From South to North: Leveraging Ground-Based LATs for Full-Sky CMB Delensing and Constraints on $r$

TL;DR

Introducing a northern hemisphere ground-based telescope significantly enhances full-sky delensing of the CMB, reducing uncertainties in primordial gravitational wave measurements and surpassing the benefits of external large-scale structure tracers.

Contribution

This work demonstrates that adding a northern hemisphere ground-based telescope enables full-sky internal delensing, substantially improving constraints on primordial gravitational waves.

Findings

Northern LAT enables full-sky internal delensing.

Full-sky delensing reduces uncertainty in r by ~40%.

Adding LATN diminishes the need for external LSS tracers.

Abstract

Delensing--the process of mitigating the lensing-induced B-mode contamination in cosmic microwave background (CMB) observations--will be a pivotal challenge for next-generation CMB experiments seeking to detect primordial gravitational waves (PGWs) through B-mode polarization. This process requires an accurate lensing tracer, which can be obtained either through internal reconstruction from high-resolution CMB observations or from external large-scale structure (LSS) surveys. Ground-based large-aperture telescopes (LATs) are crucial for internal reconstruction, yet existing and planned facilities are confined to the southern hemisphere, limiting effective delensing to that region. In this work, we assess the impact of introducing a northern hemisphere LAT, assumed to be situated near AliCPT (hence termed Ali-like LAT, or LATN), on delensing performance and PGW detection, using simulations. Our baseline setup includes a space-based small-aperture mission (LiteBIRD-like, SAT) and a southern LAT (SO-like, LATS). External LSS tracers, which have been shown to play an important role in delensing before the availability of ultra-sensitive polarization data, are also considered. We find that southern-hemisphere internal delensing reduces the uncertainty in r by approximately 21% compared to the no-delensing case. Adding LATN enables full-sky internal delensing, achieving a further ~19% reduction--comparable to that from including LSS tracers (~17%). Once LATN is included, the marginal benefit of LSS tracers drops to ~10%. These results highlight the significant role of LATN in advancing delensing capabilities and improving PGW constraints.