Researchers have addressed the inherent ambiguity in decomposing total angular momentum into its orbital and spin components. This freedom, known as pseudo-gauge ambiguity, has affected the definition of local-equilibrium density operators and, consequently, spin polarization estimates in heavy-ion collisions. The new approach reformulates this ambiguity, along with others associated with improvements of conserved currents, in terms of spurious symmetries corresponding to conserved currents with vanishing total charge.

The central problem lies in the existence of multiple valid ways to separate the total angular momentum, which introduces an indeterminacy in how the state of a system in local equilibrium is described. This indeterminacy is particularly relevant in the study of phenomena such as heavy-ion collisions, where the spin polarization of produced particles is a key observable for understanding the properties of the quark-gluon plasma.

To resolve this issue, a prescription has been introduced for the unambiguous definition of a local-equilibrium density operator. This prescription is based on the use of currents associated with genuine symmetries of the system. The resulting density operator is invariant under transformations that add improvement terms to local currents, including the energy-momentum tensor. This advance promises greater precision in characterizing local equilibrium states and their spin properties.