A demonstration of improved constraints on primordial gravitational waves with delensing

We present a constraint on the tensor-to-scalar ratio, $r$, derived from measurements of cosmic microwave background (CMB) polarization $B$-modes with ``delensing,'' whereby the uncertainty on $r$ contributed by the sample variance of the gravitational lensing $B$-modes is reduced by cross-correlating against a lensing $B$-mode template. This template is constructed by combining an estimate of the polarized CMB with a tracer of the projected large-scale structure. The large-scale-structure tracer used is a map of the cosmic infrared background derived from Planck satellite data, while the polarized CMB map comes from a combination of South Pole Telescope, bicep/Keck, and Planck data. We expand the bicep/Keck likelihood analysis framework to accept a lensing template and apply it to the bicep/Keck dataset collected through 2014 using the same parametric foreground modeling as in the previous analysis. From simulations, we find that the uncertainty on $r$ is reduced by $\ensuremath{\sim}10%$, from $\ensuremath{\sigma}(r)=0.024$ to 0.022, which can be compared with a $\ensuremath{\sim}26%$ reduction obtained when using a perfect lensing template or if there were zero lensing $B$-modes. Applying the technique to the real data, the constraint on $r$ is improved from ${r}_{0.05}<0.090$ to ${r}_{0.05}<0.082$ (95% C.L.). This is the first demonstration of improvement in an $r$ constraint through delensing.