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Register a new userA calibration of the Belle II hadronic tag-side reconstruction algorithm with $B rightarrow Xell u$ decays
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Tag-side reconstruction is an important method for reconstructing $B$ meson decays with missing energy. The Belle II tag-side reconstruction algorithm, Full Event Interpretation, relies on a hierarchical reconstruction of $B$ meson decays with multivariate classification employed at each stage of reconstruction. Given the large numbers of classifiers employed and decay chains reconstructed, the performance of the algorithm on data and simulation differs significantly. Here, calibration factors are derived for hadronic tag-side $B$ decays by measuring a signal side decay, $B rightarrow Xell u$, in $34.6$ fb$^{-1}$ of Belle II data. For a very loose selection on the tag-side $B$ multivariate classifier, the calibration factors are $0.65 pm 0.02$ and $0.83 pm 0.03$ for tag-side $B^{+}$ and $B^{0}$ mesons, respectively.
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We present measurements of the first six hadronic mass moments in semileptonic $B rightarrow X_c ell u $ decays. The hadronic mass moments, together with other observables of inclusive $B$ decays, can be used to determine the CKM matrix element $|{V_{cb}}|$ and mass of the $b$-quark $m_b$ in the context of Heavy Quark Expansions of QCD. The Belle~II data recorded at the $Upsilon (4S)$ resonance in 2019 and 2020 (March-July), corresponding to an integrated luminosity of $34.6;mathrm{fb}^{-1}$, is used for this measurement. The decay $Upsilon (4S) rightarrow B overline{B}$ is reconstructed by applying the hadronic tagging algorithm provided by the Full Event Interpretation to fully reconstruct one $B$ meson. The second $B$ meson is reconstructed inclusively by selecting a high-momentum lepton. The $X_c$ system is identified by the remaining reconstructed tracks and clusters in the electromagnetic calorimeter. We report preliminary results for the hadronic mass moments $langle M_X^n rangle $ with $n=1,dots,6$, measured as a function of a lower cut on the lepton momentum in the signal $B$ rest frame.
This contribution illustrates a new search for the flavor-changing neutral-current decay ${B^+ to K^+ u bar u}$ performed by the Belle II experiment at the SuperKEKB asymmetric-energy electron-positron collider. In this study, a sample corresponding to an integrated luminosity of $63, rm{fb}^{-1}$ collected at the $rm Upsilon(4S)$ resonance and an additional sample of $9, rm{fb}^{-1}$ collected at an energy $60, rm MeV$ below the resonance are used. A novel technique, based on an inclusive tagging method and exploiting the topological features of the ${B^+ to K^+ u bar u}$ decay, is employed and it provides a higher signal efficiency with respect to the methods used in the previous searches. No significant signal is observed. An upper limit of $4.1 times 10^{-5}$ is set on the ${B^+ to K^+ u bar u}$ branching fraction at the $90, %$ confidence level.
With the full data sample of $772 times 10^6$ $B{bar B}$ pairs recorded by the Belle detector at the KEKB electron-positron collider, the decay $bar{B} rightarrow D^* tau^- bar{ u}_tau$ is studied with the hadronic $tau$ decays $tau^- rightarrow pi^- u_tau$ and $tau^- rightarrow rho^- u_tau$. The $tau$ polarization $P_tau(D^*)$ in two-body hadronic $tau$ decays is measured, as well as the ratio of the branching fractions $R(D^{*}) = mathcal{B}(bar {B} rightarrow D^* tau^- bar{ u}_tau) / mathcal{B}(bar{B} rightarrow D^* ell^- bar{ u}_ell)$, where $ell^-$ denotes an electron or a muon. Our results, $P_tau(D^*) = -0.38 pm 0.51 {rm (stat)} ^{+0.21}_{-0.16} {rm (syst)}$ and $R(D^*) = 0.270 pm 0.035{rm (stat)} ^{+0.028}_{-0.025}{rm (syst)}$, are consistent with the theoretical predictions of the Standard Model. The polarization values of $P_tau(D^*) > +0.5$ are excluded at the 90% confidence level.
This document presents the measurement of $B^0$ meson lifetimes using the 2019 Belle II dataset that corresponds to an integrated luminosity of $8.7 pm 0.2$ fb$^{-1}$. Each candidate is fully reconstructed with hadronic decay final states on the signal side, while the rest-of-event technique allows to infer the decay vertex position on the other (tag) side. $B^0$ lifetime is extracted from an unbinned maximum likelihood fit to the distribution of the difference between the signal side $B^0$ candidate and the tag side decay times. The measured lifetime is $tau_{B^0} = 1.48 pm 0.28 pm 0.06 $ ps, where the first uncertainty is statistical and the second is systematic.
Rare and flavour-changing neutral current decays of the B meson are an important probe in the search for physics beyond the Standard Model. There have recently been several anomalies in rare B decays, and lepton-universality measurements, specifically involving the $bto sell^+ell^-$ quark transition. These results tend towards a non-Standard-Model interpretation. The Belle II experiment is a next-generation b physics experiment located at SuperKEKB, an upgraded B factory $e^+e^-$ collider, in Tsukuba, Japan. The first collisions are expected in early 2018 with full physics data expected in 2019. This document describes prospects for several rare B decays at Belle II including $bto sell^+ell^-$ processes and others, such as $bto(s,,d)gamma$ and $bto s ubar u$. Areas where the Belle II program is complementary to that of the currently running LHCb experiment are highlighted.
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