Upper limits on the Polarized Isotropic Stochastic Gravitational-Wave Background from Advanced LIGO-Virgo's First Three Observing Runs

TL;DR

This paper uses LIGO-Virgo data to set upper limits on the circular polarization of the stochastic gravitational-wave background, finding no evidence for polarization and constraining its energy density.

Contribution

It introduces a maximum-likelihood method to constrain the polarization degree of the SGWB using LIGO-Virgo data, providing the first such upper limits.

Findings

No evidence for circular polarization in SGWB

Upper limit on energy density: $ ext{Omega}_ ext{gw}(25 ext{Hz})<5.3 imes10^{-9}$

Constraints marginalized over polarization and spectral index

Abstract

Parity violation is expected to generate an asymmetry between the amplitude of left and right-handed gravitational-wave modes which leads to a circularly polarized stochastic gravitational-wave background (SGWB). Due to the three independent baselines in the LIGO-Virgo network, we focus on the amplitude difference in strain power characterized by Stokes' parameters and do maximum-likelihood estimation to constrain the polarization degree of SGWB. Our results indicate that there is no evidence for the circularly polarized SGWB in the data. Furthermore, by modeling the SGWB as a power-law spectrum, we place upper limit on the normalized energy density $\Omega_\text{gw}(25\,\text{Hz})<5.3\times10^{-9}$ at $95\%$ confidence level after marginalizing over the polarization degree and spectral index.