, which has achieved great success in solving the "spin sign puzzles"of Λ-hyperon [20,45], seems to be missing in studying the spin alignment at this moment.…”

Generating Tensor Polarization from Shear Stress

“…Introduction.-The observation of polarization phenomena in relativistic heavy-ion collisions has opened a new direction of research in the physics of the hot and dense nuclear matter [1,2]. The STAR Collaboration showed that Λ-baryons emitted in noncentral nuclear collisions are spin polarized along the direction of the global angular momentum [3,4].…”