Measurement of longitudinal spin transfer of the $\Lambda(\bar\Lambda)$ hyperon in polarized p+p collisions at $\sqrt{s}=200$ GeV at RHIC-STAR

TL;DR

This paper reports on the measurement of the longitudinal spin transfer to $\Lambda$ and $ar{\Lambda}$ hyperons in polarized proton-proton collisions at RHIC, providing insights into the strange quark's contribution to proton spin.

Contribution

It presents new measurements of the longitudinal spin transfer $D_{LL}$ for $\Lambda(ar{\Lambda})$ hyperons at higher statistics and extended kinematic range, improving understanding of strange quark polarization.

Findings

$D_{LL}$ results are consistent with theoretical predictions.

Extended kinematic coverage up to 8 GeV/c transverse momentum.

Data set is approximately twice as large as previous measurements.

Abstract

Since the first surprising results on the spin structure of the proton by the EMC experiment in the late 1980s, much progress has been made in understanding the origin of the proton spin. However, the sea quark contribution to the proton spin, for example, the helicity distributions of the strange quark (anti-quark), $s(\bar{s})$, is still not well constrained by experimental data. Since the $s(\bar{s})$ is expected to carry a substantial fraction of the spin of the $\Lambda(\bar{\Lambda})$ hyperon, measurements of the longitudinal spin transfer, $D_{LL}$, of the $\Lambda(\bar{\Lambda}$) hyperon can thus shed light on the helicity distribution of the $s(\bar{s})$ and the longitudinally polarized fragmentation functions. In these proceedings, we will present the status of the $\Lambda(\bar{\Lambda}$) $D_{LL}$ analysis using data collected at RHIC-STAR in 2015, for the pseudo-rapidity $|\eta| < 1.2$ and transverse momenta up to $8.0$ GeV$/c$. The $D_{LL}$ as a function of the longitudinal momentum fraction of the $\Lambda(\bar{\Lambda})$ hyperon in the jet is also investigated. This data set is about twice as large as the 2009 data used for the previously published $D_{LL}$ results.