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 $c to u u {bar u}$ decays to the coefficients in the $c to u ell^+ ell^-$ Hamiltonian. We analyze the $B_c to B^{(*)+} u {bar u}$ decays, for which branching fractions of at most ${cal O}(10^{-16})$ 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 $c to u ell^+ ell^-$ imply that ${cal B}(B_c to B^{(*)+} u {bar u})$ could even reach ${cal O}(10^{-6})$, an extremely large enhancement. A less pronounced effect is found in the 331 model, with ${cal O}(10^{-11})$ predicted branching fractions. Within the 331 model correlations exist among the $B_c to B^{(*)+} u bar u$ and $Kto pi u bar u$, $Bto (X_s, K, K^*) u bar u$ channels.
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