Unveiling V Modes: Enhancing CMB Sensitivity to BSM Physics with a Non-Ideal Half-Wave Plate

TL;DR

This paper explores how non-ideal half-wave plates in upcoming CMB experiments can detect V-mode polarization, offering a new way to probe beyond-standard-model physics with significantly improved sensitivity.

Contribution

It demonstrates that realistic half-wave plates can enhance sensitivity to V modes in CMB experiments, enabling better constraints on certain BSM physics models.

Findings

New-generation experiments can improve current limits by 1-3 orders of magnitude.

Including V-mode data significantly increases sensitivity to BSM models.

Half-wave plates can detect V modes without compromising linear polarization measurements.

Abstract

V-mode polarization of the cosmic microwave background is expected to be vanishingly small in the $\Lambda$CDM model and, hence, usually ignored. Nonetheless, several astrophysical effects, as well as beyond standard model physics could produce it at a detectable level. A realistic half-wave plate - an optical element commonly used in CMB experiments to modulate the polarized signal - can provide sensitivity to V modes without significantly spoiling that to linear polarization. We assess this sensitivity for some new-generation CMB experiments, such as the LiteBIRD satellite, the ground-based Simons Observatory and a CMB-S4-like experiment. We forecast the efficiency of these experiments to constrain the phenomenology of certain classes of BSM models inducing mixing of linear polarization states and generation of V modes in the CMB. We find that new-generation experiments can improve current limits by 1-to-3 orders of magnitude, depending on the data combination. The inclusion of V-mode information dramatically boosts the sensitivity to these BSM models.