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

TL;DR

The paper addresses the risk of beam-induced depolarization of the HJET hydrogen jet target in the EIC environment, where higher beam current and shorter bunches could affect the jet polarization used for absolute beam polarization calibration. It treats the jet as a four-level ground-state hydrogen hyperfine system in a holding field and computes depolarization by time-dependent perturbation theory under the time-varying magnetic field from bunched proton beams, performing numerical tracking along atomic trajectories. The main result is that, for a holding field around 120–129 mT and nominal EIC beam parameters, the depolarization of the jet target is negligibly small, | abla_ ext{dep} P_ ext{jet}| lesssim 0.01 %|, and remains stable under plausible variations such as reduced σ_t, increased beam current, and elliptical beam profiles. This supports the feasibility of precise proton beam polarization calibration at the EIC with HJET, while highlighting the importance of detailed tracking simulations to correctly quantify depolarization effects and to compare with alternative modeling approaches.

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.

Figures (9)

• Figure 1: Breit--Rabi diagram for ground-state hydrogen ($1S_{1/2}$). The projection of the total spin $F$ onto the holding magnetic field $B_\text{hold}$ is given by $m_F=m_I+m_J$, where $m_I$ and $m_J$ are the proton and electron spin projections, respectively, in the high-field limit ($\sin\theta\to0$ in Eq. \ref{['eq:Hstates']}).
• Figure 2: Beam view of the Polarized Atomic Hydrogen Gas Jet target polarimeter at RHIC.
• Figure 3: Unity-normalized radial profile of the beam-induced magnetic field at the EIC flattop.
• Figure 4: Fourier spectrum of the proton beam longitudinal profile planned for the EIC. The red dashed line represents the Gaussian envelope defined in Eq. \ref{['eq:sf']}.
• Figure 5: Tracking of a hydrogen atom through the $z$-directed proton beam.