Flavor physics at high-energy muon colliders

A bstract 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.