We re-analyze published proton beam dump data taken at the U70 accelerator at IHEP Serpukhov with the \nu -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 , 2 ] before .
More recently , limits on dark gauge forces have been derived from this data set , considering a dark photon production from \pi ^ { 0 } -decay [ 3 ] .
Here we determine extended mass and coupling exclusion bounds for dark gauge bosons ranging to masses m _ { \gamma ^ { \prime } } of 624 ~ { } MeV at admixture parameters \varepsilon \simeq 10 ^ { -6 } considering high-energy Bremsstrahlung of the U -boson off the initial proton beam and different detection mechanisms .