Improving the detection sensitivity to primordial stochastic gravitational waves with reduced astrophysical foregrounds

One of the primary targets of third-generation (3G) ground-based gravitational wave (GW) detectors is detecting the stochastic GW background (SGWB) from early universe processes. The astrophysical foreground from compact binary mergers will be a major contamination to the background, which must be reduced to high precision to enable the detection of primordial background. In this work, we revisit the limit of foreground reduction computed in previous studies, point out potential problems in previous foreground cleaning methods and propose a novel cleaning method subtracting the approximate signal strain and removing the average residual power. With this method, the binary black hole foreground is reduced with fractional residual energy density below $10^{-4}$ for frequency $f\in (10, 10^2)$ Hz, below $10^{-3}$ for frequency $f\in (10^2, 10^3)$ Hz and below the detector sensitivity limit for all relevant frequencies in our simulations. Similar precision is achieved to clean the foreground from binary neutron stars (BNSs) that are above the detection threshold, so that the residual foreground is dominated by sub-threshold BNSs, which will be the next critical problem to solve for detecting the primordial SGWB in the 3G era.