Evaluation of the beam-induced depolarization of the HJET target at the EIC

TL;DR

This study assesses the potential beam-induced depolarization of the HJET target at the EIC, finding it negligible under planned operational conditions, thus supporting its use for precise polarization measurements.

Contribution

The paper provides a quantitative quantum-mechanical analysis of hydrogen atom depolarization due to EIC beam parameters, confirming minimal depolarization risk for the HJET target.

Findings

Depolarization at EIC is negligibly small, less than 0.01%.

Results are stable across plausible variations in beam parameters.

Supports the use of HJET for accurate polarization calibration at EIC.

Abstract

The Polarized Atomic Hydrogen Gas Jet Target (HJET) has played a central role in the absolute calibration of proton beam polarization at RHIC and is foreseen as a key element of the hadron polarimetry program at the future Electron--Ion Collider (EIC). The substantially higher beam current, reduced bunch spacing, and shorter bunch length planned for EIC operation motivate a careful reassessment of possible beam-induced depolarization of the jet target. In this paper, the depolarization of ground-state hydrogen atoms caused by the time-dependent magnetic field of the circulating polarized proton beam is quantitatively evaluated. The hydrogen atom is treated as a four-level hyperfine system in a holding magnetic field, and transitions driven by harmonic components of the bunch-induced magnetic field are analyzed using time-dependent quantum-mechanical evolution along atomic trajectories. Numerical tracking of hydrogen atoms through the beam region is performed using nominal EIC beam parameters. It is shown that, for a holding field of $120\,\mathrm{mT}$ (as used at RHIC), the resulting depolarization of the jet target at the EIC is negligibly small, $\lesssim 0.01\%$, and well below the level relevant for EIC polarization accuracy requirements. The stability of this result with respect to plausible variations of the EIC proton beam parameters is also evaluated.