Extrapolation of the current known black hole (BH) transients (only 17 confirmed after decades monitoring the X-ray sky), together with Population Synthesis studies, suggest the Milky Way contains ~10^4 quiescent BH binaries. However, the full extent of this population is extremely uncertain, yet it is critical for understanding close binary evolution and BH formation.

Attempts to find new hibernating BHs have so far failed. Through this Ministry Action I seek for finantial support to develop a new strategy for an ERC-Advanced project. The goal is to undertake a deep survey of selected regions in the northern Galactic Plane to unveil a hitherto hidden population of quiescent BH binaries. To do so I will develop and exploit a revolutionary approach: a new photometric technique based on a combination of customized Halpha filters, optimized to select targets with broad Halpha emission. The combination of Halpha widths and orbital periods, from time-series photometry, will yield "Photometric Mass Functions".

This novel concept will allow an efficient selection of dynamical BHs in large volumes, as a direct spectroscopic survey would be impossibly time-consuming. Four-five areas of 200 deg² will be surveyed to a depth of r'=22 along different longitudes across the Galactic Plane to probe the density and distribution of hibernating BHs. This strategy will allow for a four-fold increase in the number of BHs in only a few years. A large boost in the statistics will revolutionise our knowledge of their period distribution and mass spectrum, key properties for understanding BH formation and the EOS of nuclear matter.

This programme has also an excellent synergy with other frontier surveys (LSST, SKA, Gaia, eROSITA) from which key information on the energy distribution and distance/kinematics of the BH population will be extracted. Finally, this programme will act as a pathfinder for surveys of hibernating BHs in nearby galaxies with the next generation of ELTs.