Hunting for new physics in $B$ meson $b\to u$ transitions: A fresh look with Belle II data

Abstract

We present an analysis of the decays $B^{-}\to {\ell }^{-}{\bar{\nu }}_{\ell }$, ${\bar{B}}^0\to {\pi }^{+}{\ell }^{-}{\bar{\nu }}_{\ell }$, and $B^{-}\to {\rho }^0{\ell }^{-}{\bar{\nu }}_{\ell }$ leveraging the precise measurements of binned differential branching fractions recently reported by the Belle II Collaboration for the semileptonic modes. We adopt a generalized low-energy Hamiltonian that incorporates all dimension-six operators with left-handed neutrinos -- consistent with the Standard Model effective field theory (SMEFT) -- featuring complex, lepton-flavor-dependent Wilson coefficients. For three representative values of $|V_{ub}|$, we perform a fit to the measured $q^2$-binned observables to constrain the coupling parameter space and determine the corresponding confidence regions. Finally, we perform a global fit for the three decay channels, determining $|V_{ub}|$ together with the new physics couplings.

Comments: 12 pages, 4 figures