LiteBIRD Science Goals and Forecasts: Improving Sensitivity to Inflationary Gravitational Waves with Multitracer Delensing

TL;DR

This paper evaluates how combining multiple external data sets for delensing can improve LiteBIRD's sensitivity to primordial gravitational waves, emphasizing the importance of multi-tracer approaches and foreground mitigation.

Contribution

It extends previous forecasts by analyzing multi-tracer delensing techniques, demonstrating a 20% improvement in constraining the tensor-to-scalar ratio r for LiteBIRD.

Findings

Multi-tracer delensing improves r constraints by about 20%.

Combining CMB lensing maps, CIB, and galaxy surveys enhances delensing efficiency.

Foreground residuals significantly impact B-mode measurements, emphasizing the need for better component separation.

Abstract

We estimate the efficiency of mitigating the lensing $B$-mode polarization, the so-called delensing, for the $LiteBIRD$ experiment with multiple external data sets of lensing-mass tracers. The current best bound on the tensor-to-scalar ratio, $r$, is limited by lensing rather than Galactic foregrounds. Delensing will be a critical step to improve sensitivity to $r$ as measurements of $r$ become more and more limited by lensing. In this paper, we extend the analysis of the recent $LiteBIRD$ forecast paper to include multiple mass tracers, i.e., the CMB lensing maps from $LiteBIRD$ and CMB-S4-like experiment, cosmic infrared background, and galaxy number density from $Euclid$- and LSST-like survey. We find that multi-tracer delensing will further improve the constraint on $r$ by about $20\%$. In $LiteBIRD$, the residual Galactic foregrounds also significantly contribute to uncertainties of the $B$-modes, and delensing becomes more important if the residual foregrounds are further reduced by an improved component separation method.