Recently, the STAR Collaboration reported the first measurement of transverse-spin effects in one-point and two-point energy correlators inside jets, using transversely polarized proton-proton collision data at a center-of-mass energy of 200 GeV from the Relativistic Heavy Ion Collider (RHIC). The study was published on September 8, 2026, in Physical Review Letters (Phys. Rev. Lett. 137, 111902 (2026), DOI: 10.1103/fgsf-gx2q). Dr. Lin Ting, from the Institute of Frontier and Interdisciplinary Science at Shandong University, is a principal author of the paper. He led the study and independently completed the full data analysis and manuscript preparation.

Understanding how quarks and gluons contribute to the proton's spin is a major goal of quantum chromodynamics (QCD). In high-energy collisions, quarks and gluons produce jets—narrow sprays of particles that carry information about their underlying dynamics. Energy correlators measure how energy is distributed among particles within a jet at different angles, providing a new way to study spin effects while reducing uncertainties associated with how quarks and gluons transform into observable particles.

Figure: STAR measured how the sideways spin of a proton influences particle production at different angular separations inside a jet. The six panels show increasing jet momentum, from upper left to lower right. Red circles represent correlations between positive-negative pion pairs, while green and blue points represent positive and negative pions. The lines and shaded bands show theoretical calculations for the single-pion measurements.

STAR observed significant transverse single-spin asymmetries for positive and negative pions and for oppositely charged pion pairs. The spin effects appear at specific angular scales inside the jet and evolve systematically with jet transverse momentum. These results provide new insight into how quark spin information is transferred through parton showering and hadronization, and demonstrate energy correlators as a new probe of the proton's spin structure. The technique also offers a promising approach for future three-dimensional studies of the proton at the Electron-Ion Collider.