A new limit on CMB circular polarization from SPIDER

We present a new upper limit on CMB circular polarization from the 2015 flight of SPIDER, a balloon-borne telescope designed to search for $B$-mode linear polarization from cosmic inflation. Although the level of circular polarization in the CMB is predicted to be very small, experimental limits provide a valuable test of the underlying models. By exploiting the non-zero circular-to-linear polarization coupling of the HWP polarization modulators, data from SPIDER's 2015 Antarctic flight provide a constraint on Stokes $V$ at 95 and 150 GHz from $33
Comment: 9 pages, 4 figures, 5 tables - Updated to version accepted for publication in ApJ, modified acknowledgements