64 Search Results

The K-long and muon detector (KLM) constitutes the outer-most volume of the Belle II spectrometer at the interaction region of the SuperKEKB collider in Tsukuba, Japan. The KLM detector was partially upgraded since the Belle experiment by replacing many of its resistive-plate chambers with scintillators containing wavelength-shifting fibers and instrumenting it with silicon photomultipliers. Here, we describe the readout electronics, firmware, and software created to control and acquire data from the scintillators and resistive-plate chambers.

Here, we report on the construction, operation, and performance of the Time-of-Propagation detector with imaging used for the Belle II experiment running at the Super-KEKB e⁺e^- collider. This detector is located in the central barrel region and uses Cherenkov light to provide particle identification among hadrons. The Cherenkov light is radiated in highly polished bars of synthetic fused silica (quartz) and transported to the ends of the bars via total internal reflection. One bar end is instrumented with finely segmented micro-channel-plate photomultiplier tubes to record the light, while the other end has a mirror attached to reflect the photons backmore » to the instrumented end. Both the propagation times and hit positions of the Cherenkov photons are measured; these depend on the Cherenkov angle and together provide good discrimination among charged pions, kaons, and protons with momenta up to around 4 GeV/c. To date, the detector has been used to record and analyze almost 600 fb⁻¹ of Belle II data.« less

We report a measurement of decay-time-dependent charge-parity ( $CP$ ) asymmetries in $B^0\to K_S^0{K}_S^0{K}_S^0$ decays. We use $387\times {10}^6$ $B\overline{B}$ pairs collected at the $\mathrm{\Upsilon }(4more »$ S ) resonance with the Belle II detector at the SuperKEKB asymmetric-energy electron-positron collider. We reconstruct 220 signal events and extract the $CP$ -violating parameters $S$ and $C$ from a fit to the distribution of the decay-time difference between the two $B$ mesons. The resulting confidence region is consistent with previous measurements in $B^0\to K_S^0{K}_S^0{K}_S^0$ and $B^0\to \left(c\overline{c}\right)K^0$ decays and with predictions based on the standard model. Published by the American Physical Society 2024« less

We search for the rare decay $B^{+}\to K^{+}\nu \overline{\nu }$ in a $362{\mathrm{fb}}^{-1}$ sample of electron-positron collisions at the $\mathrm{\Upsilon }\left(4S\right)$ resonance collected with the Belle II detector at the SuperKEKB collider. We use the inclusive properties of the accompanying $B$ meson in $\mathrm{\Upsilon }\left(4S\right)\to B\overline{B}$ events tomore » suppress background from other decays of the signal $B$ candidate and light-quark pair production. We validate the measurement with an auxiliary analysis based on a conventional hadronic reconstruction of the accompanying $B$ meson. For background suppression, we exploit distinct signal features using machine learning methods tuned with simulated data. The signal-reconstruction efficiency and background suppression are validated through various control channels. The branching fraction is extracted in a maximum likelihood fit. Our inclusive and hadronic analyses yield consistent results for the $B^{+}\to K^{+}\nu \overline{\nu }$ branching fraction of $[2.7\pm 0.5(\mathrm{stat})\pm 0.5(\mathrm{syst}\left)\right]\times {10}^{-5}$ and $\left[{1.1}_{-0.8}^{+0.9}\right(\mathrm{stat}{)}_{-0.5}^{+0.8}\left(\mathrm{syst}\right)]\times {10}^{-5}$ , respectively. Combining the results, we determine the branching fraction of the decay $B^{+}\to K^{+}\nu \overline{\nu }$ to be $[2.3\pm 0.5(\mathrm{stat}{)}_{-0.4}^{+0.5}\left(\mathrm{syst}\right)]\times {10}^{-5}$ , providing the first evidence for this decay at 3.5 standard deviations. The combined result is 2.7 standard deviations above the standard model expectation. Published by the American Physical Society 2024« less

We report measurements of the branching fractions and direct $CP$ asymmetries of the decays $B^0\to K^{+}{\pi }^{-}$ , $B^{+}\to K^{+}{\pi }^0$ , $B^{+}\to K^0{\pi }^{+}$ , and $B^0\to K^0{\pi }^0$ , and use these for testing the standard model through an isospin-based sum rule. In addition, we measure themore » branching fraction and direct $CP$ asymmetry of the decay $B^{+}\to {\pi }^{+}{\pi }^0$ and the branching fraction of the decay $B^0\to {\pi }^{+}{\pi }^{-}$ . The data are collected with the elle II detector from $e^{+}{e}^{-}$ collisions at the $\mathrm{\Upsilon }\left(4S\right)$ resonance produced by the SuperKEKB asymmetric-energy collider and contain $387\times {10}^6$ bottom-antibottom meson pairs. Signal yields are determined in two-dimensional fits to background-discriminating variables, and range from 500 to 3900 decays, depending on the channel. We obtain $-0.03\pm 0.13\pm 0.04$ for the sum rule in agreement with the standard model expectation of zero and with a precision comparable to the best existing determinations. Published by the American Physical Society 2024« less

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The first simultaneous determination of the absolute value of the Cabibbo-Kobayashi-Maskawa matrix element V_ub using inclusive and exclusive decays is performed with the full Belle data set at the $$Υ$$(4S) resonance, corresponding to an integrated luminosity of 711 fb^-1. We analyze collision events in which one B meson is fully reconstructed in hadronic modes. This allows for the reconstruction of the hadronic X_u system of the semileptonic b → $$πℓe_ℓ$$ decay. We separate exclusive B → $$πℓ\overline{ν}_ℓ$$ decays from other inclusive B → $$X_uℓ\overline{ν}_ℓ$$ and backgrounds with a two-dimensional fit that utilizes the number of charged pions in the X_umore » system and the four-momentum transfer q² between the B and X_u systems. Combining our measurement with information from lattice QCD and QCD calculations of the inclusive partial rate as well as external experimental information on the shape of the B → $$πℓe_ℓ$$ form factor, we determine |$$V^{excl}_{ub}$$| = (3.78 ± 0.23 ± 0.16 ± 0.14) × 10^-3 and |$$V^{incl}_{ub}$$| = (3.88 ± 0.20 ± 0.31 ± 0.09) x 10^-3, respectively, with the uncertainties being the statistical error, systematic errors, and theory errors. The ratio of |$$V^{excl}_{ub}$$|/|$$V^{incl}_{ub}$$| 0.97 ± 0.12 is compatible with unity.« less

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