$\Xi_{cc}^{++}-\Xi_{cc}^{+}$ Transitions as a Two-Charm-Selective Portal to Ultra-Low-$Q$ Charged Currents

Abstract

The recent LHCb observation of $\Xi_{cc}^{+}$ opens the ultra-low-$Q$ transition $\Xi_{cc}^{++}\to\Xi_{cc}^{+}$ as an experimentally motivated null test of charged-current new physics. Our two- and three-body analyses show sensitivity to effective baryon-level couplings of $\mathcal{O}(10^{-6}-10^{-7})$ for MeV-scale recoil momenta. We establish a practical no-go result for a universal light charged scalar $\phi^+$: with a first-generation $\phi^+\bar u d$ coupling, existing electroweak-precision and beta-decay constraints are parametrically stronger than the projected LHCb sensitivity. We then identify a two-charm-selective charged-current portal whose leading operator has a nonzero matrix element in doubly charmed baryons but vanishes at leading order in pions, nucleons, nuclei, and singly charmed mesons. In this class of models, $\Xi_{cc}^{++}\to\Xi_{cc}^{+}$ transitions can provide the leading direct probe of the portal interaction.