We search for the rare decay B+ -> K+ v (v) over bar in a 362 fb(-1) sample of electron-positron collisions at the Upsilon(4S) resonance collected with the Belle II detector at the SuperKEKB collider. We use the inclusive properties of the accompanying B meson in Upsilon(4S) -> B (B) over bar events to 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+ -> K+ v (v) over bar branching fraction of [2.7 +/- 0.5(stat) +/- 0.5(syst)] x 10(-5) and [1.1(-0.8)(+0.9)(stat)(-0.5)(+0.8)(syst)] x 10(-5), respectively. Combining the results, we determine the branching fraction of the decay B+ -> K+ v (v) over bar to be [2.3 +/- 0.5_stat)(-0.4)(+0.5)(syst)] x 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.

We report measurements of the e(+)e(-) -> B (B) over bar, B (B) over bar*, and B*(B) over bar* cross sections at four energies, 10653, 10701, 10746 and 10805 MeV, using data collected by the Belle II experiment. We reconstruct one B meson in a large number of hadronic final states and use its momentum to identify the production process. In the first 2 - 5 MeV above B* B* threshold, the e(+)e(-) -> B*(B) over bar* cross section increases rapidly. This may indicate the presence of a pole close to the threshold.

Abstract We report results from a study of B±→ DK± decays followed by D decaying to the CP-even final state K+K− and CP-odd final state $$ {K}_S^0{\pi}^0 $$ K S 0 π 0 , where D is an admixture of D0 and $$ {\overline{D}}^0 $$ D ¯ 0 states. These decays are sensitive to the Cabibbo-Kobayashi-Maskawa unitarity-triangle angle ϕ3. The results are based on a combined analysis of the final data set of 772 × 106$$ B\overline{B} $$ B B ¯ pairs collected by the Belle experiment and a data set of 198 × 106$$ B\overline{B} $$ B B ¯ pairs collected by the Belle II experiment, both in electron-positron collisions at the Υ(4S) resonance. We measure the CP asymmetries to be $$ \mathcal{A} $$ A CP+ = (+12.5 ± 5.8 ± 1.4)% and $$ \mathcal{A} $$ A CP− = (−16.7 ± 5.7 ± 0.6)%, and the ratios of branching fractions to be $$ \mathcal{R} $$ R CP+ = 1.164 ± 0.081 ± 0.036 and $$ \mathcal{R} $$ R CP− = 1.151 ± 0.074 ± 0.019. The first contribution to the uncertainties is statistical, and the second is systematic. The asymmetries $$ \mathcal{A} $$ A CP+ and $$ \mathcal{A} $$ A CP− have similar magnitudes and opposite signs; their difference corresponds to 3.5 standard deviations. From these values we calculate 68.3% confidence intervals of (8.5° < ϕ3 < 16.5°) or (84.5° < ϕ3 < 95.5°) or (163.3° < ϕ3 < 171.5°) and 0.321 < rB < 0.465.

We report measurements of the e+e− → BB , BB^∗ , and B^∗B^∗ cross sections at four energies, 10653, 10701, 10746 and 10805 MeV, using data collected by the Belle II experiment. We reconstruct one B meson in a large number of hadronic final states and use its momentum to identify the production process. In the first 2 – 5 MeV above B^∗B^∗ threshold, the e+e− → B^∗B^∗ cross section increases rapidly. This may indicate the presence of a pole close to the threshold.

We report a measurement of decay-time dependent charge-parity (CP) asymmetries in B^0 → K^0_S K^0_S K^0_S decays. We use 387 × 10^6 BB̅ pairs collected at the Υ(4S) 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 → K^0_S K^0_S K^0_S and B^0 → (cc̅)K^0 decays, and with predictions based on the standard model.

