Results from the Atacama B-mode Search (ABS) experiment

The Atacama B-mode Search is an experiment designed to measure the cosmic microwave background polarization at large angular scales (l > 40). It observes at 145 GHz from a site at 5,190 m elevation in northern Chile. The noise equivalent polarization temperature, or NEQ, is 41 mu K root s. One of the unique features of ABS is its use of a rapidly rotating ambient-temperature half-wave plate (HWP) as the first optical element. The HWP spins at 2.55 Hz to modulate the incident polarized signal at frequencies above where instrument white noise dominates over atmospheric fluctuations and other sources of low-frequency noise. We report here on the analysis of data from a 2,400 deg(2) region of sky. We perform a blind analysis to reduce potential bias. After unblinding, we find agreement with the Planck TE and EE measurements on the same region of sky, with a derived calibration factor of 0.89 +/- 0.10. We marginally detect polarized dust emission (at 3.2 sigma for EE and 2.2 sigma for BB) and give an upper limit on the tensor-to-scalar ratio of r < 2.3 (95% confidence level) with the equivalent of 100 on-sky days of observation. We also present a new measurement of the polarization of Tau A and introduce new methods for calibration and data analysis associated with HWP-based observations.