Search for B0 →τ±∓ (ℓ=e, μ) with a hadronic tagging method at Belle

We present a search for the lepton-flavor-violating decays B0→τ±∓, where ℓ=(e,μ), using the full data sample of 772×106 BB¯ pairs recorded by the Belle detector at the KEKB asymmetric-energy e+e- collider. We use events in which one B meson is fully reconstructed in a hadronic decay mode. The τ± lepton is reconstructed indirectly using the momentum of the reconstructed B and that of the ∓ from the signal decay. We find no evidence for B0→τ±∓ decays and set upper limits on their branching fractions at 90% confidence level of B(B0→τ±μ∓)<1.5×10-5 and B(B0→τ±e∓)<1.6×10-5. © 2021 authors. Published by the American Physical Society. Published by the American Physical Society under the terms of the "https://creativecommons.org/licenses/by/4.0/"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

Measurement of the branching fractions of the B+ -> eta l(+)nu(l) and B+ -> eta ' l(+)nu(l) decays with signal-side only reconstruction in the full q(2) range

The branching fractions of the decays B+ -> eta l(+)nu(l) and B+ -> eta'l(+)nu(l) are measured, where l is either an electron or a muon, using a data sample of 711 fb(-1) containing 772 x 10(6)B (B) over bar pairs collected at the Upsilon(4S) resonance with the Belle detector at the KEKB asymmetric-energy e(+)e(-) collider. To reduce the dependence of the result on the form factor model, the measurement is performed over the entire q(2) range. The resulting branching fractions are B(B+ -> eta l(+)nu(l)) = (2.83 +/- 0.55((stat.)) +/- 0.34((syst.))) x 10(-5) and B(B+ -> eta'l(+)nu(l)) = (2.79 +/- 1.29((stat.)) +/- 0.30((syst.))) x 10(-5).journal articl

Search for the dark photon in B \u3csup\u3e0\u3c/sup\u3e → A′A′, A′ → e \u3csup\u3e+\u3c/sup\u3e e \u3csup\u3e −\u3c/sup\u3e, μ \u3csup\u3e+\u3c/sup\u3e μ \u3csup\u3e −\u3c/sup\u3e, and π \u3csup\u3e+\u3c/sup\u3e π \u3csup\u3e −\u3c/sup\u3e decays at Belle

We present a search for the dark photon A′ in the B0 → A′A′ decays, where A′ subsequently decays to e+e−, μ+μ−, and π+π−. The search is performed by analyzing 772 × 106BB¯ events collected by the Belle detector at the KEKB e+e− energy-asymmetric collider at the ϒ(4S) resonance. No signal is found in the dark photon mass range 0.01 GeV/c2 ≤ mA′ ≤ 2.62 GeV/c2, and we set upper limits of the branching fraction of B0 → A′A′ at the 90% confidence level. The products of branching fractions, ℬ(B0→A′A′)×ℬ(A′→e+e−)2 and ℬ(B0→A′A′)×ℬ(A′→μ+μ−)2, have limits of the order of 10−8 depending on the A′ mass. Furthermore, considering A′ decay rate to each pair of charged particles, the upper limits of ℬ (B→ A′ A′) are of the order of 10−8–10−5. From the upper limits of ℬ (B→ A′ A′) , we obtain the Higgs portal coupling for each assumed dark photon and dark Higgs mass. The Higgs portal couplings are of the order of 10−2–10−1 at mh′≃mB0 ± 40 MeV/c2 and 10−1–1 at mh′≃mB0 ± 3 GeV/c2. [Figure not available: see fulltext.

Search for lepton-flavor-violating tau-lepton decays to ℓγ at Belle

Charged lepton flavor violation is forbidden in the Standard Model but possible in several new physics scenarios. In many of these models, the radiative decays τ± → ℓ±γ (ℓ = e, μ) are predicted to have a sizeable probability, making them particularly interesting channels to search at various experiments. An updated search via τ± → ℓ±γ using full data of the Belle experiment, corresponding to an integrated luminosity of 988 fb−1, is reported for charged lepton flavor violation. No significant excess over background predictions from the Standard Model is observed, and the upper limits on the branching fractions, B(τ± → μ±γ) ≤ 4.2 × 10−8 and B(τ± → e±γ) ≤ 5.6 × 10−8, are set at 90% confidence level. [Figure not available: see fulltext.]

Search for the decay Bs0 →η′η

We report the results of the first search for the decay Bs0→η′η using 121.4 fb-1 of data collected at the I (5S) resonance with the Belle detector at the KEKB asymmetric-energy e+e-collider. We observe no significant signal and set a 90% confidence-level upper limit of 6.5×10-5 on the branching fraction of this decay

Test of lepton flavor universality and search for lepton flavor violation in B → Kℓℓ decays

We present measurements of the branching fractions for the decays B → Kμ+μ− and B → Ke+e−, and their ratio (RK), using a data sample of 711 fb−1 that contains 772 × 106BB¯ events. The data were collected at the ϒ(4S) resonance with the Belle detector at the KEKB asymmetric-energy e+e− collider. The ratio RK is measured in five bins of dilepton invariant-mass-squared (q2): q2 ∈ (0.1, 4.0), (4.00, 8.12), (1.0, 6.0), (10.2, 12.8) and (\u3e 14.18) GeV2/c4, along with the whole q2 region. The RK value for q2 ∈ (1.0, 6.0) GeV2/c4 is 1.03−0.24+0.28 ± 0.01. The first and second uncertainties listed are statistical and systematic, respectively. All results for RK are consistent with Standard Model predictions. We also measure CP-averaged isospin asymmetries in the same q2 bins. The results are consistent with a null asymmetry, with the largest difference of 2.6 standard deviations occurring for the q2 ∈ (1.0, 6.0) GeV2/c4 bin in the mode with muon final states. The measured differential branching fractions, dℬ /dq2, are consistent with theoretical predictions for charged B decays, while the corresponding values are below the expectations for neutral B decays. We have also searched for lepton-flavor-violating B → Kμ±e∓ decays and set 90% confidence-level upper limits on the branching fraction in the range of 10−8 for B+ → K+μ±e∓, and B0 → K0μ±e∓ modes. [Figure not available: see fulltext.

