A search for the flavor-changing neutral-current decay B+ →K+$ν\bar{ν}$ is performed at the Belle II experiment at the SuperKEKB asymmetric energy electron-positron collider. The data sample ...corresponds to an integrated luminosity of 63 fb-1 collected at the Υ (4S) resonance and a sample of 9 fb-1 collected at an energy 60 MeV below the resonance. Because the measurable decay signature involves only a single charged kaon, a novel measurement approach is used that exploits not only the properties of the B+ →K+$ν\bar{ν}$ decay, but also the inclusive properties of the other B meson in the Υ(4S) $B\bar{B}$ event, to suppress the background from other B meson decays and light-quark pair production. This inclusive tagging approach offers a higher signal efficiency compared to previous searches. No significant signal is observed. An upper limit on the branching fraction of B+ →K+$ν\bar{ν}$ of 4.1 × 10-5 is set at the 90% confidence level.
We present measurements of partial branching fractions of inclusive semileptonic B → Xuℓ+ νℓ decays using the full Belle dataset of 711 fb−1 of integrated luminosity at the Υ(4S) resonance and for ℓ ...= e, μ. Inclusive semileptonic B → Xuℓ+ νℓ decays are Cabibbo-Kobayashi-Maskawa (CKM) suppressed and measurements are complicated by the large background from CKM favored B → Xcℓ+ νℓ transitions, which have a similar signature. Using machine learning techniques, we reduce this and other backgrounds effectively, while retaining access to a large fraction of the B → Xuℓ+νℓ phase space and high signal efficiency. We measure partial branching fractions in three phase-space regions covering about 31% to 86% of the accessible B → Xuℓ+νℓ phase space. The most inclusive measurement corresponds to the phase space with lepton energies of EBℓ > 1 GeV, and we obtain ΔB ( B → Xuℓ+νℓ) = (1.59 ± 0.07 ± 0.16) × 10−3 from a two-dimensional fit of the hadronic mass spectrum and the four-momentum-transfer squared distribution, with the uncertainties denoting the statistical and systematic error. We find |Vub| = (4.10 ± 0.09 ± 0.22 ± 0.15) × 10−3 from an average of four calculations for the partial decay rate with the third uncertainty denoting the average theory error. This value is higher but compatible with the determination from exclusive semileptonic decays within 1.3 standard deviations. In addition, we report charmless inclusive partial branching fractions separately for B + and B 0 mesons as well as for electron and muon final states. No isospin breaking or lepton flavor universality violating effects are observed.
We report results from a study of the spin and parity of Ξ c ( 2970 ) + using a 980 fb −1 data sample collected by the Belle detector at the KEKB asymmetric-energy e+e− collider. The decay angle ...distributions in the chain Ξc (2970)+→ Ξc (2645) 0π+ → Ξ+c π− π+ are analyzed to determine the spin of this charmed-strange baryon. The angular distributions strongly favor the Ξc (2970)+ spin J = 1/2 over 3/2 or 5/2, under an assumption that the lowest partial wave dominates in the decay. We also measure the ratio of Ξc (2970)+ decay branching fractions R = B Ξc (2970)+ → Ξc (2645)0π+/B Ξc (2970)+ → Ξ0c π+ = 1.67 ± 0.29 (stat)+0.15 −0.09 (syst) ± 0.25 (IS), where the last uncertainty is due to possible isospin-symmetry-breaking effects. This R value favors the spin-parity J P = 1/2+ with the spin of the light-quark degrees of freedom sl = 0. This is the first determination of the spin and parity of a charmed-strange baryon.
We report measurements of the production cross sections of charged pions, kaons, and protons as a function of fractional energy, the event-shape variable called thrust, and the transverse momentum ...with respect to the thrust axis. These measurements access the transverse momenta created in the fragmentation process, which are of critical importance to the understanding of any transverse-momentum-dependent distribution and fragmentation functions. The low transverse-momentum part of the cross sections can be well described by Gaussians in transverse momentum as is generally assumed but the fractional-energy dependence is nontrivial and different hadron types have varying Gaussian widths. The width of these Gaussians decreases with thrust and shows an initially rising, then decreasing fractional-energy dependence. The widths for pions and kaons are comparable within uncertainties, while those for protons are significantly narrower. These single-hadron cross sections and Gaussian widths are obtained from a 558 fb−1 data sample collected at the ϒ(4S) resonance with the Belle detector at the KEKB asymmetric-energy e+e− collider.
We present the first measurements of the absolute branching fractions of Ξc+ decays into Ξ−π+π+ and pK−π+ final states. Our analysis is based on a data set of (772±11)×106 BB¯ pairs collected at the ...ϒ(4S) resonance with the Belle detector at the KEKB e+e− collider. We measure the absolute branching fraction of B¯0→Λ¯c−Ξc+ with the Ξc+ recoiling against Λ¯c− in B¯0 decays resulting in B(B¯0→Λ¯c−Ξc+)=1.16±0.42(stat.)±0.15(syst.)×10−3. We then measure the product branching fractions B(B¯0→Λ¯c−Ξc+)B(Ξc+→Ξ−π+π+) and B(B¯0→Λ¯c−Ξc+)B(Ξc+→pK−π+). Dividing these product branching fractions by B¯0→Λ¯c−Ξc+ yields B(Ξc+→Ξ−π+π+)=2.86±1.21(stat.)±0.38(syst.)% and B(Ξc+→pK−π+)=0.45±0.21(stat.)±0.07(syst.)%. Our result for B(Ξc+→Ξ−π+π+) can be combined with Ξc+ branching fractions measured relative to Ξc+→Ξ−π+π+ to set the absolute scale for many Ξc+ branching fractions.
Using a data sample of 921.9 fb-1 collected with the Belle detector, we study the process of $e^+e^- → D^+_s D_{s1}(2536)^-+c.c.$ via initial-state radiation. We report the first observation of a ...vector charmoniumlike state decaying to $D^+_s D_{s1}(2536)^-+c.c.$ with a significance of $5.9σ$, including systematic uncertainties. The measured mass and width are $(4625.9^{+6.2}_{-6.0}$(stat)$±0.4$(syst)) MeV/$c_2$ and ($49.8^{+13.9}_{-11.5}$(stat)$±4.0$(syst)) MeV, respectively. The product of the $e^+e^- → D^+_s D_{s1}(2536)^-+c.c.$ cross section and the branching fraction of $D_{s1}(2536)^- → \bar{D}^{*0}K^-$ is measured from the $D_s \bar{D}_{s1}(2536)$ threshold to $5.59$ GeV.
We report measurements of the branching fractions of singly Cabibbo-suppressed decays Λc+ → pη and Λc+ → pπ0 using the full Belle data sample corresponding to an integrated luminosity of 980.6 ...fb−1. The data were collected by the Belle detector at the KEKB e+ e− asymmetric energy collider. A clear Λ + c signal is seen in the invariant mass distribution of p η . The fitted number of signal events of the Λc+ → pη process is 7734 ± 263 ; from this, we measure the ratio of branching fractions B ( Λc+→ pη ) / B ( Λc+ → pK− π+ ) = 2.258 ± 0. 077 ( stat ) ± 0.122 ( syst ) × 10 − 2 , from which we infer the branching fraction B ( Λc+ → pη ) = 1.42 ± 0.05 ( stat ) ± 0.11 ( syst ) × 10−3. In addition, no significant signal for Λc+ → pπ0 is found, so an upper limit on the branching fraction of B ( Λc+ → pπ0) < 8.0 × 10−5 at a 90% credibility level is set, more than 3 times better than the best current upper limit.
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