Leptoquark-mediated Dirac neutrino mass and its impact on $B \to K \nu \bar{\nu}$ and $K \to \pi \nu \bar{\nu}$ decays

Abstract

Right-handed neutrinos $\nu_R$ play a crucial role in flavor-changing neutral-current processes with missing energy, such as $b\to s + \slashed{E} $ and $d\to s + \slashed{E} $, where Belle-II reports unexpectedly large branching fraction in $B\to K \nu \bar\nu$ decays. Assuming $\nu_R$ is the partner of the active neutrino $\nu_L$ 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 $B\to K^{(*)} \nu\bar{\nu}$ and $K^+\to \pi^+ \nu\bar{\nu}$. Additionally, the model predicts excesses in $R_D$ from $B\to D\tau\bar{\nu}$ that remain within $1\sigma$ of current experimental data.