Cosmic Birefringence from Planck Data Release 4
  We search for the signature of parity-violating physics in the cosmic
microwave background, called cosmic birefringence, using the Planck data
release 4. We initially find a birefringence angle of $\beta=0.30\pm0.11$ (68%
C.L.) for nearly full-sky data. The values of $\beta$ decrease as we enlarge
the Galactic mask, which can be interpreted as the effect of polarized
foreground emission. Two independent ways to model this effect are used to
mitigate the systematic impact on $\beta$ for different sky fractions. We
choose not to assign cosmological significance to the measured value of $\beta$
until we improve our knowledge of the foreground polarization.
