The directional neighbor discovery problem,i.e.,spatial rendezvous,is a fundamental problem in millimeter wave (mmWave) wireless networks,where directional transmissions are used to overcome the high attenuation.The challenge is how to let the transmitter and the receiver beams meet in space under deafness caused by directional transmission and reception,where no control channel,prior information,and coordination are available.In this paper,we present a Hunting-based Directional Neighbor Discovery (HDND) scheme for ad hoc mmWave networks,where a node follows a unique sequence to determine its transmission or reception mode,and continuously rotates its directional beam to scan the neighborhood for other mmWave nodes.Through a rigorous analysis,we derive the conditions for ensured neighbor discovery,as well as a bound for the worst-case discovery time and the impact of sidelobes.We validate the analysis with extensive simulations and demonstrate the superior performance of the proposed scheme over several baseline schemes.