Measurement of inclusive $B \to X_{u} \ell \nu$ partial branching fractions and $|V_{ub}|$ at Belle II
| Abstract | A measurement of partial branching fractions of charmless semileptonic $B \to X_{u} \ell \nu$ decays using an inclusively reconstructed hadronic system is presented for $\ell = e, \mu$. The measurement uses the full Run~1 Belle~II sample corresponding to 365 fb$^{-1}$ of data. Events containing a signal electron or muon $\ell$ and a hadronic $B$ decay that constrains the signal kinematics are selected, while the rest of the event defines the hadronic system $X_u$ associated with the signal. To discriminate the signal from the 50-times larger background originating from Cabibbo-favoured semileptonic decays, a template fit is performed in both signal and control regions after applying an optimised selection. The partial branching fraction measured for lepton energies greater than 1 GeV in the signal $B$ meson rest frame is $\mathcal{B}(B \to X_u \ell \nu) = (1.54 \pm 0.08 \, {\rm (stat.)} \pm 0.12 \, {\rm (syst.)}) \times 10^{-3}$. From this measurement, using the Gambino, Giordano, Ossola, Uraltsev theoretical framework, the strength of the beauty-to-up quark coupling is determined to be $|V_{ub}| = (4.01 \pm 0.19 ^{+0.07} _{-0.08}) \times 10^{-3}$ where the uncertainties are experimental and theoretical, respectively. This value is in good agreement with the world average of $|V_{ub}| = (4.06 \pm 0.16) \times 10^{-3}$. Partial branching fractions and corresponding values of $|V_{ub}|$ are also obtained for other phase-space regions, defined by additional selections on the $X_u$ and leptonic system masses, and by using different theoretical models. |
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| Tags | Run 1 |
| Working group | Semileptonic and Missing Energy Decay |
| Principal authors | @ Tommy Martinov, Marcel Hohmann, Lu Cao, Kerstin Tackmann, Florian Bernlochner, Merle Graf-Schreiber, Munira Khan, Martin Angelsmark, Phillip Urquijo |
| Reference | submitted to PRD |
| Links | arXiv:2512.08056, |