Exploration of the Muon $g-2$ and Light Dark Matter explanations in NA64 with the CERN SPS high energy muon beam

TL;DR

This paper reports a search for a new light Z' boson at NA64 using a high-energy muon beam, setting new limits on its parameters and exploring implications for muon g-2 and dark matter.

Contribution

First search for a $Z'$ boson at NA64 with a muon beam, providing new constraints on its parameters relevant to muon g-2 and dark matter explanations.

Findings

No signal events observed in the expected region.

Set new exclusion limits on $Z'$ parameter space.

Excluded part of the parameter space related to dark matter models.

Abstract

We report on a search for a new $Z'$ ($L_\mu-L_\tau$) vector boson performed at the NA64 experiment employing a high energy muon beam and a missing energy-momentum technique. Muons from the M2 beamline at the CERN Super Proton Synchrotron with a momentum of 160 GeV/c are directed to an active target. A signal event is a single scattered muon with momentum $<$ 80 GeV/c in the final state, accompanied by missing energy, i.e. no detectable activity in the downstream calorimeters. For a total statistic of $(1.98\pm0.02)\times10^{10}$ muons on target, no event is observed in the expected signal region. This allows us to set new limits on part of the remaining $(m_{Z'},\ g_{Z'})$ parameter space which could provide an explanation for the muon $(g-2)_\mu$ anomaly. Additionally, our study excludes part of the parameter space suggested by the thermal Dark Matter relic abundance. Our results pave the way to explore Dark Sectors and light Dark Matter with muon beams in a unique and complementary way to other experiments.