This paper studies how the future space gravitational-wave observatory LISA (Laser Interferometer Space Antenna) could look for very light particles called ultralight bosons. The authors focus on two complementary methods. The first uses precise measurements of black hole spins from mergers. The second attempts targeted follow-up searches for continuous gravitational waves from the merged black hole’s remnant. They find that spin measurements could rule out or detect scalar bosons with masses roughly between 5×10^−18 and 1×10^−14 electronvolts and vector bosons between about 6×10^−19 and 2×10^−14 eV. Targeted post-merger searches are sensitive to a narrower vector-boson window near 3×10^−17 to 3×10^−15 eV, and for vector masses around 10^−16 to 2×10^−15 eV the chance of seeing a detectable event ranges from near zero to nearly certain depending on the astrophysical model used.

Why black hole mergers help. A rotating black hole can amplify waves of certain boson fields through a process called superradiance. That process can take angular momentum from the hole and grow a long-lived boson “cloud” around it. The cloud can both spin the hole down (so observed holes would be less rapidly rotating than expected) and emit long-lived, nearly monochromatic gravitational waves. LISA will observe mergers of massive black holes (roughly 10^4 to 10^7 times the Sun’s mass) with high signal strength, so it can measure final masses and spins precisely and search for the predicted continuous waves from any post-merger boson cloud.

What the authors did. They used three different astrophysical population models for massive black holes (including variants based on heavy or light formation seeds) and catalogs updated in light of recent pulsar timing array results. The analysis uses SuperRad1, an open-source waveform model that includes theoretical predictions for how superradiance changes black hole spins and for the gravitational-wave signals from boson clouds. For each population model they forecasted how often LISA could exclude boson masses or produce detections using either spin information or targeted follow-up searches of merger remnants.