Cosmic Birefringence from Planck Public Release 4
  We search for the signature of parity-violating physics in the Cosmic
Microwave Background using Planck polarization data from the Public Release 4
(PR4 or $\mathtt{NPIPE}$). For nearly full-sky data, we initially find a
birefringence angle $\beta=0.30^\circ\pm0.11^\circ$ ($68\%$~C.L.). We also find
that the values of $\beta$ decrease as we enlarge the Galactic mask, which can
be interpreted as the effect of polarized foreground emission. We use two
independent approaches to model this effect and mitigate its impact on $\beta$.
Although results are promising, and the good agreement between both models is
encouraging, we do not assign cosmological significance to the measured value
of $\beta$ until we improve our knowledge of the foreground polarization.
Acknowledging that the miscalibration of polarization angles is not the only
instrumental systematic that can create spurious TB and EB correlations, we
also perform a detailed study of $\mathtt{NPIPE}$ end-to-end simulations to
prove that our measurements of $\beta$ are not significantly affected by any of
the known systematics.
