${\Xi }_{cc}^{++}-{\Xi }_{cc}^{+}$ 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 ${\varphi }^{+}$: with a first-generation ${\varphi }^{+}\bar{u}d$ 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.

Comments: v2: refined discussion on the UV + extended discussion on LHCb simulation. Conclusion remains unchanged