UV cut-off of the Standard Model and proton decays

TL;DR

The study argues for a UV completion of the SM at a cutoff scale $\Lambda \sim 10^{11}$ GeV in which higher-dimension operators inherit SM Yukawa flavor structure via an $\epsilon$-scheme arising from partial compositeness. Neutrino masses are generated by the $\ell\ell HH/\Lambda$ term, while proton decay proceeds through dimension-six BNV operators whose flavor couplings are suppressed by $\epsilon$ factors. The authors compute proton lifetimes using SMEFT-to-LEFT matching and lattice/QCD inputs, finding that lifetimes are consistent with current Super-Kamiokande bounds and that the $p\to \pi^0\mu^+$ channel could be within reach of Hyper-Kamiokande. This framework links fermion hierarchies, neutrino masses, and baryon-number violation into a testable high-scale scenario with clear experimental signatures. The results motivate continued proton-decay searches and further explorations of UV completions with flavor-structured higher-dimensional operators.

Abstract

Non-observation of proton decays as well as the smallness of the neutrino masses can naturally be explained by the accidental baryon and lepton number symmetry in the Standard Model, where the approximate symmetries are a consequence of the absence of the baryon or lepton number violating operators at the renormalizable level. The neutrino masses at sub-eV scales can be explained by the presence of the dimension-five, $\ell\ell HH/Λ$, term in the Lagrangian, suggesting that a more fundamental theory takes over beyond the energy scale $Λ$. We consider the possibility that the theory above the scale $Λ$ generates general higher dimensional operators with the flavor structure implied by the Yukawa interactions in the Standard Model. Such a set-up can be realized, for example, in the composite Higgs scenario with partial compositeness of fermions. The fermion masses and the neutrino masses are explained for $Λ\sim 10^{11}$GeV. The lifetime of proton in this scenario is, interestingly, consistent with the observed event of the $p \to π^0 μ^+$ decay at the Super-Kamiokande experiment. The Hyper-Kamiokande experiments should see a large number of events soon after the data taking.

Figures (1)

• Figure 1: The $\epsilon_{q_3}$-dependence of the proton lifetime for each decay mode. The other free parameters are fixed at $\Lambda_{11}=1$, $\lambda_{{\rm yuk}}=4\pi$, and $c_{qqq\ell}=c_{qque}=c_{duue}=c_{duq\ell}=1$.