$c\to u\nu \bar{\nu }$ transitions of $B_c$ mesons: 331 model facing Standard Model null tests

The Glashow-Iliopoulos-Maiani mechanism is extremely efficient to suppress the flavour-changing neutral current decays of charmed hadrons induced by the $c\to u$ transitions, making such processes particularly sensitive to phenomena beyond the Standard Model. In particular, $c\to u$ decays with a neutrino pair in the final state are theoretically appealing due to the small long-distance contributions. Moreover, in the framework of the Standard Model Effective Field Theory (SMEFT), the $SU(2{)}_L$ invariance allows to relate the Wilson coefficients in the effective Hamiltonian governing the $c \to u \nu {\bar \nu}$decaystothecoefficientsinthe$c \to u \ell^+ \ell^-$Hamiltonian\mathrm{.}Weanalyzethe$B_c \to B^{()+} \nu {\bar \nu}$ decays, for which branching fractions of at most $\mathcal{O}({10}^{-16})$ are predicted in the Standard Model including short- and long-distance contributions, so small that they can be considered as null tests. Using SMEFT and the relation to the $c \to u \ell^+ \ell^-$ processes we study the largest enhancement achievable in generic new physics scenarios. Then we focus on a particular extension of the Standard Model, the 331 model. SMEFT relations and the connection with $c \to u \ell^+ \ell^-$implythat${\cal B}(B_c \to B^{()+} \nu {\bar \nu})$ could even reach $\mathcal{O}({10}^{-6})$, an extremely large enhancement. A less pronounced effect is found in the 331 model, with $\mathcal{O}({10}^{-11})$ predicted branching fractions. Within the 331 model correlations exist among the $B_c\to B^{(\ast )+}\nu \bar{\nu }$ and $K\to \pi \nu \bar{\nu }$, $B\to (X_s, K, K^*) \nu \bar \nu$ channels.