Abstract We report a search for the charged-lepton flavor violation in Υ(2S) → ℓ∓τ± (ℓ = e, μ) decays using a 25 fb−1 Υ(2S) sample collected by the Belle detector at the KEKB e+e− asymmetric-energy collider. We find no evidence for a signal and set upper limits on the branching fractions ($$ \mathcal{B} $$ B ) at 90% confidence level. We obtain the most stringent upper limits: $$ \mathcal{B} $$ B (Υ(2S) → μ∓τ±) < 0.23 × 10−6 and $$ \mathcal{B} $$ B (Υ(2S) → e∓τ±) < 1.12 × 10−6.

We report a study of the hadronic transitions χ_bJ(2P)(1S), with ω^+π^-π^0, using 28.2×10^6 Υ(3S) mesons recorded by the Belle detector. We present the first evidence for the near–threshold transition χ_b0(2P)(1S), the analog of the near-threshold charm sector decay χ_c1(3872)/ψ, with a branching fraction of B(χ_b0(2P)(1S)) = (0.55±0.19±0.07)%. We also obtain branching fractions of B(χ_b1(2P)(1S)) = (2.39^+0.20_-0.19±0.24)% and B(χ_b2(2P)(1S)) = (0.47^+0.13_-0.12±0.06)%, confirming the measurement of the ω transitions of the J=1,2 P–wave states. The ratio for the J=2 to J=1 transitions is also measured and found to differ by 3.3 standard deviations from the expected value in the QCD multipole expansion.

A dark leptophilic scalar (ϕL) is a hypothetical particle that couples only to leptons rather than quarks. We report on a search for ϕL in the e+e−→τ+τ−ϕL, ϕL→ℓ+ℓ−(ℓ=e,μ) process using 626 fb−1 of data collected by the Belle experiment near the ϒ(4S) resonance. We validate the backgrounds with multiple control regions in data, using a novel multiclass multivariate event classifier. In absence of a signal, we quote upper limits at the 90% confidence level on the coupling between ϕL and leptons. Our bounds, obtained in a blinded approach, are 19% more constraining than the previous limits, averaged over the mass range 0.04≤mϕL≤6.5 GeV. We exclude the parameter space below 4 GeV favored by measurement of the anomalous magnetic moment of the muon.Received 15 July 2023Revised 11 October 2023Accepted 13 December 2023DOI:https://doi.org/10.1103/PhysRevD.109.032002Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP3.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasLepton productionParticle dark matterPhysical SystemsLepton collidersTau leptonsParticles & Fields

We search for the B^0→ pΣ̅^0π^- decay with Σ̅^0 →Λ̅γ, where the γ is not measured, using a data sample corresponding to an integrated luminosity of 711 fb^-1 which contains 772 × 10^6 BB̅ pairs, collected around the Υ(4S) resonance with the Belle detector at the KEKB asymmetric-energy e^+e^- collider. We measure for the first time the B^0→ pΣ̅^0π^- branching fraction to be ℬ(B^0 → p Σ̅^0 π^-) = (1.17^+0.43_-0.40(stat)± 0.07(syst)) × 10^-6 with a significance of 3.0σ. We simultaneously measure the branching fraction for the related channel B^0→ pΛ̅π^- with much improved precision.

We report the results of the first search for the decay B_s^0→π^0π^0 using 121.4 fb^-1 of data collected at the Υ(5 S) resonance with the Belle detector at the KEKB asymmetric-energy e^+e^- collider. We observe no signal and set a 90% confidence level upper limit of 7.7× 10^-6 on the B_s^0→π^0π^0 decay branching fraction.

