A search for a new $Z^{\prime }$ gauge boson associated with (un)broken $B-L$ symmetry in the keV–GeV mass range is carried out for the first time using the missing-energy technique in the NA64 experiment at the CERN SPS. From the analysis of the data with $3.22\times {10}^{11}$ electrons on target collected during 2016–2021 runs, no signal events were found. This allows us to derive new constraints on the $Z^{\prime }-e$ coupling strength, which, for the mass range $0.3\lesssim m_{Z^{\prime }}\lesssim 100\mathrm{MeV}$, are more stringent compared to those obtained from the neutrino-electron scattering data.

• Received 22 July 2022
• Accepted 14 September 2022
• Corrected 27 January 2023

DOI:https://doi.org/10.1103/PhysRevLett.129.161801

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Published by the American Physical Society