SPT-3G D1: Constraints on inflationary gravitational waves with two years of SPT-3G data

We present a measurement of the B-mode polarization power spectrum of the cosmic microwave background anisotropies at 32 ≤ l < 502 for three bands centered at 95, 150, and 220 GHz using data from the SPT-3G receiver on the South Pole Telescope. This work uses SPT-3G observations from the 2019 and 2020 winter observing seasons of a ∼1500 deg² patch of sky that directly overlaps with fields observed with the BICEP/Keck family of telescopes and covers part of the proposed Simons Observatory and CMB-S4 deep fields. Employing new techniques for mitigating polarized atmospheric noise, the SPT-3G data demonstrates a white noise level of 9.3 (6.7) μK-arcmin at l ∼ 500 for the 95 GHz (150 GHz) data, with a 1=l noise knee at l = 128 (182). We fit the observed six auto- and cross-frequency B-mode power spectra to a model including lensed ΛCDM B-modes and a combination of Galactic and extragalactic foregrounds. This work characterizes foregrounds in the vicinity of the BICEP/Keck survey area, finding foreground power consistent with that reported by the BICEP/Keck collaboration within the same region and a factor of ∼3 higher power over the full SPT-3G survey area. Using SPT-3G data over the BICEP/Keck survey area, we place a 95% upper limit on the tensor-to-scalar ratio of r < 0.25 and find the statistical uncertainty on r to be σ(r) = 0.067.