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Eliminating beam-induced depolarizing effects in the hydrogen jet target for high-precision proton beam polarimetry at the electron-ion collider

F. Rathmann1,*, A. Nass2, K. O. Eyser1, V. Shmakova1, E. C. Aschenauer1, G. Atoian1, A. Cannavo1, X. Chu1, K. Hock1 et al.

H. Huang1, H. Lovelace1, G. Mahler1, N. N. Nikolaev3,4,5, J. Ritter1, G. Robert-Demolaize1, V. Schoefer1, P. Shanmuganathan1, E. Shulga1, H. Soltner6, and Z. Zhang1

  • *Contact author: frathmann@bnl.gov
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Phys. Rev. Accel. Beams 29, 021001 – Published 10 February, 2026

DOI: https://doi.org/10.1103/8nh5-l63q

Abstract

We analyze beam-induced depolarizing effects in the hydrogen jet target (HJET) at the relativistic heavy ion collider (RHIC), which 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 (23.5 GeV) and flattop (275 GeV), beam-induced depolarization through the bunch structure renders operation at the current RHIC magnetic guide field at the target (B0=120  mT) untenable. Increasing the magnetic guide field at the target to B0≈400  mT moves all hyperfine transition frequencies to at least 3 times the cutoff frequency, ensuring reliable absolute beam polarimetry with the required 1% precision at the EIC.

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Phys. Rev. Accel. Beams 29, 021001– Published 10 February, 2026
  • Received 10 September 2025
  • Accepted 7 January 2026
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