Abstract We report measurements of the e + e − → B B ¯ $$ B\overline{B} $$ , B B ¯ ∗ $$ B{\overline{B}}^{\ast } $$ , and B ∗ B ¯ ∗ $$ {B}^{\ast }{\overline{B}}^{\ast } $$ cross sections at four energies, 10653, 10701, 10746 and 10805 MeV, using data collected by the Belle II experiment. We reconstruct one B meson in a large number of hadronic final states and use its momentum to identify the production process. In the first 2 – 5 MeV above B ∗ B ¯ ∗ $$ {B}^{\ast }{\overline{B}}^{\ast } $$ threshold, the e + e − → B ∗ B ¯ ∗ $$ {B}^{\ast }{\overline{B}}^{\ast } $$ cross section increases rapidly. This may indicate the presence of a pole close to the threshold.

We report measurements of the branching fractions and direct $\it{CP}$ 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 the branching fraction and direct $\it{CP}$ 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 Belle II detector from $e^+e^-$ collisions at the $\Upsilon(4S)$ 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.

We search for the rare decay B+→K+νν¯ in a 362 fb−1 sample of electron-positron collisions at the ϒ(4S) resonance collected with the Belle II detector at the SuperKEKB collider. We use the inclusive properties of the accompanying B meson in ϒ(4S)→BB¯ events to 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+→K+νν¯ branching fraction of [2.7±0.5(stat)±0.5(syst)]×10−5 and [1.1−0.8+0.9(stat)−0.5+0.8(syst)]×10−5, respectively. Combining the results, we determine the branching fraction of the decay B+→K+νν¯ to be [2.3±0.5(stat)−0.4+0.5(syst)]×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

We study the processes e^{+}e^{-}→ωχ_{bJ}(1P) (J=0, 1, or 2) using samples at center-of-mass energies sqrt[s]=10.701, 10.745, and 10.805 GeV, corresponding to 1.6, 9.8, and 4.7  fb^{-1} of integrated luminosity, respectively. These data were collected with the Belle II detector during special operations of the SuperKEKB collider above the ϒ(4S) resonance. We report the first observation of ωχ_{bJ}(1P) signals at sqrt[s]=10.745  GeV. By combining Belle II data with Belle results at sqrt[s]=10.867  GeV, we find energy dependencies of the Born cross sections for e^{+}e^{-}→ωχ_{b1,b2}(1P) to be consistent with the shape of the ϒ(10753) state. These data indicate that the internal structures of the ϒ(10753) and ϒ(10860) states may differ. Including data at sqrt[s]=10.653  GeV, we also search for the bottomonium equivalent of the X(3872) state decaying into ωϒ(1S). No significant signal is observed for masses between 10.45 and 10.65  GeV/c^{2}.

We present a measurement of time-dependent rate asymmetries in B^0→ϕ K^0_S decays to search for non-standard-model physics in b→ q qs transitions. The data sample is collected with the Belle II detector at the SuperKEKB asymmetric-energy e^+e^- collider in 2019-2022 and contains (387± 6)× 10^6 bottom-antibottom mesons from Υ(4S) resonance decays. We reconstruct 162±17 signal events and extract the charge-parity (CP) violating parameters from a fit to the distribution of the proper-decay-time difference of the two B mesons. The measured direct and mixing-induced CP asymmetries are A=0.31±0.20±0.05 and S=0.54±0.26^+0.06_-0.08, respectively, where the first uncertainties are statistical and the second are systematic. The results are compatible with the CP asymmetries observed in b→ cc s transitions.

The L_{μ}-L_{τ} extension of the standard model predicts the existence of a lepton-flavor-universality-violating Z^{'} boson that couples only to the heavier lepton families. We search for such a Z^{'} through its invisible decay in the process e^{+}e^{-}→μ^{+}μ^{-}Z^{'}. We use a sample of electron-positron collisions at a center-of-mass energy of 10.58 GeV collected by the Belle II experiment in 2019-2020, corresponding to an integrated luminosity of 79.7  fb^{-1}. We find no excess over the expected standard-model background. We set 90%-confidence-level upper limits on the cross section for this process as well as on the coupling of the model, which ranges from 3×10^{-3} at low Z^{'} masses to 1 at Z^{'} masses of 8  GeV/c^{2}.

