New exclusion limits on dark gauge forces from proton Bremsstrahlung in beam-dump data

We re-analyze published proton beam dump data taken at the U70 accelerator at IHEP Serpukhov with the ν-calorimeter I experiment in 1989 to set mass-coupling limits for dark gauge forces. The corresponding data have been used for axion and light Higgs particle searches in Refs. [1] and [2] before. More recently, limits on dark gauge forces have been derived from this data set, considering a dark photon production from π0π0-decay [3]. Here we determine extended mass and coupling exclusion bounds for dark gauge bosons ranging to masses mγ′mγ′ of 624 MeV624 MeV at admixture parameters ε≃10−6ε≃10−6 considering high-energy Bremsstrahlung of the γ′γ′-boson off the initial proton beam and different detection mechanisms.