Eliminating beam-induced depolarizing effects in the hydrogen jet target for high-precision proton beam polarimetry at the Electron-Ion Collider

TL;DR

This paper analyzes and mitigates beam-induced depolarizing effects in the hydrogen jet target to enable high-precision proton beam polarimetry at the Electron-Ion Collider by adjusting the magnetic guide field.

Contribution

It introduces a frequency-domain analysis method to identify a photon emission threshold and proposes increasing the magnetic guide field to prevent depolarization at EIC energies.

Findings

Beam-induced depolarization is problematic at current RHIC magnetic fields for EIC energies.

Increasing the magnetic guide field to approximately 400 mT effectively suppresses hyperfine transition resonances.

Adjusting the magnetic guide field ensures reliable 1% precision in beam polarimetry at the EIC.

Abstract

We analyze beam-induced depolarizing effects in the hydrogen jet target (HJET) at the Relativistic Heavy Ion Collider (RHIC) that has been used for absolute hadron beam polarimetry and shall be employed at the Electron-Ion Collider (EIC). The EIC's higher bunch repetition frequencies and shorter bunch durations shift beam harmonics to frequencies that can resonantly drive hyperfine transitions in hydrogen, threatening to depolarize the target atoms. Using frequency-domain analysis of beam harmonics and hyperfine transition frequencies, we establish a photon emission threshold above which beam-induced fields are too weak to cause significant depolarization. For EIC injection (\SI{23.5}{\GeV}) and flattop (\SI{275}{\GeV}), beam-induced depolarization through the bunch structure renders operation at the current RHIC magnetic guide field at the target of $B_0 = \SI{120}{\milli\tesla}$ untenable. Increasing the magnetic guide field at the target to $B_0 \approx \SI{400}{\milli\tesla}$ moves all hyperfine transition frequencies to at least three times the cutoff frequency, ensuring reliable absolute beam polarimetry with the required 1\% precision at the EIC.