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

TL;DR

This paper reports on the search for a light muon-philic Z' boson using CERN's NA64-e experiment data, setting limits on its properties and exploring its potential to explain muon g-2 anomalies.

Contribution

The study provides the first rigorous upper limits on a muon-philic Z' boson from NA64-e data and discusses future sensitivity improvements.

Findings

Limits on Z' parameter space close to muon g-2 favored region

Data constrains Z' mass around 1 MeV

Future runs could enhance detection sensitivity

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 $2.84\times10^{11}$ 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.