Searches for Binary Mergers with Sub-solar Mass Components in Data from the First Part of LIGO--Virgo--KAGRA's Fourth Observing Run

We report on a gravitational wave search for compact binary coalescences involving at least one component with mass between $0.2 M_\odot$ to $1 M_\odot$, and ratio of component masses between 0.1 and 1. The analysis uses data collected by the LIGO detectors between May 24 2023 15:00 UTC and January 16 2024 16:00 UTC. No statistically significant sub-solar mass candidates were identified by the participating search algorithms. We report the detection sensitivity of the current searches to the target sub-solar mass black hole population, while also reporting the sensitivity of the search to low-mass neutron star binaries for the first time. With the absence of detections, we place upper limits on the merger rate of sub-solar mass black holes, ranging from $110$ $\mathrm{Gpc^{-3}yr^{-1}}$ to $10000$ $\mathrm{Gpc^{-3}yr^{-1}}$ at 90% confidence. We use the merger rate limits to constrain two illustrative dark matter scenarios that can form sub-solar mass compact objects: primordial black holes, and dark black holes forming in a dissipative dark matter model. For late-forming primordial black hole binaries, our search excludes the fraction of dark matter in primordial black holes to be 1 for masses above $0.9 M_\odot$. In the early-formation scenario, we limit this fraction to be $\leq$ 7% at $1 M_\odot$, and $\leq$ 40% at $0.35 M_\odot$. For the dissipative model, the excluded region in the parameter space of dark matter fraction in dark black holes and their minimum possible mass extends down to (1.2 to 1.3) $\times10^{-5}$ when the minimum mass is $1 M_\odot$. For binary neutron stars that include sub-solar mass components, we estimate the sensitive space-time hypervolume to be $10^{-3}$ $\mathrm{Gpc^3yr}$, and report the upper limit on their merger rate for a simple, fixed population as ~86 $\mathrm{Gpc^{-3}yr^{-1}}$.