We present results from an analysis of all data taken by the BICEP2, Keck Array, and BICEP3 CMB polarization experiments up to and including the 2018 observing season. We add additional Keck Array observations at 220 GHz and BICEP3 observations at 95 GHz to the previous $95/150/220\mathrm{GHz}$ dataset. The $Q/U$ maps now reach depths of 2.8, 2.8, and $8.8\mu {\mathrm{K}}_{\mathrm{CMB}}\mathrm{arcmin}$ at 95, 150, and 220 GHz, respectively, over an effective area of $\approx 600$ square degrees at 95 GHz and $\approx 400$ square degrees at 150 and 220 GHz. The 220 GHz maps now achieve a signal-to-noise ratio on polarized dust emission exceeding that of Planck at 353 GHz. We take auto- and cross-spectra between these maps and publicly available WMAP and Planck maps at frequencies from 23 to 353 GHz and evaluate the joint likelihood of the spectra versus a multicomponent model of lensed $\mathrm{\Lambda }\mathrm{CDM}+r+\mathrm{dust}+\mathrm{synchrotron}+\mathrm{noise}$. The foreground model has seven parameters, and no longer requires a prior on the frequency spectral index of the dust emission taken from measurements on other regions of the sky. This model is an adequate description of the data at the current noise levels. The likelihood analysis yields the constraint $r_{0.05}<0.036$ at 95% confidence. Running maximum likelihood search on simulations we obtain unbiased results and find that $\sigma (r)=0.009$. These are the strongest constraints to date on primordial gravitational waves.

• Received 21 June 2021
• Accepted 12 August 2021

DOI:https://doi.org/10.1103/PhysRevLett.127.151301