BICEP / Keck XIV: Improved constraints on axion-like polarization
  oscillations in the cosmic microwave background
  We present an improved search for axion-like polarization oscillations in the
cosmic microwave background (CMB) with observations from the Keck Array. An
all-sky, temporally sinusoidal rotation of CMB polarization, equivalent to a
time-variable cosmic birefringence, is an observable manifestation of a local
axion field and potentially allows a CMB polarimeter to detect axion-like dark
matter directly. We describe improvements to the method presented in previous
work, and we demonstrate the updated method with an expanded dataset consisting
of the 2012-2015 observing seasons. We set limits on the axion-photon coupling
constant for mass $m$ in the range $10^{-23}$-$10^{-18}~\mathrm{eV}$, which
corresponds to oscillation periods on the order of hours to years. Our results
are consistent with the background model. For periods between $1$ and
$30~\mathrm{d}$ ($1.6 \times 10^{-21} \leq m \leq 4.8 \times
10^{-20}~\mathrm{eV}$), the $95\%$-confidence upper limits on rotation
amplitude are approximately constant with a median of $0.27^\circ$, which
constrains the axion-photon coupling constant to $g_{\phi\gamma} < (4.5 \times
10^{-12}~\mathrm{GeV}^{-1}) m/(10^{-21}~\mathrm{eV}$), if axion-like particles
constitute all of the dark matter. More than half of the collected BICEP
dataset has yet to be analyzed, and several current and future CMB polarimetry
experiments can apply the methods presented here to achieve comparable or
superior constraints. In the coming years, oscillation measurements can achieve
the sensitivity to rule out unexplored regions of the axion parameter space.
