An improved upper limit to the CMB circular polarization at large
  angular scales
  Circular polarization of the Cosmic Microwave Background (CMB) offers the
possibility of detecting rotations of the universe and magnetic fields in the
primeval universe or in distant clusters of galaxies. We used the Milano
Polarimeter (MIPOL) installed at the Testa Grigia Observatory, on the italian
Alps, to improve the existing upper limits to the CMB circular polarization at
large angular scales. We obtain 95% confidence level upper limits to the degree
of the CMB circular polarization ranging between 5.0x10^{-4} and 0.7x10^{-4} at
angular scales between 8 and 24 deg, improving by one order of magnitude
preexisting upper limits at large angular scales. Our results are still far
from the nK region where today expectations place the amplitude of the V Stokes
parameter used to characterize circular polarization of the CMB but improve the
preexisting limit at similar angular scales. Our observations offered also the
opportunity of characterizing the atmospheric emission at 33 GHz at the Testa
Grigia Observatory.
