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Search for subsolar primordial black holes in low mass-ratio binaries with LIGO-Virgo O2 data and implications for the primordial black hole dark matter fraction

Khun Sang Phukon ^1,2,3,4,*, Gregory Baltus ⁵, Sarah Caudill^3,1,6, Sebastien Clesse ^7,†, Antoine Depasse ⁸, Maxime Fays ⁵, Heather Fong^9,10, Shasvath J. Kapadia^11,12, Ryan Magee¹³ et al.

Andres Jorge Tanasijczuk⁸

¹Nikhef—National Institute for Subatomic Physics, Science Park, 1098 XG Amsterdam, The Netherlands
²Institute for High-Energy Physics, University of Amsterdam, Science Park, 1098 XG Amsterdam, The Netherlands
³Institute for Gravitational and Subatomic Physics, Utrecht University, Princetonplein 1, 3584 CC Utrecht, The Netherlands
⁴School of Physics and Astronomy and Institute for Gravitational Wave Astronomy, University of Birmingham, Edgbaston, Birmingham, B15 9TT, United Kingdom
⁵Department of Astrophysics, Geophysics and Oceanography (GEO), Space sciences, Technologies and Astrophysics Research (STAR), Université de Liège, Allée du six Août 19, 4000 Liège, Belgium
⁶Department of Physics, University of Massachusetts, Dartmouth, Massachusetts 02747, USA
⁷Service de Physique Théorique, Université Libre de Bruxelles (ULB), Boulevard du Triomphe, CP225, B-1050 Brussels, Belgium
⁸Center of Cosmology, Phenomenology and Particle Physics, Research Institute in Mathematics and Physics (IRMP), Louvain University, 2 Chemin du Cyclotron, 1348 Louvain-la-Neuve, Belgium
⁹Research Center for the Early Universe (RESCEU), University of Tokyo, Tokyo, 113-0033, Japan
¹⁰University of British Columbia, Vancouver Campus, 2329 West Mall, Vancouver, British Columbia V6T 1Z4, Canada
¹¹International Centre for Theoretical Sciences, Tata Institute of Fundamental Research, Bangalore 560089, India
¹²Inter-University Centre for Astronomy and Astrophysics, Post Bag 4, Ganeshkhind, Pune 411 007, India
¹³LIGO, California Institute of Technology, Pasadena, California 91125, USA

^*Contact author: k.s.phukon@bham.ac.uk
^†Contact author: sebastien.clesse@ulb.be

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Phys. Rev. D 114, 063529 – Published 15 September, 2026

DOI: https://doi.org/10.1103/cpjc-qvwl

Abstract

We perform a search for binary black hole mergers with one subsolar mass (SSM) black hole and a primary component above $\sim 2 M_{⊙}$ in data from the second observing run (O2) of the LIGO-Virgo detectors. Our analysis extends the parameter space explored by previous LIGO-Virgo Collaboration searches for binaries containing SSM components into a region of parameter space motivated by broad mass distributions of primordial black holes (PBHs) exhibiting a peak around $[2 - 3] M_{⊙}$ , which can arise from the reduction of the equation of state during the QCD phase transition in the early Universe. Four candidate events are found passing a signal-to-noise ratio (SNR) threshold of 8 and a false alarm rate (FAR) threshold of 2 per year, although none are statistically significant enough to constitute a confident detection. Assuming a null result for the search, we derive PBH model-independent 90% confidence upper limits on the PBH merger rates by estimating the sensitive volume-time of the search using simulated gravitational-wave signal injections. We interpret these observational limits using a representative broad PBH mass function bearing imprints of the thermal history of the early Universe and considering both early and late PBH binary formation channels. The resulting constraints on the PBH dark-matter fraction, $f_{PBH}$ , depend on the assumed mass function and merger-rate prescription. For all considerations, the upper limits remain above $f_{PBH} = 1$ , indicating the O2 data from LIGO-Virgo are not sensitive enough to place meaningful constraints within the assumptions of the PBH mass model and PBH binary mergers.

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Search for subsolar primordial black holes in low mass-ratio binaries with LIGO-Virgo O2 data and implications for the primordial black hole dark matter fraction

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