000004507 001__ 4507
000004507 005__ 20240908132733.0
000004507 037__ $$aBELLE2-PTHESIS-2024-017
000004507 041__ $$aeng
000004507 100__ $$aHenrik Junkerkalefeld
000004507 245__ $$aTests of lepton universality in the rates of inclusive semileptonic B-meson decays at Belle II
000004507 260__ $$aBonn$$bPhysikalisches Institut$$c2024
000004507 300__ $$amult. p
000004507 500__ $$aPresented on 28 06 2024
000004507 502__ $$aPhD$$bBonn, University of Bonn$$c2024
000004507 520__ $$aThis thesis presents two precision tests of lepton universality: the first measurement of the ratio of light-lepton branching fractions of inclusive semileptonic $B$-meson decays, $R(X_{e/mu}) = \mathcal{B}(B \to Xe\nu) / \mathcal{B}(B \to X\mu\nu)$, and the first successful measurement of inclusive $B \to X\tau\nu$ decays, measured in a ratio relative to $B \to X\ell\nu$ decays ($\ell = e, \mu$), $R(X_{\tau/\ell}) = \mathcal{B}(B \to X\tau\nu) / \mathcal{B}(B \to X\ell\nu)$. Experimental data from electron-positron collisions at the $\Upsilon(4S)$ resonance collected with the Belle II detector is used, that corresponds to an integrated luminosity of $\SI{189}{\invfb}$. Continuum $e^+e^- \to \qqbar$ events are constrained using $\SI{18}{\invfb}$ of off-resonance data, collected \SI{60}{\MeV} below the $\Upsilon(4S)$ resonance. One $B$ meson from the $\Upsilon(4S) \to \BBbar$ decay is fully reconstructed in a hadronic decay mode. In the signal  or normalization decay of the second $B$ meson, $B \to X[\tau\to\ell\nu\nu]\nu$ or $B \to X\ell\nu$, a charged lepton candidate is required. The hadronic system $X$ is left unconstrained, rendering this analysis sensitive to all possible hadronic final states. The light-lepton ratio $R(X_{e/mu})$ is extracted in a binned maximum-likelihood fit to the lepton-momentum spectra above \SI{1.3}{\GeV}. The fit is performed simultaneously for both lepton flavors, resulting in a substantial cancellation of correlated systematic uncertainties. The yields from \BBbar backgrounds are constrained in a fit to control-sample data defined by the presence of two $B$-meson candidates with the same flavor. The value of $R(X_{e/mu}) = 1.007 \pm 0.009 (stat.) \pm 0.019 (syst.)$ is obtained, which aligns with the Standard-Model expectation. This is the most precise branching fraction-based test of lepton universality in semileptonic $B$-meson decays to date. For the measurement of $R(X_{\tau/\ell})$, significant discrepancies between experimental data and simulation, mostly attributed to the modeling of $D$-meson decays within the hadronic system $X$, are revealed. A new data-driven method to identify, quantify, and correct these modeling inaccuracies is developed, which simultaneously ameliorates the modeling of several observables that are associated with the hadronic system. This technique enables the extraction of $R(X_{\tau/\ell})$ in a two-dimensional binned maximum-likelihood fit to the lepton-momentum spectra and the squared missing mass in the event. The resulting $R(X_{\tau/\ell})$ values for electrons and muons are $R(X_{\tau/e}) = 0.232 \pm 0.020 (stat.) \pm 0.037 (syst.)$, and $R(X_{\tau/\mu}) = 0.222 \pm 0.027 (stat.) \pm 0.050 (syst.)$, respectively. The combination of lepton flavors in a weighted average of correlated values yields $R(X_{\tau/\ell}) = 0.228 \pm 0.016 (stat.) \pm 0.036 (syst.)$. The results are in agreement with the Standard-Model predictions and represent a complementary test of the longstanding flavor anomaly observed in the exclusive ratios $R(D^{(*)})$.
000004507 700__ $$aJochen Dingfelder$$edir.
000004507 700__ $$aPeter Lewis$$edir.
000004507 700__ $$aFlorian Bernlochner$$edir.
000004507 8560_ $$fjunkerkalefeld@physik.uni-bonn.de
000004507 8564_ $$uhttps://docs.belle2.org/record/4507/files/BELLE2-PTHESIS-2024-017.pdf
000004507 980__ $$aTHESIS