We report the measurement of branching fractions and $CP$-violation asymmetries in $B\to \phi \phi K$ decays based on a $711\,{\rm fb}^{-1}$ data sample containing $772\times 10^6$ $B\bar{B}$ events. The data were recorded at the $\Upsilon (4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^+ e^-$ collider. For $B^+ \to \phi \phi K^+$, the branching fraction and $CP$-violation asymmetry measured below the $\eta_{c}$ threshold ($m_{\phi\phi}<2.85\,{\rm GeV}/c^2$) are $[3.43^{\,+\,0.48}_{\,-\,0.46}({\rm stat})\pm 0.22({\rm syst})] \times10^{-6}$ and $-0.02\pm0.11({\rm stat})\pm0.01({\rm syst})$, respectively. Similarly, the branching fraction obtained for $B^0 \to\phi\phi K^0$ below the $\eta_{c}$ threshold is $[3.02^{\,+\,0.75}_{\,-\,0.66} ({\rm stat})\pm \,0.20({\rm syst})]\times10^{-6}$. We also measure the $CP$-violation asymmetry for $B^+ \to\phi\phi K^+$ within the $\eta_{c}$ region ($m_{\phi\phi}\in [2.94,3.02]\,{\rm GeV}/c^2$) to be $+0.12\pm0.12({\rm stat})\pm0.01({\rm syst})$.

Abstract We report the measurement of the two-photon decay width of χc2(1P) in two-photon processes at the Belle experiment. We analyze the process γγ → χc2(1P) → J/ψγ, J/ψ → ℓ+ℓ− (ℓ = e or μ) using a data sample of 971 fb−1 collected with the Belle detector at the KEKB e+e− collider. In this analysis, the product of the two-photon decay width of χc2(1P) and the branching fraction is determined to be $$ {\Gamma}_{\gamma \gamma}\left({\chi}_{c2}(1P)\right)\mathcal{B}\left({\chi}_{c2}(1P)\to J/\psi \gamma \right)\mathcal{B}\left(J/\psi \to {\ell}^{+}{\ell}^{-}\right)=14.8\pm 0.3\left(\textrm{stat}.\right)\pm 0.7\left(\textrm{syst}.\right) $$ Γ γγ χ c 2 1 P B χ c 2 1 P → J / ψγ B J / ψ → ℓ + ℓ − = 14.8 ± 0.3 stat . ± 0.7 syst . eV, which corresponds to Γγγ(χc2(1P)) = 653 ± 13(stat.) ± 31(syst.) ± 17(B.R.) eV, where the third uncertainty is from $$ \mathcal{B} $$ B (χc2(1P) → J/ψγ) and $$ \mathcal{B} $$ B (J/ψ → ℓ+ℓ−).

Using a data sample of 980 fb(-1) collected with the Belle detector at the KEKB asymmetric-energy e(+)e(-) collider, we study for the first time the singly Cabibbo-suppressed decays omega(0)(c) -(sic) -pi(+) and omega K--(+) and the doubly Cabibbo-suppressed decay omega(0)(c) -(sic)K--(+). Evidence for an Q0 c signal in the omega(0)(c) -E-pi+ mode is reported with a significance of 4.5 sigma including systematic uncertainties. The ratio of branching fractions to the normalization mode omega(0)(c) -omega(-)pi(+ )is measured to beB(omega(0)(c) -(sic)(-)pi(+))/B(omega(0)(c) -Q-pi+) = 0.253 +/- 0.052(stat.) +/- 0.030(syst.).No significant signals of omega(0)(c) -(sic)K--(+) and omega K--(+) modes are found. The upper limits at 90% confidence level on ratios of branching fractions are determined to beB(omega(0)(c) -> (sic)K--(+))/B(omega(0)(c) -> omega-pi(+)) < 0.070 andB(omega(0)(c) -> omega K--(+))/B(omega(0)(c) -> omega-pi(+)) < 0.29.

