Search for a new gauge boson in a fixed-target experiment at CERN SPS

A fixed-target experiment at the CERN SPS utilizing the active beam dump and missing energy approach for the searches of a new gauge boson (A') with sub-GeV mass and mixing with ordinary photon is discussed. Electrons impinging on a high-Z target produce bremsstrahlung A's, decaying either to an e$^+$e$^-$ pair or invisibly to dark sector particles. The status of the experiment and the expected sensitivity in the test run in 2015 are presented.