Abstract
Abstract
Precessing binary black holes generically produce an imbalance of right- and left-handed gravitational waves (GWs), reflecting the breaking of mirror symmetry by the merger dynamics. We study this phenomenon using the observer-independent quantity
V
GW
, a gravitational analog of the optical Stokes parameter that quantifies the intrinsic handedness of the emitted radiation. Using 91 LIGO–Virgo–KAGRA black hole mergers from the O1–O4a observing runs, we find that, at least, 92% of these events must have nonvanishing
V
GW
even though only one event shows strong individual evidence, indicating a predominance of precessing dynamics across the events. Through a relation between
V
GW
and the remnant black hole recoil recently established by S. H. Leong et al., we further constrain the retention of merger remnants in dense stellar environments, finding that at most 8% could remain gravitationally bound to globular or nuclear star clusters and subsequently participate in hierarchical merger channels. We finally investigate the cosmological distribution of black hole merger handedness. The observed GW distribution is consistent with symmetry under
V
GW
→ −
V
GW
, and yields an average value
〈
V
GW
〉
=
−
1
.
9
−
6.6
+
6.1
×
1
0
−
3
(90% credibility), consistent with the absence of a preferred handedness and with expectations from large-scale statistical isotropy. In particular, the inclusion of O4a events reduces uncertainties in 〈
V
GW
〉 by ∼40% with respect to O1–O3 events. These results establish black hole merger handedness as a unified probe of orbital precession, remnant recoil, hierarchical formation, and cosmological mirror symmetry.