First bounds on effective muon interactions using the NA64μ experiment at CERN

A bstract We analyze how NA64 μ can contribute to the global SMEFT program demonstrating that it can probe two effective four-lepton operators completely unbounded so far and break one of the current flat directions. Furthermore, we also study an extension of SMEFT that includes fermion singlets of the SM gauge group in the low energy field content. This effective field theory, usually dubbed ν SMEFT, is well motivated by the observation of light neutrino masses and leptonic mixing. We find that NA64 μ can constrain three unbounded four-fermion operators of the ν SMEFT. We derive the current bounds on these operators and compute the future sensitivity.