Title: BICEP2 / Keck Array IX: New Bounds on Anisotropies of CMB Polarization Rotation and Implications for Axion-Like Particles and Primordial Magnetic Fields,
Abstract: We present the strongest constraints to date on anisotropies of CMB
polarization rotation derived from $150$ GHz data taken by the BICEP2 & Keck
Array CMB experiments up to and including the 2014 observing season (BK14). The
definition of polarization angle in BK14 maps has gone through self-calibration
in which the overall angle is adjusted to minimize the observed $TB$ and $EB$
power spectra. After this procedure, the $QU$ maps lose sensitivity to a
uniform polarization rotation but are still sensitive to anisotropies of
polarization rotation. This analysis places constraints on the anisotropies of
polarization rotation, which could be generated by CMB photons interacting with
axion-like pseudoscalar fields or Faraday rotation induced by primordial
magnetic fields. The sensitivity of BK14 maps ($\sim 3\mu$K-arcmin) makes it
possible to reconstruct anisotropies of polarization rotation angle and measure
their angular power spectrum much more precisely than previous attempts. Our
data are found to be consistent with no polarization rotation anisotropies,
improving the upper bound on the amplitude of the rotation angle spectrum by
roughly an order of magnitude compared to the previous best constraints. Our
results lead to an order of magnitude better constraint on the coupling
constant of the Chern-Simons electromagnetic term $f_a \geq 1.7\times
10^2\times (H_I/2\pi)$ ($2\sigma$) than the constraint derived from uniform
rotation, where $H_I$ is the inflationary Hubble scale. The upper bound on the
amplitude of the primordial magnetic fields is 30nG ($2\sigma$) from the
polarization rotation anisotropies.