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We present the strongest constraints to date on anisotropies of cosmicmicrowave background (CMB) polarization rotation derived from 150 GHz datataken by the BICEP2/Keck Array CMB experiments up to and including the 2014observing season (BK14). The definition of the polarization angle in BK14 mapshas gone through self-calibration in which the overall angle is adjusted tominimize the observed TB and EB power spectra. After this procedure, the QUmaps lose sensitivity to a uniform polarization rotation but are stillsensitive to anisotropies of polarization rotation. This analysis placesconstraints on the anisotropies of polarization rotation, which could begenerated by CMB photons interacting with axionlike pseudoscalar fields orFaraday rotation induced by primordial magnetic fields. The sensitivity of BK14maps ($\sim 3\mu$K-arcmin) makes it possible to reconstruct anisotropies of thepolarization rotation angle and measure their angular power spectrum much moreprecisely than previous attempts. Our data are found to be consistent with nopolarization rotation anisotropies, improving the upper bound on the amplitudeof the rotation angle spectrum by roughly an order of magnitude compared to theprevious best constraints. Our results lead to an order of magnitude betterconstraint on the coupling constant of the Chern-Simons electromagnetic term$g_{a\gamma}\leq 7.2\times 10^{-2}/H_I$ (95% confidence) than the constraintderived from the B-mode spectrum, where $H_I$ is the inflationary Hubble scale.This constraint leads to a limit on the decay constant of $10^{-6}\lesssimf_a/M_{\rm pl}$ at mass range of $10^{-33}< m_a< 10^{-28}$ eV for $r=0.01$,assuming $g_{a\gamma}\sim\alpha/(2\pi f_a)$ with $\alpha$ denoting the finestructure constant. The upper bound on the amplitude of the primordial magneticfields is 30nG (95% confidence) from the polarization rotation anisotropies.

physical review. d/physical review. d.

BICEP2 / Keck Array IX: New Bounds on Anisotropies of CMB Polarization  Rotation and Implications for Axion-Like Particles and Primordial Magnetic  Fields