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 data set collected through 2014 using the same parametric foreground modelling as in the previous analysis. From simulations, we find that the uncertainty on \(r\) is reduced by \(\sim10\%\), from \(\sigma(r)\)= 0.024 to 0.022, which can be compared with a \(\sim26\%\) reduction obtained when using a perfect lensing template. 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.