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Register a new userB lifetime and $B^0-bar B^0$ mixing results from early Belle II data
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The Belle II experiment at the SuperKEKB energy-asymmetric $e^+ e^-$ collider is a substantial upgrade of the B factory facility at the Japanese KEK laboratory. The design luminosity of the machine is $8times 10^{35}$ cm$^{-2}$s$^{-1}$ and the Belle II experiment aims to record 50 ab$^{-1}$ of data, a factor of 50 more than its predecessor. From February to July 2018, the machine has completed a commissioning run, achieved a peak luminosity of $5.5times 10^{33}$ cm$^{-2}$s$^{-1}$, and Belle II has recorded a data sample of about 0.5 fb$^{-1}$. Main operation of SuperKEKB has started in March 2019. We use this dataset to characterize the performance of the detector regarding the tracking of charged particles, the reconstruction of known resonances, and the capability of identifying displaced decay vertices. To assess the B Physics capabilities of the experiment, one of the first benchmarks consists in the measurement of the lifetime of B mesons and of the $B^0-bar B^0$ mixing frequency. We present the first results, based on samples of B mesons that decay to hadronic and semileptonic final states.
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The Belle II experiment at the SuperKEKB energy-asymmetric $e^+e^-$ collider is a substantial upgrade of the B factory facility at the Japanese KEK laboratory. The design luminosity of the machine is $8times10^{35}, mathrm{cm}^{-2}mathrm{s}^{-1}$ and the Belle II experiment aims to record $50, mathrm{ab}^{-1}$ of data, a factor of 50 more than its predecessor. From February to July 2018, SuperKEKB has completed a commissioning run, achieved a peak luminosity of $5.5 times 10^{33}, mathrm{cm}^{-2}mathrm{s}^{-1}$, and Belle II recorded a data sample of about $0.5, mathrm{fb}^{-1}$. In this presentation we show first results from studying missing energy signatures, such as leptonic and semileptonic B meson decays based on this early Belle II data. We report first studies on re-measuring important standard candle processes, such as the abundant inclusive $Brightarrow X l u$ and $Bto D^*ell u$ decays, and evaluate the performance of machine learning-based tagging algorithms. Furthermore, we also present an overview of the semileptonic B decays that will be measured in the upcoming years at Belle II and discuss prospects for important B-anomalies like R$(D)$ and R$(D^*)$, as well as other tests of lepton flavor universality.
We perform a quantitative analysis of the $bbbar{b}bar{b}$ tetraquark decays into hidden- and open-bottom mesons and calculate, for the first time, the $bbbar{b}bar{b}$ tetraquark total decay width. On the basis of our results, we propose the $bbbar{b}bar{b} to B^{+} B^{-} (B^0 bar{B}^0) (B_s^0 bar{B}_s^0) to l^{+} l^{-}+text{X}$ decays as the most suitable channels to observe the $bbbar{b}bar{b}$ tetraquark states, since the calculated two-lepton cross section upper limit, $simeq 39 $ fb, is so large as to be potentially detectable with the 2018 LHCb sensitivity, paving the way to the observation of the $bbbar{b}bar{b}$ tetraquark in the future LHCb upgrade. The $4mu$ signal for the ground state, $J^{PC}=0^{++}$, is likely to be too small even for the upgraded LHCb, but it may not be hopeless for the $J^{PC}=2^{++}$ fully-bottom state.
The first observation of the decay $bar{B}^0_s to D^0 K^{*0}$ using $pp$ data collected by the LHCb detector at a centre-of-mass energy of 7 TeV, corresponding to an integrated luminosity of 36 pb$^{-1}$, is reported. A signal of $34.4 pm 6.8$ events is obtained and the absence of signal is rejected with a statistical significance of more than nine standard deviations. The $bar{B}^0_s to D^0 K^{*0}$ branching fraction is measured relative to that of $bar{B}^0 to D^0 rho^0$: $frac{{cal B}(bar{B}^0_s to D^0 K^{*0})}{{cal B}(bar{B}^0 to D^0 rho^0)} = 1.48 pm 0.34 pm 0.15 pm 0.12$, where the first uncertainty is statistical, the second systematic and the third is due to the uncertainty on the ratio of the $B^0$ and $B^0_s$ hadronisation fractions.
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.
The $B^0$--$bar B^0$ oscillation frequency $Delta m_d$ is measured by the LHCb experiment using a dataset corresponding to an integrated luminosity of $1.0,$fb$^{-1}$ of proton-proton collisions at $sqrt{s} = 7,$TeV, and is found to be $Delta m_d = 0.5156 pm 0.0051,($stat.$) pm 0.0033,($syst.$),$ps$^{-1}$. The measurement is based on results from analyses of the decays $B^0 to D^- pi^+$ ($D^- to K^+ pi^- pi^-$) and $B^0 to J/ psi K^{*0}$ ($ J/ psi to mu^+ mu^-$, $K^{*0} to K^+ pi^-$) and their charge conjugated modes.
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