Flavor physics at high-energy muon colliders

TL;DR

This work investigates flavor-breaking heavy new physics using a 10 TeV muon collider by mapping high-energy difermion production onto SMEFT operators containing a muon bilinear. It combines exclusive, semi-inclusive, and with-radiation observables, powered by electroweak Sudakov double-log resummation, and employs jet flavor tagging to disentangle operator structures. The analysis finds sensitivity to NP scales around $\Lambda\sim100$ TeV for a broad set of four-fermion operators, including those involving the top quark that are inaccessible at low energies, with competitive or superior reach compared to low-energy flavor measurements. It also demonstrates reduced theoretical uncertainties at high energy and highlights the muon collider’s role as a pathway to explore flavor physics at the energy frontier, complementing the intensity frontier. The results motivate further work on explicit BSM models, higher-order corrections, and realistic detector performance to solidify the projected sensitivities.

Abstract

Flavor-breaking interactions due to heavy new physics can be probed at a 10 TeV muon collider in the high-energy production of quarks and leptons. The high collision energy mitigates the suppression of the new interactions, offering sensitivity to interaction scales well above 100 TeV both in the lepton and in the quark sector. We investigate all possible deformations of the Standard Model that produce quadratic growth with energy of the four-fermion scattering amplitudes at the muon collider, and we derive sensitivity projections. Electroweak radiation emission gives access to new observable final states like for instance the production of a charged pair of fermions by the emission of at least one W boson. This, combined with jet flavor tagging, improves the sensitivity and the ability to disentangle different interactions. Currently, the best probes of flavor-breaking new physics are high-intensity low-energy measurements of lepton or hadron decays or oscillations. The high-energy probes at the muon collider, of the interactions containing a muon bilinear, are competitive with current bounds and strongly superior for some class of transitions. The sensitivity extends to operators involving the top quark that cannot be tested at low energy. In addition, muon collider probes are generically less exposed to experimental or theoretical mismodeling uncertainties as they do not target the observation of extremely rare phenomena, nor they rely on extremely accurate measurements and theoretical predictions: the energy enhancement makes the putative effects relatively easy to observe. Muon collider measurements offer a novel pathway towards the exploration of flavor physics at the energy rather than at the intensity frontier.

Figures (6)

• Figure 1: Exclusive cross sections for the $bj$ (left) and $bb$ (right) flavor-tagged dijet final states as a function of the collider center of mass energy. The black lines display the SM prediction, while the EFT interaction effects are reported in red for $\Lambda=100$ TeV interaction scale. The bottom panels show the relative deviations from the SM due to the effective operator, in comparison with the estimates in Eqs. (\ref{['eq:estimate_FV_EFT0']}) and (\ref{['eq:sigma_EFTeffect']}) (blue). On the right panel, the EFT prediction excluding the quadratic terms are shown as a red dotted line.
• Figure 2: Single-operator sensitivity---namely, the sensitivity when all other operators are set to zero---at the 68% CL from the 3 TeV (orange) and 10 TeV (red) MuC on semileptonic flavor-violating EFT coefficients, compared to present limits from low-energy measurements (gray). The hashed gray bars correspond to the expected sensitivity in the 2040s, with full HL-LHC, Belle-II, and NA62 results.
• Figure 3: Illustrative fits of pairs of EFT coefficients at the 10 TeV MuC. Dark and light blue are 68 and 95% CL regions from the global analysis, respectively. Other contours are 68% CL regions from individual flavor-tagged dijet categories, as indicated in the plots. The solid lines indicate current 68% CL bounds from low-energy flavor observables.
• Figure 4: Single-operator sensitivity at the 68% CL of the 10 TeV (red) and 3 TeV (orange) MuC, and from $\tau$ decays (gray), on LFV EFT coefficients. The hashed gray bars correspond to the expected sensitivity with the full Belle-II dataset.
• Figure 5: Single-operator sensitivity at the 68% CL on flavor-preserving four-lepton operators with different lepton flavor of the 3 TeV (orange) and 10 TeV (red) MuC, compared to current constraints from low-energy measurements (gray).