Leptoquark-mediated Dirac neutrino mass and its impact on B→ Kν ν̄ and K→ π ν ν̄ decays

A bstract Right-handed neutrinos ν R play a crucial role in flavor-changing neutral-current processes with missing energy, such as "Image missing" and "Image missing", where Belle-II reports unexpectedly large branching fraction in B→ Kν ν̄ B → Kν ν ¯ decays. Assuming ν R is the partner of the active neutrino ν L in the standard model, a Dirac-type neutrino framework emerges. We investigate a scenario of radiative Dirac neutrino mass generation in a scalar leptoquark (LQ) model with a global U(1) X symmetry to suppress Majorana mass, tree-level Dirac mass, and diquark couplings. The simplest LQ realization consists of two S 1 = (3 , 1 , −1 / 3) LQs with distinct U(1) X charges. A non-Casas-Ibarra parametrization is proposed to match neutrino data with fewer model parameters. Imposing current experimental constraints from meson mixing and lepton flavor-violating processes, we find that right-handed neutrino effects can significantly enhance B→ K( )ν ν̄ B → K ∗ ν ν ¯ and K⁺→ π⁺ν ν̄ K + → π + ν ν ¯ . Additionally, the model predicts excesses in R D from B→ Dτ ν̄ B → Dτ ν ¯ that remain within 1 σ of current experimental data.