Spin Femtoscopy: A Framework for Revealing Genuine Spin Correlations

Spin correlations are among the most fundamental quantum observables in many-body systems, yet they remain difficult to access experimentally in relativistic heavy-ion collisions.

Existing spin measurements, including hyperon polarization and vector-meson spin alignment, have revealed important single-particle spin phenomena, but genuine two-particle spin correlations in the produced hadronic system remain largely unexplored.

Here we propose spin femtoscopy, a framework for accessing genuine two-particle spin correlations through spin-resolved femtoscopic measurements.

The key principle is that different two-particle spin configurations can give rise to different femtoscopic correlation functions because of quantum statistics, spin-dependent final-state interactions.

Using $\Lambda\Lambda$ pairs as a proof of principle, we exploit the self-analyzing weak decay of $\Lambda$ hyperons to construct spin-sensitive femtoscopic correlation functions with different singlet and triplet admixtures.

We show that these observables provide experimental access to the spin-state populations of the pair and allow genuine spin correlations to be separated from spin-dependent femtoscopic mixing caused by quantum statistics and final-state interactions.

This work extends femtoscopy from a probe of source geometry and final-state interactions to a framework for revealing the quantum spin structure of strongly interacting matter.