Abstract We report the first measurement of the inclusive e+e−→$$ b\overline{b} $$ b b ¯ →$$ {D}_s^{\pm } $$ D s ± X and e+e−→$$ b\overline{b} $$ b b ¯ → D0/$$ {\overline{D}}^0 $$ D ¯ 0 X cross sections in the energy range from 10.63 to 11.02 GeV. Based on these results, we determine σ(e+e−→$$ {B}_s^0{\overline{B}}_s^0 $$ B s 0 B ¯ s 0 X) and σ(e+e−→$$ B\overline{B} $$ B B ¯ X) in the same energy range. We measure the fraction of $$ {B}_s^0 $$ B s 0 events at Υ(10860) to be fs = ($$ {22.0}_{-2.1}^{+2.0} $$ 22.0 − 2.1 + 2.0 )%. We determine also the ratio of the $$ {B}_s^0 $$ B s 0 inclusive branching fractions $$ \mathcal{B} $$ B ($$ {B}_s^0 $$ B s 0 → D0/$$ {\overline{D}}^0 $$ D ¯ 0 X)/$$ \mathcal{B} $$ B ($$ {B}_s^0 $$ B s 0 →$$ {D}_s^{\pm } $$ D s ± X) = 0.416 ± 0.018 ± 0.092. The results are obtained using the data collected with the Belle detector at the KEKB asymmetric-energy e+e− collider.

We present the first measurement of the Michel parameter $\xi^\prime$ in the $\tau^-\to\mu^-\bar{\nu}_\mu\nu_\tau$ decay using the full data sample of $988\,\text{fb}^{-1}$ collected by the Belle detector operating at the KEKB asymmetric energy $e^+ e^-$ collider. The method is based on the reconstruction of the $\mu^- \to e^- \bar{\nu}_e\nu_\mu$ decay-in-flight in the Belle central drift chamber and relies on the correlation between muon spin and its daughter electron momentum. We study the main sources of the background that can imitate the signal decay, such as kaon and pion decays-in-flight and charged particle scattering on the detector material. Highly efficient methods of their suppression are developed and applied to select 165 signal-candidate events. We obtain $\xi^\prime=0.22\pm0.94\pm0.42$ where the first uncertainty is statistical, and the second one is systematic. The result is in agreement with the Standard Model prediction of $\xi^\prime=1$.

We present measurements of the branching fractions for the singly Cabibbo-suppressed decays $D^+\to K^{+}K^{-}\pi^{+}\pi^{0}$ and $D_s^{+}\to K^{+}\pi^{-}\pi^{+}\pi^{0}$, and the doubly Cabibbo-suppressed decay $D^{+}\to K^{+}\pi^{-}\pi^{+}\pi^{0}$, based on 980 ${\rm fb}^{-1}$ of data recorded by the Belle experiment at the KEKB $e^{+}e^{-}$ collider. We measure these modes relative to the Cabibbo-favored modes $D^{+}\to K^{-}\pi^{+}\pi^{+}\pi^{0}$ and $D_s^{+}\to K^{+}K^{-}\pi^{+}\pi^{0}$. Our results for the ratios of branching fractions are $B(D^{+}\to K^{+}K^{-}\pi^{+}\pi^{0})/B(D^{+}\to K^{-}\pi^{+}\pi^{+}\pi^{0}) = (11.32 \pm 0.13 \pm 0.26)\%$, $B(D^{+}\to K^{+}\pi^{-}\pi^{+}\pi^{0})/B(D^{+}\to K^{-}\pi^{+}\pi^{+}\pi^{0}) = (1.68 \pm 0.11\pm 0.03)\%$, and $B(D_s^{+}\to K^{+}\pi^{-}\pi^{+}\pi^{0})/B(D_s^{+}\to K^{+}K^{-}\pi^{+}\pi^{0}) = (17.13 \pm 0.62 \pm 0.51)\%$, where the uncertainties are statistical and systematic, respectively. The second value corresponds to $(5.83\pm 0.42)\times\tan^4\theta_C$, where $\theta_C$ is the Cabibbo angle; this value is larger than other measured ratios of branching fractions for a doubly Cabibbo-suppressed charm decay to a Cabibbo-favored decay. Multiplying these results by world average values for $B(D^{+}\to K^{-}\pi^{+}\pi^{+}\pi^{0})$ and $B(D_s^{+}\to K^{+}K^{-}\pi^{+}\pi^{0})$ yields $B(D^{+}\to K^{+}K^{-}\pi^{+}\pi^{0})= (7.08\pm 0.08\pm 0.16\pm 0.20)\times10^{-3}$, $B(D^{+}\to K^{+}\pi^{-}\pi^{+}\pi^{0})= (1.05\pm 0.07\pm 0.02\pm 0.03)\times10^{-3}$, and $B(D_s^{+}\to K^{+}\pi^{-}\pi^{+}\pi^{0}) = (9.44\pm 0.34\pm 0.28\pm 0.32)\times10^{-3}$, where the third uncertainty is due to the branching fraction of the normalization mode. The first two results are consistent with, but more precise than, the current world averages. The last result is the first measurement of this branching fraction.

