Filling the Gap: Hunting for Vector Bosons at the MUonE Experiment with Displaced Decay Signature

Abstract

The upcoming MUonE experiment aims to precisely measure the running of the fine structure constant via elastic muon-electron scattering, to shed light on the current tension in the muon's anomalous magnetic moment. In addition to its primary function as a precision experiment, MUonE also offers a unique testing ground to probe long-lived vector bosons. Such vector bosons can be produced via $μe \to μe V$ or $μN \to μN V$ scattering and decay into an electron/positron pair a few centimeters away from the interaction point. With its high-resolution tracking system and unique geometric design, MUonE is well-suited to reconstruct displaced vertices close to the target, allowing it to probe parameter space previously unattainable at colliders and longer-baseline beam dump experiments. We present a comprehensive study of the discovery potential of BSM vector boson mediators at the MUonE experiment. We show that MUonE can fill the long-standing gap in the parameter space of vector boson mediators with masses up to around 100 MeV.

Figures (9)

• Figure 1: Feynman diagrams of vector boson production at MUonE.
• Figure 2: A schematic diagram for the MUonE apparatus with displaced vertex signature, borrowed from Fig. Krnjaic:2024ols. Orange line: incoming muon beam and outgoing scattered muon. Blue dashed line: displaced decaying vector boson. Green lines: decay products of the vector boson. Black squares: tracking stations. Gray squares: Beryllium targets. Blue and dark blue squares: the ECAL and the muon filter.
• Figure 3: Signal event geometry in the corresponding volume, borrowed from Fig. Krnjaic:2024ols. We show the ECAL and muon filter after the module, but note that this can be a few meters away from the module that we consider.
• Figure 4: Production cross sections per unit charge, normalized to $g_V^2 = 1$ (Table. \ref{['tab:model']}). At low masses, production from scattering off electrons is much more likely, as the electron is more likely to radiate out a gauge boson than the heavy nucleus, leading to a larger cross section. At large vector boson masses, production from nuclear scattering dominates, extending the experimental reach of MUonE to $\sim$ 100-150 MeV.
• Figure 5: Energy and angle distribution for the produced vector boson and deflected muon for $m_{A'} = 12$ MeV.