Axion Portal Dark Matter and the LUX-ZEPLIN High-Recoil Event

Abstract

LUX-ZEPLIN (LZ) has reported one event compatible with a $248\mathrm{keV}$ nuclear recoil. While initial attention has focused on inelastic dark matter, that interpretation depends sensitively on the uncertain high-speed tail of the Galactic halo. We develop instead an elastic realisation of the pseudoscalar-pseudoscalar interaction $L_4=(\bar{\chi }i{\gamma }^5\chi )(\bar{N}i{\gamma }^{5}N)$, whose contact templates give one of the largest local significances identified by LZ. The minimal model presented involves Dirac fermion dark matter and a pseudoscalar mediator. The pseudoscalar arises from a complex singlet and the minimal gauge-invariant UV completion also requires a single multi-TeV vector-like heavy quark. A representative model which realises thermal freeze-out has $m_{\chi }\sim 400\mathrm{GeV}$ and $m_a\sim 1\mathrm{GeV}$. The simplest model breaks from the isoscalar/isovector assumption, and the recoil spectrum lies between LZ's elastic $O_4^v$ and contact $L_4^v$ templates, suggesting a local significance of $2.6$-$3.2\sigma$.

arXiv comment5 pages, 2 figure