Search for light Dark Sectors with GeV Muon Beams

TL;DR

The paper investigates light dark-sector mediators in the $L_m-L_ au$ gauge framework, focusing on a $Z'$ boson that can couple to muons and taus and decay invisibly. It proposes a muon-on-target search using a tunable $E_\mu$ beam (1–10 GeV) from HIAF-HIRIBL with a PKMu-based tracking target and a BDT-based analysis to distinguish $\mu e^-\to\mu e^- Z'$ events from backgrounds, complemented by MadGraph and GEANT4 simulations. The study demonstrates competitive 95% CL limits on $g_{Z'}$ down to $\sim10^{-3}$ for $m_{Z'}$ in the tens of MeV range, and shows that a MUonE-like 40-station setup could achieve up to three orders of magnitude higher sensitivity in the low-mass region, potentially yielding world-leading bounds. Overall, this work establishes a concrete foundation for precision muon experiments at HIAF to explore light dark forces and the associated dark-sector phenomenology.

Abstract

Sub-GeV light dark matter often requires new light mediators, such as a dark $Z$ boson in the $L_μ- L_τ$ gauge theory. We study the search potential for such a $Z^\prime$ boson via the process $μe^- \to μe^- X$, with $X$ decaying invisibly, in a muon on-target experiment using a high-intensity 1-10 GeV muon beam from facilities such as HIAF-HIRIBL. Events are identified by the scattered muon and electron from the target using silicon strip detectors in a single-station telescope system. Backgrounds are suppressed through a trained boosted decision tree (BDT) classifier, and activity in downstream subdetectors remains low. This approach can probe a $Z^\prime$ boson in the 10 MeV mass range with improved sensitivity. Nearly three orders of magnitude improvement is achievable with a full multi-telescope station system employing a 160 GeV muon beam at CERN, such as in the MUonE experiment.

Figures (8)

• Figure 1: Schematic layout of the HIRIBL.
• Figure 1: Efficient $\mu e^- \to \mu e^- Z^\prime$ event generation
• Figure 2: A proposed muon experiment to probe $Z^\prime$ through muon on-target collisions.
• Figure 3: The $\mu e^- \to \mu e^- Z^\prime$ process depicted in (a) lab and (b) center-of-mass (COM) frames.
• Figure 4: Correlation distributions for the signal process, illustrating the relations between particle energies and momentum vectors for the case of a 10 GeV muon beam and a $Z^\prime$ mass of 30 MeV. The target is a 20 mm graphite block. The yields are normalized to $2.5\times10^{14}$ muons on target. (a) Correlation between $E_\mu$ and $\left<\vec{p}_0, \vec{p}_1\right>$; (b) correlation between $E_\mu$ and $\left<\vec{p}_0, \vec{p}_2\right>$; (c) correlation between $E_e$ and $\left<\vec{p}_0, \vec{p}_1\right>$; (d) correlation between $E_e$ and $\left<\vec{p}_0, \vec{p}_2\right>$.