We present a measurement of the differential shapes of exclusive B→ D^* ℓν̅_ℓ (B = B^-, B̅^0 and ℓ = e, μ) decays with hadronic tag-side reconstruction for the full Belle data set of 711 fb^-1 integrated luminosity. We extract the Caprini-Lellouch-Neubert (CLN) and Boyd-Grinstein-Lebed (BGL) form factor parameters and use an external input for the absolute branching fractions to determine the Cabibbo-Kobayashi-Maskawa matrix element and find |V_cb|_CLN = (40.1±0.9)× 10^-3 and |V_cb|_BGL = (40.6± 0.9)× 10^-3 with the zero-recoil lattice QCD point ℱ(1) = 0.906 ± 0.013. We also perform a study of the impact of preliminary beyond zero-recoil lattice QCD calculations on the |V_cb| determinations. Additionally, we present the lepton flavor universality ratio R_eμ = ℬ(B → D^* e ν̅_e) / ℬ(B → D^* μν̅_μ) = 0.990 ± 0.021 ± 0.023, the electron and muon forward-backward asymmetry and their difference Δ A_FB=0.022±0.026± 0.007, and the electron and muon D^* longitudinal polarization fraction and their difference Δ F_L^D^* = 0.034 ± 0.024 ± 0.007. The uncertainties quoted correspond to the statistical and systematic uncertainties, respectively.

We present the analysis of two-particle angular correlations using coordinate systems defined with the conventional beam axis and the event thrust axis. We propose the latter as a good representation for the correlation structure interpretation in the e+e− collision system. The e+e− collisions to hadronic final states at center-of-mass energies of √(s) = 10.52 GeV and 10.58 GeV are recorded by the Belle detector at KEKB. In this paper, results on the first dataset are supplementary to the previous Belle publication [1]. At the same time, the latter is the first two-particle correlation measurement at collision energy on the Υ(4S) resonance and is sensitive to its decay products. Measurements are reported as a function of the charged-particle multiplicity. Finally, a qualitative understanding of the correlation structure is discussed using a combination of Monte Carlo simulations and experimental data.

We report on a search for a heavy Majorana neutrino in the decays τ^{-}→π^{-}ν_{h}, ν_{h}→π^{±}ℓ^{∓}, ℓ=e, μ. The results are obtained using the full data sample of 988  fb^{-1} collected with the Belle detector at the KEKB asymmetric energy e^{+}e^{-} collider, which contains 912×10^{6}  ττ pairs. We observe no significant signal and set 95% CL upper limits on the couplings of the heavy right-handed neutrinos to the conventional standard model left-handed neutrinos in the mass range 0.2-1.6  GeV/c^{2}. This is the first study of a mixed couplings of heavy neutrinos to τ leptons and light-flavor leptons.

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}.

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}.

