Constraints on parity violation from ACTpol and forecasts for
  forthcoming CMB experiments
  We use the ACTpol published cosmic microwave background (CMB) polarization
data to constrain cosmological birefringence, a tracer of parity violation
beyond the standard model of particle physics. To this purpose, we employ all
the polarized ACTpol spectra, including the cross-correlations between
temperature anisotropy and B mode polarization (TB) and between E mode and B
mode (EB), which are most sensitive to the effect. We build specific, so-called
D-estimators for birefringence and assess their performances and error budgets
by using realistic Monte Carlo simulations based on the experimental
characteristics provided by the ACTpol collaboration. We determine the optimal
multipole range for our analysis to be $250 < \ell < 3025$ over which we find a
null result for the uniform birefringence angle $\alpha = 0.29^\circ \pm
0.28^\circ$ (stat.) $\pm 0.5^\circ$ (syst.), the latter uncertainty being the
estimate published by the ACTpol team on their global systematic error budget.
We show that this result holds consistently when other multipole ranges are
considered. Finally, we forecast the capability of several forthcoming ground
based, balloon and space borne CMB experiments to constrain the birefringence
angle, showing, e.g., that the proposed post-Planck COrE satellite mission
could in principle constrain $\alpha$ at a level of 10 arcsec, provided that
all systematics are under control. Under the same circumstances, we find the
COrE constraints to be at least 2 or 3 times better than what could ideally be
achieved by the other experiments considered.
