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POLARBEAR is a ground-based experiment to measure polarization anisotropy in the Cosmic Microwave Background. It is designed to have a combination of sensitivity, foreground mitigation, and rejection of systematic errors to search for the B-mode signature of Inflationary gravity waves over much of the parameter range suggested by simple power-law Inflation models. POLARBEAR is designed to detect a gravitational-wave signature with a tensor-to-scalar ratio r as low as 0.025 (95% confidence). POLARBEAR will also measure polarized lensing of the Cosmic Microwave Background which will give valuable information on large-scale structure at z > 1 and bound the total mass of the neutrinos.

POLARBEAR will have a 3.5 meter primary meter giving it an angular resolution of 3.0' at its main observation frequency band centered at 150 GHz. The 250 mK focal plane design contains 637 dual-polarization pixels (1274 bolometers) that are coupled to the telescope using microlithographed planar antennas. The experiment will be sited in the Atacama Desert in Chile at 5000 meter (16,500 ft) altitude starting in 2009 after a prototype testing stage at Cedar Flats California. The first configuration of the experiment will observe at only one frequency band with the first season at 150 GHz and the second at 220 GHz. The optics will be upgraded to have simultaneous observations in those two bands in the third season of observations. POLARBEAR and QUIET Will observe the same sky patches, and together they will have frequency bands at 30, 40, 90, 150, and 220 GHz giving broad coverage of galactic foregrounds and a valuable cross-check by comparison of polarization maps. In POLARBEAR, polarization systematic errors are mitigated by a continuously rotating 50 K half-wave plate and an observation strategy that takes advantage of parallactic angle rotation to rotate the experiment relative to polarization patterns on the sky.