We report a study of $B \to J/\psi(\ell^{ +}\ell^{ -})K$ decays, where $\ell$ represents an electron or a muon, using $e^{ +}e^{ -}$ collisions at the $\Upsilon(4S)$ resonance. The data were collected by the Belle II experiment at the SuperKEKB asymmetric-energy collider during 2019-2021, corresponding to an integrated luminosity of $189$ fb$^{ -1}$. The measured quantities are the branching fractions (${ \mathcal B}$) of the decay channels $B^{ +} \to J/\psi(e^{ +}e^{ -})K^{ +}$, $B^{ +} \to J/\psi(\mu^{ +}\mu^{ -}) K^{ +}$, $B^{ 0} \to J/\psi(e^{ +}e^{ -}) K^{ 0}_{ S}$, and $B^{ 0} \to J/\psi(\mu^{ +}\mu^{ -})K^{ 0}_{ S}$; the lepton-flavor-dependent isospin asymmetries for the electron [$A_{ I}\left(B \to J/\psi(e^{ +}e^{ -}) K\right)$] and muon [$A_{ I}\left(B \to J/\psi(\mu^{ +} \mu^{ -}) K\right)$] channels; and the ratios of branching fractions between the muon and electron channels for the charged [$R_{ K^{ +}}\left(J/\psi\right)$] and neutral kaon [$R_{ K^{ 0}}\left(J/\psi\right)$] case. The measurements are consistent with the world-average values.
Categories: hep-ex