Measurement of branching fractions and search for CP violation in D \u3csup\u3e0\u3c/sup\u3e → π \u3csup\u3e+\u3c/sup\u3e π \u3csup\u3e −\u3c/sup\u3e η, D \u3csup\u3e0\u3c/sup\u3e → K \u3csup\u3e+\u3c/sup\u3e K \u3csup\u3e −\u3c/sup\u3e η, and D \u3csup\u3e0\u3c/sup\u3e → ϕη at Belle

We measure the branching fractions and CP asymmetries for the singly Cabibbo-suppressed decays D0 → π+π−η, D0 → K+K−η, and D0 → ϕη, using 980 fb−1 of data from the Belle experiment at the KEKB e+e− collider. We obtainB(D0→π+π−η)=[1.22±0.02(stat)±0.02(syst)±0.03(Bref)]×10−3,B(D0→K+K−η)=[1.80−0.06+0.07(stat)±0.04(syst)±0.05(Bref)]×10−4,B(D0→ϕη)=[1.84±0.09(stat)±0.06(syst)±0.05(Bref)]×10−4, where the third uncertainty (Bref) is from the uncertainty in the branching fraction of the reference mode D0 → K−π+η. The color-suppressed decay D0 → ϕη is observed for the first time, with very high significance. The results for the CP asymmetries areACP(D0π+π−η)=[0.9±1.2(stat)±0.5(syst)]%,ACP(D0→K+K−η)=[−1.4±3.3(stat)±1.1(syst)]%,ACP(D0→ϕη)=[−1.9±4.4(stat)±0.6(syst)]%. The results for D0 → π+π−η are a significant improvement over previous results. The branching fraction and ACP results for D0 → K+K−η, and the ACP result for D0 → ϕη, are the first such measurements. No evidence for CP violation is found in any of these decays. [Figure not available: see fulltext.

Study of π0\pi^0 pair production in single-tag two-photon collisions

We report a measurement of the differential cross section of $\pi^0$ pair production in single-tag two-photon collisions, $\gamma^* \gamma \to \pi^0 \pi^0$, in $e^+ e^-$ scattering. The cross section is measured for $Q^2$ up to 30 GeV$^2$, where $Q^2$ is the negative of the invariant mass squared of the tagged photon, in the kinematic range 0.5 GeV < W < 2.1 GeV and $|\cos \theta^*|$ < 1.0 for the total energy and pion scattering angle, respectively, in the $\gamma^* \gamma$ center-of-mass system. The results are based on a data sample of 759 fb$^{-1}$ collected with the Belle detector at the KEKB asymmetric-energy $e^+ e^-$ collider. The transition form factor of the $f_0(980)$ and that of the $f_2(1270)$ with the helicity-0, -1, and -2 components separately are measured for the first time and are compared with theoretical calculations.Comment: 36 pages, 37 figures, 11 tables, Belle Preprint 2015-15, KEK Preprint 2015-2

Observation of B0→pΛˉD(∗)−B^{0} \rightarrow p\bar{\Lambda} D^{(*)-}

We report the first observation of the decays $B^0 \rightarrow p\bar{\Lambda} D^{(*)-}$. The data sample of $711$ fb$^{-1}$ used in this analysis corresponds to $772$ million $B\bar{B}$ pairs, collected at the $\Upsilon(4S)$ resonance by the Belle detector at the KEKB asymmetric-energy $e^{+}e^{-}$ collider. We observe $19.8\sigma$ and $10.8\sigma$ excesses of events for the two decay modes and measure the branching fractions of $B^0 \rightarrow p\bar{\Lambda} D^{-}$ and $B^0 \rightarrow p\bar{\Lambda} D^{*-}$ to be $(25.1\pm2.6\pm3.5)\times10^{-6}$ and $(33.6\pm6.3\pm4.4)\times10^{-6}$, respectively, where the first uncertainties are statistical and the second are systematic. These results are not compatible with the predictions based on the generalized factorization approach. In addition, a threshold enhancement in the di-baryon ($p\bar{\Lambda}$) system is observed, consistent with that observed in similar $B$ decays.Comment: 16 pages, 3 figures and 3 tables, submitted to PR

Inclusive study of bottomonium production in association with an η\eta meson in e+e−e^+e^- annihilations near Υ(5S)\Upsilon(5S)

We study bottomonium production in association with an $\eta$ meson in $e^+e^-$ annihilations near the $\Upsilon(5S)$, at a center of mass energy of $\sqrt{s}=10.866\,$GeV. The results are based on the $121.4\,$fb$^{-1}$ data sample collected by the Belle experiment at the asymmetric energy KEKB collider. Only the $\eta$ meson is reconstructed and the missing-mass spectrum of $\eta$ candidates is investigated. We observe the $e^+e^-\to\eta\Upsilon_J(1D)$ process and find evidence for the $e^+e^-\to\eta\Upsilon(2S)$ process, while no significant signals of $\Upsilon(1S)$, $h_b(1P)$, nor $h_b(2P)$ are found. Cross sections for the studied processes are reported.Comment: Submitted to EPJ-