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

Abstract

We report on a gravitational wave search for compact binary coalescences involving at least one component with mass between $0.2M_{\odot }$ to $1M_{\odot }$, 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. With the absence of detections, we place upper limits on the merger rate of sub-solar mass black holes, ranging from 110 $\mathrm{Gp}{\mathrm{c}}^{-3}\mathrm{y}{\mathrm{r}}^{-1}$ to 10000 $\mathrm{Gp}{\mathrm{c}}^{-3}\mathrm{y}{\mathrm{r}}^{-1}$ at 90\% confidence. We constrain two illustrative dark matter scenarios that can form sub-solar mass compact objects with these searches: 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 $\le 1$ only for masses above $0.9M_{\odot }$. In the early-formation scenario, we limit this fraction to be $\le 7\%$ at $1M_{\odot }$, and $\le 40\%a$ at $0.35M_{\odot }$. 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 (0.9 to 1.2) $\times 10^{-5}$ at $1M_{\odot }$ with no constraints below $0.02M_{\odot }$. For the first time, we report the detection sensitivity of our searches to binaries with sub-solar mass neutron stars, and place the 90\% confidence merger rate limit at (570 to 710) $\mathrm{Gp}{\mathrm{c}}^{-3}\mathrm{y}{\mathrm{r}}^{-1}$ for a population with component masses distributed uniformly down to $0.5M_{\odot }$.