We present measurements of the first to fourth moments of the lepton mass squared q(2) of B -> X(c)l (nu) over bar (l) decays for l = e, mu and with X-c a hadronic system containing a charm quark. These results use a sample of electron-positron collisions at the Upsilon(4S) resonance corresponding to 62.8 fb(-1) of integrated luminosity and collected by the Belle II 2 experiment in 2019 and 2020. To identify the X-c system and reconstruct q(2), one of the B mesons from an Upsilon(4S) -> B (B) over bar decay is fully reconstructed in a hadronic decay mode using a multivariate B tagging algorithm. We report raw and central moments for q(2) > 1.5 GeV2/c(4) up to q(2) > 8.5 GeV2/c(4), probing up to 77% of the accessible B -> X(c)l (nu) over bar (l) phase space. This is the first measurement of moments in the experimentally challenging range of [1.5, 2.5] GeV2/c(4). The results can be used for a new determination of vertical bar V-cb vertical bar using inclusive B -> X(c)l (nu) over bar (l) decays.

We determine the Cabibbo-Kobayashi-Maskawa matrix-element magnitude vertical bar V-cb vertical bar using (B) over bar (0) -> D*(+)l(-)(nu) over bar (l) decays reconstructed in 189 fb(-1) of collision data collected by the Belle II experiment, located at the SuperKEKB e(+)e(-) collider. Partial decay rates are reported as functions of the recoil parameter w and three decay angles separately for electron and muon final states. We obtain vertical bar V-cb vertical bar using the Boyd-Grinstein-Lebed and Caprini-Lellouch-Neubert parametrizations, and find vertical bar V-cb vertical bar(BGL) = (40.57 +/- 0.31 +/- 0.95 +/- 0.58) x 10(-3) and vertical bar V-cb vertical bar(CLN) = (40.13 +/- 0.27 +/- 0.93 +/- 0.58) x 10(-3) with the uncertainties denoting statistical components, systematic components, and components from the lattice QCD input, respectively. The branching fraction is measured to be B((B) over bar (0) -> D*(+)l(-)(nu) over bar (l)) = (4.922 +/- 0.023 +/- 0.220)%. The ratio of branching fractions for electron and muon final states is found to be 0.998 +/- 0.009 +/- 0.020. In addition, we determine the forward-backward angular asymmetry and the D*(+) longitudinal polarization fractions. All results are compatible with lepton-flavor universality in the Standard Model.

We measure the lifetime of the D_{s}^{+} meson using a data sample of 207  fb^{-1} collected by the Belle II experiment running at the SuperKEKB asymmetric-energy e^{+}e^{-} collider. The lifetime is determined by fitting the decay-time distribution of a sample of 116×10^{3} D_{s}^{+}→ϕπ^{+} decays. Our result is τ_{D_{s}^{+}}=(499.5±1.7±0.9)  fs, where the first uncertainty is statistical and the second is systematic. This result is significantly more precise than previous measurements.

We present a measurement of the tau-lepton mass using a sample of about 175 million e(+) e(-) -> tau(+) tau(-) events collected with the Belle II detector at the SuperKEKB e(+) e(-) collider at a center-of-mass energy of 10.579 GeV. This sample corresponds to an integrated luminosity of 190 fb(-1). We use the kinematic edge of the tau pseudomass distribution in the decay tau(-) -> pi(-) pi(+) pi(-) nu(tau) and measure the t mass to be 1777.09 +/- 0.08 +/- 0.11 MeV/c(2), where the first uncertainty is statistical and the second systematic. This result is the most precise to date.

