M$^3$: A New Muon Missing Momentum Experiment to Probe $(g-2)_{\mu}$ and Dark Matter at Fermilab

TL;DR

The paper proposes M$^3$, a Fermilab-based fixed-target experiment using a muon beam to detect invisibly decaying particles that could explain the muon g-2 anomaly and dark matter interactions, covering unexplored parameter space.

Contribution

Introduction of a novel missing-momentum experimental approach with a two-phase setup to probe muon-coupled light particles and dark matter candidates.

Findings

Phase 1 can test parameter space for resolving the g-2 anomaly.

Phase 2 can explore dark matter models with muon-philic forces.

The experiment can access previously inaccessible regions of parameter space.

Abstract

New light, weakly-coupled particles are commonly invoked to address the persistent $\sim 4\sigma$ anomaly in $(g-2)_\mu$ and serve as mediators between dark and visible matter. If such particles couple predominantly to heavier generations and decay invisibly, much of their best-motivated parameter space is inaccessible with existing experimental techniques. In this paper, we present a new fixed-target, missing-momentum search strategy to probe invisibly decaying particles that couple preferentially to muons. In our setup, a relativistic muon beam impinges on a thick active target. The signal consists of events in which a muon loses a large fraction of its incident momentum inside the target without initiating any detectable electromagnetic or hadronic activity in downstream veto systems. We propose a two-phase experiment, M$^3$ (Muon Missing Momentum), based at Fermilab. Phase 1 with $\sim 10^{10}$ muons on target can test the remaining parameter space for which light invisibly-decaying particles can resolve the $(g-2)_\mu$ anomaly, while Phase 2 with $\sim 10^{13}$ muons on target can test much of the predictive parameter space over which sub-GeV dark matter achieves freeze-out via muon-philic forces, including gauged $U(1)_{L_\mu - L_\tau}$.