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We present an analysis of the charmless semileptonic decay $B^0\to\pi^-\ell^+ \nu_{\ell}$, where $\ell = e, \mu$, from 198.0 million pairs of$B\bar{B}$ mesons recorded by the Belle II detector at the SuperKEKBelectron-positron collider. The decay is reconstructed without identifying thepartner $B$ meson. The partial branching fractions are measured independentlyfor $B^0\to\pi^- e^+ \nu_{e}$ and $B^0\to\pi^- \mu^+ \nu_{\mu}$ as functions of$q^{2}$ (momentum transfer squared), using 3896 $B^0\to\pi^- e^+ \nu_{e}$ and5466 $B^0\to\pi^- \mu^+ \nu_{\mu}$ decays. The total branching fraction isfound to be $(1.426 \pm 0.056 \pm 0.125) \times 10^{-4}$ for $B^0\to\pi^-\ell^+ \nu_{\ell}$ decays, where the uncertainties are statistical andsystematic, respectively. By fitting the measured partial branching fractionsas functions of $q^{2}$, together with constraints on the nonperturbativehadronic contribution from lattice QCD calculations, the magnitude of theCabibbo-Kobayashi-Maskawa matrix element $V_{ub}$, $(3.55 \pm 0.12 \pm 0.13 \pm0.17) \times 10^{-3}$, is extracted. Here, the first uncertainty isstatistical, the second is systematic and the third is theoretical.

Determination of $|V_{ub}|$ from untagged $B^0\to\pi^- \ell^+ \nu_{\ell}$ decays using 2019-2021 Belle II data