Studies on Inclusive |Vub|: A Measurement of |Vub|/|Vcb| at the Belle Experiment and A Search for the Weak Annihilation Contribution to Charmless Semileptonic B Decays at the Belle II Experiment

Sumitted to PubDB: 2025-06-06

Category: Phd Thesis

Tags: ECL Belle Run 1

Principal Authors Marcel Hohmann
Date 2024-12-19
Belle II Number BELLE2-PTHESIS-2025-017
Abstract Two analyses related to the inclusive determination of the magnitude of the Cabbibo-Kobayashi-Maskawa matrix element $|V_{ub}|$ are presented. The first analysis presents the first measurement of the ratio of partial branching fractions of the inclusive charmless, $\overline{B} \to X_u \ell \overline{\nu}$, to the inclusive charmed, $\overline{B} \to X_c \ell \overline{\nu}$, semileptonic $B$ decay, where $\ell$ represents an electron or muon, and $X$ a hadronic system. The measurement is performed at the Belle experiment, using the full dataset collected at the $\Upsilon(4S)$ resonance, corresponding to $711fb^{-1}$. We measure: $$\frac{\Delta \mathcal{B}(\overline{B} \to X_u \ell \overline{\nu})}{\Delta \mathcal{B}(\overline{B} \to X_c \ell \overline{\nu})} = (1.99 \pm 0.17_{\rm stat} \pm 0.16_{\rm syst}) \times 10^{-2},$$ in the phase space region of lepton energies in the $B$-frame, $E_{\ell}^{B_{\rm sig}}$, above $1 {\rm GeV}$. From the partial branching fraction ratio we extract $|V_{ub}|/|V{cb}|$, finding $$\frac{|V_{ub}|}{|V_{cb}|}^{\rm BLNP} = (9.81 \pm 0.42_{\rm stat} \pm 0.38_{\rm syst} \pm 0.51_{ \Delta\Gamma(\overline{B} \to X_u \ell \overline{\nu})} \pm 0.20_{ \Delta\Gamma(\overline{B} \to X_c \ell \overline{\nu})}) \times 10^{-2},$$ This value is in excellent agreement with inclusive and exclusive world averages of $|V_{ub}|/|V_{cb}|$. We additionally report this measurement broken down by $B$ charge, determining a limit on isospin breaking in $\overline{B} \to X_u \ell \overline{\nu}$ decays, as well as by lepton flavour, finding no evidence for lepton flavour universality violation in either the charmed or charmless inclusive semileptonic $B$ decay. Furthermore, we report the differential ratio of partial branching fractions in momentum transfer squared to the lepton-neutrino system, $q^2$, and $E_{\ell}^{B_{\rm sig}}$. The second analysis presented is a search for the poorly understood weak annihilation contribution to the inclusive $\overline{B} \to X_u \ell \overline{\nu}$ decay rate, which is a source of systematic uncertainty to inclusive $|V_{ub}|$ determinations. The analysis is performed with the Belle II data sample ($364fb^{-1}$) collected at the $\Upsilon(4S)$ resonance before 2023. As little is known about the shape of the weak annihilation contribution, we search for structure at high $q^2$, where it is expected to be localised, using a broad range of hypothesised models. The analysis must still undergo internal review; we therefore investigate the sensitivity of the analysis to simulated signal samples. Depending on the shape and magnitude of the simulated weak annihilation sample we expect to place $90\%$ upper limits between $1.5$--$6.8\%$ for the weak annihilation contribution to neutral $B$ decays, and between $1.4$--$9.1\%$ for charged $B$ decays, competitive with existing estimates of the upper limit for the admixture.

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