Search for Sub-Solar Mass Binaries in the First Part of LIGO's Fourth Observing Run

Abstract: We report the first results of a sub-solar mass compact binary search using the data from the first part of the fourth observing run (O4a) of the Advanced LIGO detectors. Sub-solar mass neutron stars or primordial black holes are not expected to form via standard stellar evolution, and their observation would signify a new class of astrophysical object or the discovery of dark matter. Our search covers binaries with primary masses 0.1 to 2 $\textrm{M}\odot$ and secondary masses 0.1 to 1 $\textrm{M}\odot$. We explicitly incorporate tidal effects up to $7\times10^5$ for extremely low mass neutron stars. No statistically significant candidates are identified. The advanced sensitivity of the O4a run enables an improvement in the sub-solar mass black hole merger rate limits by more than $2 \times$ over the previous three observing runs (O1-O3b). We place a $90\%$ confidence upper limit on the merger rate $\mathcal{R}{90}$ for sub-solar mass black holes to be $< 1.77\times10^4 \textrm{Gpc}^{-3} \textrm{yr}^{-1}$ for a chirp mass of 0.2 $\textrm{M}\odot$. We place the first constraints for binary neutron stars with tidal deformabilities up to $\sim 7\times10^5$ and improve the merger rate estimate by a factor $\sim 4$ in comparison to previous O3 tidal searches for tidal deformabilities $< 10^4$. We further constrain the fraction of dark matter composed of primordial black hole $f_{\rm PBH}< 2\%$ for a chirp mass of 0.1 $\textrm{M}_\odot$. Our results significantly expand the observational search space for sub-solar binaries and provide rigorous constraints on the local abundance of compact objects that may arise from non-standard formation mechanisms.