Tests of lepton universality in the rates of inclusive semileptonic <em>B</em>-meson decays at Belle II

Abstract

This thesis presents two precision tests of lepton universality: the first measurement of the ratio of light-lepton branching fractions of inclusive semileptonic <em>B</em>-meson decays, <em>R(X_e/&mu;) = &Bscr;(B &rarr; Xe&nu;) / &Bscr;(B &rarr; X&mu;&nu;)</em>, and the first successful measurement of inclusive <em>B &rarr; X&tau;&nu;</em> decays, measured in a ratio relative to <em>B &rarr; X&ell;&nu;</em> decays (<em>&ell; = e, &mu;</em>), <em>R(X_&tau;/&ell;) = &Bscr;(B &rarr; X&tau;&nu;) / &Bscr;(B &rarr; X&ell;&nu;)</em>. <br/> Experimental data from electron-positron collisions at the <em>&upsih;(4S)</em> resonance collected with the Belle II detector is used, that corresponds to an integrated luminosity of <span>189 fb^&minus;1</span>. Continuum <em>e⁺e^&minus; &rarr; q<span style="text-decoration: overline;">q</span></em> events are constrained using <span>18 fb^&minus;1</span> of off-resonance data, collected <span>60 MeV</span> below the <em>&upsih;(4S)</em> resonance. One <em>B</em> meson from the <em>&upsih;(4S) &rarr; B<span style="text-decoration: overline;">B</span></em> decay is fully reconstructed in a hadronic decay mode. In the signal or normalization decay of the second <em>B</em> meson, <em>B &rarr; X</em>[<em>&tau;&rarr;&ell;&nu;&nu;</em>]<em>&nu;</em> or <em>B &rarr; X&ell;&nu;</em>, a charged lepton candidate is required. The hadronic system <em>X</em> is left unconstrained, rendering this analysis sensitive to all possible hadronic final states. <br/> The light-lepton ratio <em>R(X_e/&mu;)</em> is extracted in a binned maximum-likelihood fit to the lepton-momentum spectra above <span>1.3 GeV</span>. The fit is performed simultaneously for both lepton flavors, resulting in a substantial cancellation of correlated systematic uncertainties. The yields from <em>B<span style="text-decoration: overline;">B</span></em> backgrounds are constrained in a fit to control-sample data defined by the presence of two <em>B</em>-meson candidates with the same flavor. The value of <br/> <em>R(X_e/&mu;)</em> = 1.007 &plusmn; 0.009 (stat.) &plusmn; 0.019 (syst.) <br/> is obtained, which aligns with the Standard-Model expectation. This is the most precise branching fraction-based test of lepton universality in semileptonic <em>B</em>-meson decays to date.<br/> For the measurement of <em>R(X_&tau;/&ell;)</em>, significant discrepancies between experimental data and simulation, mostly attributed to the modeling of <em>D</em>-meson decays within the hadronic system <em>X</em>, 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 <em>R(X_&tau;/&ell;)</em> in a two-dimensional binned maximum-likelihood fit to the lepton-momentum spectra and the squared missing mass in the event.<br/> The resulting <em>R(X_&tau;/&ell;)</em> values for electrons and muons are <em>R(X_&tau;/e)</em> = 0.232 &plusmn; 0.020 (stat.) &plusmn; 0.037 (syst.), and <em>R(X_&tau;/&mu;)</em> = 0.222 &plusmn; 0.027 (stat.) &plusmn; 0.050 (syst.), respectively. The combination of lepton flavors in a weighted average of correlated values yields <br/> <em>R(X_&tau;/&ell;)</em> = 0.228 &plusmn; 0.016 (stat.) &plusmn; 0.036 (syst.). <br/> 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 <em>R(D^(*))</em>.