Search for a light muon-philic $Z^{\prime }$ with the NA64-$e$ experiment at CERN

Abstract

The extension of Standard Model made by inclusion of additional $U(1)$ gauge $L_{\mu }-L_{\tau }$ symmetry can explain the difference between the measured and the predicted value of the muon magnetic moment and solve the tension in $B$ meson decays. This model predicts the existence of a new, light $Z^{\prime }$ vector boson, predominantly coupled to second and third generation leptons, whose interaction with electrons is due to a loop mechanism involving muons and taus. In this work, we present a rigorous evaluation of the upper limits in the $Z^{\prime }$ parameter space, obtained from the analysis of the data collected by the NA64-$e$ experiment at CERN SPS, that performed a search for light dark matter with $2.84\times 10^{11}$ electrons impinging with 100 GeV on an active thick target. The resulting limits, despite being included in a region already investigated by neutrino experiments,touch the muon $g-2$ preferred band for values of the $Z^{\prime }$ mass of order of 1 MeV. The sensitivity projections for the future high-statistics NA64-$e$ runs demonstrate the power of the electrons/positron beam approach in this theoretical scenario.

11 pages, 9 figures. This version was accepted for publication

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• Search for an invisible $Z^{\prime }$ in a final state with two muons and missing energy at Belle II
• Supernova Limits on Muonic Dark Forces
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• Search for a ${\mu }^{+}{\mu }^{-}$ resonance in four-muon final states at Belle II
• Dark SU(2) Stueckelberg portal
• Resonant probing spin-0 and spin-2 dark matter mediators with fixed target experiments
• Prospects of five-dimensional $L_{\mu }-L_{\tau }$ gauge interactions in the light of elastic neutrino-electron scatterings: The scope of the DUNE near detector