Shedding light on Dark Sectors with high-energy muons at the NA64 experiment at the CERN SPS

TL;DR

This paper reports a search for dark sector particles using high-energy muons at CERN, analyzing missing energy signatures to probe various muon-philic mediators, including vector and scalar particles, expanding dark matter exploration.

Contribution

It introduces a novel muon-based search technique at CERN for dark sector particles, complementing existing electron-positron experiments and exploring new parameter spaces.

Findings

No dark sector particles observed within sensitivity limits.

Constraints set on muon-philic mediators, including $L_-L_$ $Z'$ and dark photons.

Technique demonstrates the potential for high-energy muon beams to probe dark matter models.

Abstract

A search for Dark Sectors is performed using the unique M2 beam line at the CERN Super Proton Synchrotron. New particles ($X$) could be produced in the bremsstrahlung-like reaction of high energy 160 GeV muons impinging on an active target, $\mu N\rightarrow\mu NX$, followed by their decays, $X\rightarrow\text{invisible}$. The experimental signature would be a scattered single muon from the target, with about less than half of its initial energy and no activity in the sub-detectors located downstream the interaction point. The full sample of the 2022 run is analyzed through the missing energy/momentum channel, with a total statistics of $(1.98\pm0.02)\times10^{10}$ muons on target. We demonstrate that various muon-philic scenarios involving different types of mediators, such as scalar or vector particles, can be probed simultaneously with such a technique. For the vector-case, besides a $L_\mu-L_\tau$ $Z'$ vector boson, we also consider an invisibly decaying dark photon ($A'\rightarrow\text{invisible}$). This search is complementary to NA64 running with electrons and positrons, thus, opening the possibility to expand the exploration of the thermal light dark matter parameter space by combining the results obtained with the three beams.