First constraints on the L_μ− L_τ explanation of the muon g-2 anomaly from NA64-e at CERN

The inclusion of an additional U(1) gauge L_μ − L_τ symmetry would release the tension between the measured and the predicted value of the anomalous muon magnetic moment: this paradigm assumes the existence of a new, light Z′ vector boson, with dominant coupling to μ and τ leptons and interacting with electrons via a loop mechanism. The L_μ − L_τ model can also explain the Dark Matter relic abundance, by assuming that the Z′ boson acts as a “portal” to a new Dark Sector of particles in Nature, not charged under known interactions. In this work we present the results of the Z′ search performed by the NA64-e experiment at CERN SPS, that collected ~ 9 × 10¹¹ 100 GeV electrons impinging on an active thick target. Despite the suppressed Z′ production yield with an electron beam, NA64-e provides the first accelerator-based results excluding the g − 2 preferred band of the Z′ parameter space in the 1 keV <mZ_′ ≲ 2 MeV range, in complementarity with the limits recently obtained by the NA64-μ experiment with a muon beam.