Search for Axions in Magnetic White Dwarf Polarization at Lick and Keck Observatories

We present the most sensitive search to date for light axion-like particles with masses below a micro-eV, using spectropolarimetric data collected from the Lick and Keck Observatories. The conversion of optical photons emitted from the surface of a magnetic white dwarf (MWD) into axions in the strong magnetic field around the star induces a nearly wavelength-independent linear polarization in the observed starlight. We analyze the Stokes parameters $(U,Q,I)$ measured with the Kast spectrograph at the Lick Observatory toward the MWDs SDSS J033320+000720 and ZTF J190132+145807, and with the LRISp-ADC instrument at the Keck Observatory toward ZTF J190132+145807, SDSS J002129+150223, and SDSS J100356+053825 to search for this effect. The data show no evidence of axion-induced linear polarization, and we set world-leading constraints on the axion-photon coupling $\mathrm{\mid }{g}_{a\gamma \gamma }\mathrm{\mid }\lesssim 1.7\times {10}^{-12}{\mathrm{G}\mathrm{e}\mathrm{V}}^{-1}$ at the $95\mathrm{\%}$ confidence level for masses $m_a\lesssim 2\times {10}^{-7}\mathrm{e}\mathrm{V}$.