Abstract

The inclusion of an additional U(1) gauge L-mu - L-tau 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 mu and tau leptons and interacting with electrons via a loop mechanism. The L-mu - L-tau 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 similar to 9 x 10(11) 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 < m(Z ') less than or similar to 2 MeV range, in complementarity with the limits recently obtained by the NA64-mu experiment with a muon beam.