Lepton Flavor Violation: From Muon Decays to Muon Colliders

Abstract

We investigate the unique potential of a high-energy muon collider to probe lepton-flavor-violating signals arising from physics beyond the Standard Model (SM). Low-energy, precision searches for charged lepton-flavor violation (LFV) are projected to dramatically improve their sensitivity in the coming years, and could provide the first evidence of new physics. We interpret the sensitivity of these searches in terms of a set of LFV operators in the SM Effective Field Theory. The same operators are then probed at the TeV scale by evaluating the projected reach of a muon collider while accounting for SM backgrounds and using a parameterized detector simulation. We find that for most operators, a muon collider could confirm signals if they are seen at future low energy experiments, whereas for certain flavor combinations it extends the reach to scales well beyond those accessible at lower energies. We also project the sensitivity of a muon collider to lepton flavor-violating decays of the SM Higgs boson and demonstrate improved sensitivity to $h \to eτ$ and $h \to μτ$ by an order of magnitude compared to the High-Luminosity LHC. The importance of having multiple, complementary probes is illustrated by considering both various combinations of operators and relative sizes of flavor-violating transitions between generations under various assumptions for the flavor structure of new physics.

Figures (17)

• Figure 1: Feynman diagrams for the VBF production of a Higgs, decaying to lepton-flavor-violating final states (left) and for SM backgrounds that produce different flavor final state leptons (middle and right).
• Figure 2: Histograms of kinematic observables from toy datasets of the $h \to l_i l_j$ signal and the different backgrounds, after applying basic pre-selection cuts demanding the correct number of reconstructed particle objects. The top row shows the invariant mass of the lepton pair for the $\mu e$ and $\tau\mu$ analyses (the plot for the $\tau e$ analysis is essentially identical to the latter). The bottom row shows the $p_T$ of the reconstructed Higgs and the $M_{T2}$ variable from the $\mu e$ analysis. The $\tau\tau$ background contribution is both small in magnitude and sufficiently diffuse to remain undetectable in the bottom-left and bottom-right histograms. The vertical dashed lines denote the cuts on the variable, see the text for details.
• Figure 3: Summary of the constraints on flavor-violating Higgs Yukawa couplings. We show the constraints for $\mu$ -- $e$, $\tau$ -- $e$, and $\tau$ -- $\mu$ flavor violation in the top-left, top-right and bottom panels, respectively. Each plot shows the constraints in the plane of the two off-diagonal couplings; the opposite axes show the constraints interpreted as constraints on the scale of the effective operator $\mathcal{O}_{eH}$. The shaded region in each panel indicates the region excluded by various low-energy constraints (with projected low-energy constraints indicated by the corresponding dashed lines), while the solid (dashed) orange line show the current LHC bounds (HL-LHC projections). The solid red curve shows the projected muon collider constraint. The dashed gray lines indicate a "fine-tuning" constraint on the size of the off-diagonal elements, as discussed in the text.
• Figure 4: Top row: Example diagrams contributing to the $\mu^+ \mu^- \to \mu^\pm \tau^\mp h$ scattering process at a high-energy muon collider, arising from insertions of dimension-6 lepton-flavor-violating operators. The first diagram includes the $\mu V \to \tau h$ sub-process of interest, with the additional muon traveling in the forward direction. The other diagrams contribute at the same order, and are included in our simulations for consistency. Bottom row: Example diagrams for processes that serve as irreducible backgrounds to the $\mu \mu \to \mu \tau h$ signal.
• Figure 5: Histograms of the $p_T$ and mass of reconstructed Higgs from a fat jet system with two $b$-tags for both the signal (red) and dominant $\tau\nu_{\tau}h$ (orange) and $\tau\nu_{\tau}b\bar{b}$ (green) backgrounds with each process independently normalized. The vertical dashed lines denote the cuts on the variable, see the text for details.