Improved measurement of the longitudinal spin transfer to $\Lambda$ and $\bar \Lambda$ hyperons in polarized proton-proton collisions at $\sqrt s$ = 200 GeV

TL;DR

This paper reports an improved measurement of the longitudinal spin transfer to Lambda and anti-Lambda hyperons in polarized proton-proton collisions at 200 GeV, providing more precise data to understand proton spin structure.

Contribution

The study presents a significantly more precise measurement of $D_{LL}$, covering a broad kinematic range, and compares results with theoretical models to enhance understanding of spin transfer mechanisms.

Findings

Measured $D_{LL}$ for Lambda hyperons: -0.036 ± 0.048 (stat) ± 0.013 (sys)

Measured $D_{LL}$ for anti-Lambda hyperons: 0.032 ± 0.043 (stat) ± 0.013 (sys)

Results are consistent with previous data but with improved precision.

Abstract

The longitudinal spin transfer $D_{LL}$ to $\Lambda$ and $\bar{\Lambda}$ hyperons produced in high-energy polarized proton--proton collisions is expected to be sensitive to the helicity distribution functions of strange quarks and anti-quarks of the proton, and to longitudinally polarized fragmentation functions. We report an improved measurement of $D_{LL}$ from data obtained at a center-of-mass energy of $\sqrt{s}$ = 200 GeV with the STAR detector at RHIC. The data have an approximately twelve times larger figure-of-merit than prior results and cover $|\eta|<$ 1.2 in pseudo-rapidity with transverse momenta $p_T$ up to 6 GeV/c. In the forward scattering hemisphere at largest $p_T$, the longitudinal spin transfer is found to be $D_{LL}$ = -0.036 $\pm$ 0.048 (stat) $\pm$ 0.013(sys) for $\Lambda$ hyperons and $D_{LL}$ = 0.032 $\pm$ 0.043\,(stat) $\pm$ 0.013\,(sys) for $\bar{\Lambda}$ anti-hyperons. The dependences on $\eta$ and $p_T$ are presented and compared with model evaluations.