Polarization phenomenon in heavy-ion collisions

• Published in: International journal of modern physics. E, Nuclear physics Vol. 33; no. 9
• Main Authors: Niida, Takafumi, Voloshin, Sergei A.
• Format: Journal Article
• Language: English
• Published: Singapore World Scientific Publishing Company 01.09.2024; World Scientific Publishing Co. Pte., Ltd; World Scientific
• Subjects: Atomic collisions; Heavy ions; Ideal fluids; Ionic collisions; Orbital mechanics; Particle spin; Physics; Polarization (spin alignment); Spin dynamics; Spin-orbit interactions; Velocity distribution; Vorticity; Polarization; vorticity; nuclear collisions
• ISSN: 0218-3013; 1793-6608
• DOI: 10.1142/S0218301324300108

The strongly interacting system created in ultrarelativistic nuclear collisions behaves almost as an ideal fluid with rich patterns of the velocity field exhibiting strong vortical structure. Vorticity of the fluid, via spin-orbit coupling, leads to particle spin polarization. Due to the finite orbital momentum of the system, the polarization on average is not zero. It depends on the particle momenta reflecting the spatial variation of the local vorticity. In the last few years, this field experienced a rapid growth due to experimental discoveries of the global and local polarizations. Recent measurements triggered further development of the theoretical description of the spin dynamics and suggestions of several new mechanisms for particle polarization. In this review, we focus mostly on the experimental results. We compare the measurements with the existing theoretical calculations but try to keep the discussion of possible underlying physics at the qualitative level. Future measurements and how they can help to answer open theoretical questions are also discussed. We pay a special attention to the employed experimental methods, as well as to the detector effects and associated corrections to the measurements.