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Register a new userStudies of the semileptonic $bar B^0to D^{*+}ell^-bar u_ell$ and $B^-to D^{0}ell^-bar u_ell$ decay processes with 34.6 fb$^{-1}$ of Belle II data
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We report measurements of the $bar{B}^0 to D^{*+} ell^{-} bar{ u}_l$ and $B^- to D^{0} ell^{-} bar{ u}_l$ processes using 34.6 fb$^{-1}$ of collision events recorded by the Belle II experiment at the SuperKEKB asymmetric-energy $e^+ e^-$ collider. For the $B^-to D^{0}ell^-bar u_ell$ channel, we present first studies that isolate this decay from other semileptonic processes and backgrounds. We report a measurement of the $bar{B}^0 to D^{*+} ell^{-} bar{ u}_l$ branching fraction and obtain ${cal B}(bar{B}^0 to D^{*+} ell^{-} bar{ u}_l) = left(4.60 pm 0.05_{mathrm{stat}}pm0.17_{mathrm{syst}} pm 0.45_{pi_s}right) %$, in agreement with the world average. Here, the uncertainties are statistical, systematic, and related to slow pion reconstruction, respectively. The systematic uncertainties are limited by the statistics of auxiliary measurements and will improve in the future. We also report differential branching fractions in five bins of the hadronic recoil parameter $w$ for $bar{B}^0 to D^{*+} ell^{-} bar{ u}_l$, unfolded to account for resolution and efficiency effects.
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We measure the branching fractions for the decays $bar{B}^0 to D^{*+} e^{-} bar{ u}_e$ and $bar{B}^0 to D^{*+} mu^{-} bar{ u}_mu$ using $8.70 pm 0.09~{rm fb}^{-1}$ of data collected by the Belle II experiment at the SuperKEKB asymmetric-energy $e^+ e^-$ collider. Candidate signal decays are reconstructed with the subsequent decays $D^{*+}to D^0 pi^+$ and $D^0to K^-pi^+$. We obtain the results ${cal B}(bar{B}^0 to D^{*+} e^{-} bar{ u}_e)$ = $(4.55pm0.14(mathrm{stat})pm0.35 (mathrm{syst})) %$ and ${cal B}(bar{B}^0 to D^{*+} mu^{-} bar{ u}_mu)$ = $(4.84pm 0.13(mathrm{stat})pm0.37(mathrm{syst})) %$, in agreement with the world averages. The measurements serve to validate the full chain of detector operation and calibration, data collection and processing, and production of physics results in the case of semileptonic $B$-meson decays.
Besides being important to determine Standard Model parameters such as the CKM matrix elements $|V_{cb}|$ and $|V_{ub}|$, semileptonic $B$ decays seem also promising to reveal new physics (NP) phenomena, in particular in connection with the possibility of uncovering lepton flavour universality (LFU) violating effects. In this view, it could be natural to connect the tensions in the inclusive versus exclusive determinations of $|V_{cb}|$ to the anomalies in the ratios $R(D^{(*)})$ of decay rates into $tau$ vs $mu, e$. However, the question has been raised about the role of the parametrization of the hadronic $B to D^{(*)}$ form factors in exclusive $B$ decay modes. We focus on the fully differential angular distributions of $bar B to D^* ell^-{bar u}_ell$ with $D^* to D pi$ or $D^* to D gamma$, the latter mode being important in the case of $B_s to D_s^*$ decays. We show that the angular coefficients in the distributions can be used to scrutinize the role of the form factor parametrization and to pin down deviations from SM. As an example of a NP scenario, we include a tensor operator in the $b to c$ semileptonic effective Hamiltonian, and discuss how the angular coefficients allow to construct observables sensitive to this structure, also defining ratios useful to test LFU.
We present a first measurement of the $bar{B^{0}} rightarrow D^{*+} ell^{-} u_{ell}$ branching fraction using fully reconstructed $B$ meson decays employing the Full Event Interpretation algorithm. Collision events corresponding to an integrated luminosity of lumi are analyzed, which were recorded by the Belle~II detector operated at the SuperKEKB accelerator complex. We measure$cal{B}(bar{B^{0}} rightarrow D^{*+} ell^{-} u_{ell}) =4.51 pm 0.41_{stat}pm0.27_{syst} pm0.45_{pi_s}$, with the first and second error denoting the statistical and systematic uncertainty, respectively, and the third dominant uncertainty is from the slow pion reconstruction efficiency.
We report a measurement of ratio ${cal R}(D^*) = {cal B}(bar{B}^0 rightarrow D^{*+} tau^- bar{ u}_{tau})/{cal B}(bar{B}^0 rightarrow D^{*+} ell^- bar{ u}_{ell})$, where $ell$ denotes an electron or a muon. The results are based on a data sample containing $772times10^6$ $Bbar{B}$ pairs recorded at the $Upsilon(4S)$ resonance with the Belle detector at the KEKB $e^+ e^-$ collider. We select a sample of $B^0 bar{B}^0$ pairs by reconstructing both $B$ mesons in semileptonic decays to $D^{*mp} ell^{pm}$. We measure ${cal R}(D^*)= 0.302 pm 0.030({rm stat)} pm 0.011({rm syst)}$, which is within $1.6 sigma$ of the Standard Model theoretical expectation, where $sigma$ is the standard deviation including systematic uncertainties.
We report a measurement of the ratio ${cal R}(D^*) = {cal B}(bar{B}^0 rightarrow D^{*+} tau^- bar{ u}_{tau})/{cal B}(bar{B}^0 rightarrow D^{*+} ell^- bar{ u}_{ell})$, where $ell$ denotes an electron or a muon. The results are based on a data sample containing $772times10^6$ $Bbar{B}$ pairs recorded at the $Upsilon(4S)$ resonance with the Belle detector at the KEKB $e^+ e^-$ collider. We select a sample of $B^0 bar{B}^0$ pairs by reconstructing both $B$ mesons in semileptonic decays to $D^{*mp} ell^{pm}$. We measure ${cal R}(D^*)= 0.302 pm 0.030({rm stat)} pm 0.011({rm syst)}$, which is within $1.6 sigma$ of the Standard Model theoretical expectation, where the standard deviation $sigma$ includes systematic uncertainties. We use this measurement to constrain several scenarios of new physics in a model-independent approach.
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