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→uℓν[over ¯]_{ℓ} decay. We separate exclusive B→πℓν[over ¯]_{ℓ} decays from other inclusive B→X_{u}ℓν[over ¯]_{ℓ} and backgrounds with a two-dimensional fit that utilizes the number of charged pions in the X_{u} system and the four-momentum transfer q^{2} 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→πℓν[over ¯]_{ℓ} form factor, we determine |V_{ub}^{excl}|=(3.78±0.23±0.16±0.14)×10^{-3} and |V_{ub}^{incl}|=(3.88±0.20±0.31±0.09)×10^{-3}, respectively, with the uncertainties being the statistical error, systematic errors, and theory errors. The ratio of |V_{ub}^{excl}|/|V_{ub}^{incl}|=0.97±0.12 is compatible with unity.

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 present measurements of the branching fractions for the singly Cabibbo-suppressed decays D+ -K+K-x-+x-0 and D+s -K+x--x-+x-0, and the doubly Cabibbo-suppressed decay D+ -K+x--x-+x-0, based on 980 fb-1 of data recorded by the Belle experiment at the KEKB e+e- collider. We measure these modes relative to the Cabibbo-favored modes D+ -K-x-+x-+x-0 and D+s -K+K-x-+x-0. Our results for the ratios of branching fractions are B(D+ -K+K-x-+x-0)/B(D+ -K-x-+x-+x-0) = (11.32 +/- 0.13 +/- 0.26)%, B(D+ -K+x--x-+x-0)/B(D+ -K-x-+x-+x-0) = (1.68 +/- 0.11 +/- 0.03)%, and B(D+s -K+x--x-+x-0)/ B(D+s -K+K-x-+x-0) = (17.13 +/- 0.62 +/- 0.51)%, where the uncertainties are statistical and systematic, respectively. The second value corresponds to (5.83 +/- 0.42) x tan4 BC, where BC is the Cabibbo angle; this value is larger than other measured ratios of branching fractions for a doubly Cabibbo-suppressed charm decay to a Cabibbo-favored decay. Multiplying these results by world average values for B(D+ -K-x-+x-+x-0) and B(D+s -K+K-x-+x-0) yields B(D+ -K+K-x-+x-0) = (7.08 +/- 0.08 +/- 0.16 +/- 0.20) x 10-3, B(D+ -K+x--x-+x-0) = (1.05 +/- 0.07 +/- 0.02 +/- 0.03) x 10-3, and B(D+s -K+x--x-+x-0) = (9.44 +/- 0.34 +/- 0.28 +/- 0.32) x 10-3, where the third uncertainty is due to the branching fraction of the normalization mode. The first two results are consistent with, but more precise than, the current world averages. The last result is the first measurement of this branching fraction.

Using (771.6 +/- 10.6)x106 B over line B meson pairs recorded by the Belle experiment at the KEKB e+e- collider, we report the branching fractions B( over line B0 -> D*+pi-)=(2.62 +/- 0.02 +/- 0.09)x10-3 and B( over line B0 -> D*+K-)=(2.22 +/- 0.06 +/- 0.08)x10-4; the quoted uncertainties are statistical and systematic, respectively. A measurement of the ratio of these branching fractions is also presented, RK/pi=B( over line B -> D*+K-)/B( over line B -> D*+pi-)=(8.41 +/- 0.24 +/- 0.13)x10-2, where systematic uncertainties due to the D*+ meson reconstruction cancel out. Furthermore, we report a new QCD factorization test based on the measured ratios for over line B -> D*+h- and over line B -> D*+l-nu decays at squared momentum transfer values equivalent to the mass of the h=pi or K hadron. The parameters |a1(h)| are measured to be |a1(pi)|=0.884 +/- 0.004 +/- 0.003 +/- 0.016 and |a1(K)|=0.913 +/- 0.019 +/- 0.008 +/- 0.013, where the last uncertainties account for all external inputs. These values are approximately 15% lower than those expected from theoretical predictions. Subsequently, flavor SU(3) symmetry is tested by measuring the ratios for pions and kaons, |a1(K)|2/|a1(pi)|2=1.066 +/- 0.042 +/- 0.018 +/- 0.023, as well as for different particle species. The ratio is consistent with unity and therefore no evidence for SU(3) symmetry breaking effects is found at the 5% precision level.