Current Status of Transverse Spin at STAR

TL;DR

This paper reviews recent measurements of transverse spin asymmetries at STAR, highlighting detector upgrades and their impact on understanding proton spin structure at high energies.

Contribution

It provides an update on transverse spin measurements at STAR, emphasizing new detector capabilities and extended kinematic coverage for probing proton spin phenomena.

Findings

Large transverse single spin asymmetries observed for neutral pions.

Enhanced detector coverage enables higher transverse momentum measurements.

Improved analysis of spin-dependent jet structures.

Abstract

The origins of the proton spin remain an area of active investigation. The Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Lab uniquely provides polarized proton data at high center of mass energies. STAR has previously reported large transverse single spin asymmetries for inclusive neutral pion production in the forward direction at square root of s of 200 GeV, measured with a modular Pb-glass calorimeter called the "Forward Pion Detector" (FPD). Beginning in 2008 (Run 8), STAR's forward calorimetry was significantly enhanced with the commissioning of the "Forward Meson Spectrometer" (FMS), which has full azimuthal coverage in the pseudo-rapidity range 2.5 to 4.2. This detector has extended asymmetry measurements to higher transverse momentum, and its large areal coverage enables measurements of higher mass resonances and "jet-like" correlations. STAR also provides excellent tracking and full azimuthal electromagnetic calorimeter coverage at mid-rapidity, which facilitates analyses of spin-dependent jet structure. Here we review recent transverse spin measurements with the STAR detector and also present an update on some existing analyses.