Are the Circular Polarizations of Observed Gravitational-Waves Even-Handed?

Abstract

We study whether gravitational waves (GWs) from binary black hole (BBH) mergers show a difference between right- and left-handed circular polarizations. Such a difference could point to a violation of parity symmetry in gravity. We analyze publicly available data from the collaboration of the LIGO-Virgo-KAGRA (LVK) detectors, focusing on events with accurate sky localization using all three detectors. This allows us to separate the circular polarization modes and measure their amplitudes. Out of 15 well-localized events, 7 show a dominant right-handed mode and 8 a left-handed one. This small difference is consistent with statistical fluctuations. A Bayesian analysis shows that the probability $ p_R $ of a right-handed signal follows a $Beta(8,9)$ distribution, with no strong evidence for asymmetry. We also estimate how many future events would be needed to detect a small asymmetry like $p_R = 7 / 15$ with high confidence. To reach 2$\sigma$ or 3$\sigma$ significance, about 900 to 2000 well-localized events would be required. These numbers could be reached in upcoming observation runs. Our results support parity symmetry for now, but future data may allow us to test it more precisely.