We study the processes e(+)e(-)+ omega chi(bJ)(1P) (J = 0, 1, or 2) using samples at center-of-mass energies p = 10.701, 10.745, and 10.805 GeV, corresponding to 1.6, 9.8, and 4.7 fb(-1) of integrated luminosity, ffiffis respectively. These data were collected with the Belle II detector during special operations of the SuperKEKB collider above the gamma(4S) resonance. We report the first observation of omega chi(bJ)(1P) signals at p = 10.745 GeV. By combining Belle II data with Belle results at ffis ffi ffiffis p = 10.867 GeV, we find energy dependencies of the Born cross sections for e(+)e(-)+ omega chi(b1;b2)(1P) to be consistent with the shape of the gamma(10753) state. These data indicate that the internal structures of the gamma(10753) and gamma(10860) states may ffiffidiffer. Including data at root s = 10.653 GeV, we also search for the bottomonium equivalent of the X(3872) state decaying into omega gamma(1S). No significant signal is observed for masses between 10.45 and 10.65 GeV/c(2).

An absolute measurement of the Λ_{c}^{+} lifetime is reported using Λ_{c}^{+}→pK^{-}π^{+} decays in events reconstructed from data collected by the Belle II experiment at the SuperKEKB asymmetric-energy electron-positron collider. The total integrated luminosity of the data sample, which was collected at center-of-mass energies at or near the ϒ(4S) resonance, is 207.2  fb^{-1}. The result, τ(Λ_{c}^{+})=203.20±0.89±0.77  fs, where the first uncertainty is statistical and the second systematic, is the most precise measurement to date and is consistent with previous determinations.

We report measurements of the branching fraction and CP asymmetry in B-0 -> pi(0)pi(0) decays reconstructed at Belle II in an electron-positron collision sample containing 198 x 10(6) BB pairs. We measure a branching fraction B(B-0 -> pi(0)pi(0)) = (1.38 +/- 0.27 +/- 0.22) x 10(-6) and a CP asymmetry A(CP)(B-0 -> pi(0)pi(0)) = 0.14 +/- 0.46 +/- 0.07, where the first uncertainty is statistical and the second is systematic.

Additional spin-0 particles appear in many extensions of the standard model. We search for long-lived spin-0 particles $S$ in $B$-meson decays mediated by a $b\to s$ quark transition in $e^+e^-$ collisions at the $\Upsilon(4S)$ resonance at the Belle II experiment. Based on a sample corresponding to an integrated luminosity of $189 \mathrm{\,fb}^{-1}$, we observe no evidence for signal. We set model-independent upper limits on the product of branching fractions $\mathrm{Br}(B^0\to K^*(892)^0(\to K^+\pi^-)S)\times \mathrm{Br}(S\to x^+x^-)$ and $\mathrm{Br}(B^+\to K^+S)\times \mathrm{Br}(S\to x^+x^-)$, where $x^+x^-$ indicates $e^+e^-, \mu^+\mu^-, \pi^+\pi^-$, or $K^+K^-$, as functions of $S$ mass and lifetime at the level of $10^{-7}$.

We report the first search for a nonstandard-model resonance decaying into τ pairs in e^{+}e^{-}→μ^{+}μ^{-}τ^{+}τ^{-} events in the 3.6-10  GeV/c^{2} mass range. We use a 62.8  fb^{-1} sample of e^{+}e^{-} collisions collected at a center-of-mass energy of 10.58 GeV by the Belle II experiment at the SuperKEKB collider. The analysis probes three different models predicting a spin-1 particle coupling only to the heavier lepton families, a Higgs-like spin-0 particle that couples preferentially to charged leptons (leptophilic scalar), and an axionlike particle, respectively. We observe no evidence for a signal and set exclusion limits at 90% confidence level on the product of cross section and branching fraction into τ pairs, ranging from 0.7 to 24 fb, and on the couplings of these processes. We obtain world-leading constraints on the couplings for the leptophilic scalar model for masses above 6.5  GeV/c^{2} and for the axionlike particle model over the entire mass range.