Spin in quantum chromodynamics

This article surveys the role of spin in quantum chromodynamics (QCD), tracing its manifestations from the internal structure of the nucleon to the collective dynamics of strongly interacting matter. We review the decomposition of the proton spin into the spin and orbital angular momentum of quarks and gluons and discuss the theoretical and experimental progress that has shaped our modern understanding of polarized parton structure. We then examine spin transport phenomena in hot and dense QCD matter, highlighting theoretical developments of phase structures under rotation and spin hydrodynamics theory as well as the experimental discovery of global polarization and other related phenomena. Last, we present anomalous transport and the chiral magnetic effect as paradigmatic examples of how quantum anomalies convert chirality and topology into macroscopic, nondissipative currents whose signal can be measured in experiments. Together, these topics illustrate how spin provides a unifying framework linking hadronic structure, gauge field topology, quark field chirality, and relativistic many-body dynamics in QCD.