Direct constraints on ultra-light boson mass from searches for continuous gravitational waves

\textit{Superradiance} can trigger the formation of an ultra-light boson cloud around a spinning black hole. Once formed, the boson cloud is expected to emit a nearly periodic, long-duration, gravitational-wave signal. For boson masses in the range $(10^{-13}-10^{-11})$ eV, and stellar mass black holes, such signals are potentially detectable by gravitational wave detectors, like Advanced LIGO and Virgo. In this {\it Letter} we present full band upper limits for a generic all-sky search for periodic gravitational waves in LIGO O2 data, and use them to derive - for the first time - direct constraints on the ultra-light scalar boson field mass.