Search for: All records

We report a search for a heavy neutral lepton (HNL) that mixes predominantly with ${\nu }_{\tau }$. The search utilizes data collected with the Belle detector at the KEKB asymmetric energy $e^{+}{e}^{-}$collider. The data sample was collected at and just below the center-of-mass energies of the $\mathrm{\Upsilon }\left(4S\right)$and $\mathrm{\Upsilon }\left(5S\right)$resonances and has an integrated luminosity of $915{\mathrm{fb}}^{-1}$, corresponding to $(836\pm 12)\times {10}^6$ $e^{+}{e}^{-}\to {\tau }^{+}{\tau }^{-}$events. We search for production of the HNL (denoted $N$) in the decay ${\tau }^{-}\to {\pi }^{-}N$followed by its decay via $N\to {\mu }^{+}{\mu }^{-}{\nu }_{\tau }$. The search focuses on the parameter-space region in which the HNL is long-lived, so that the ${\mu }^{+}{\mu }^{-}$originate from a common vertex that is significantly displaced from the collision point of the KEKB beams. Consistent with the expected background yield, one event is observed in the data sample after application of all the event-selection criteria. We report limits on the mixing parameter of the HNL with the $\tau$neutrino as a function of the HNL mass. Published by the American Physical Society2024 

A<sc>bstract</sc> We report a search for the charged-lepton flavor violation in Υ(2S) →ℓ^∓τ^±(ℓ=e, μ) decays using a 25 fb⁻¹Υ(2S) sample collected by the Belle detector at the KEKBe⁺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^±)<0.23×10⁻⁶and$$ \mathcal{B} $$ $B$(Υ(2S)→ e^∓τ^±)<1.12×10⁻⁶. 

A<sc>bstract</sc> We report the first measurement of the inclusivee⁺e⁻→$$ b\overline{b} $$ $b\overline{b}$→$$ {D}_s^{\pm } $$ $D_s^{\pm }$Xande⁺e⁻→$$ b\overline{b} $$ $b\overline{b}$→ D⁰/$$ {\overline{D}}^0 $$ ${\overline{D}}^0$Xcross sections in the energy range from 10.63 to 11.02 GeV. Based on these results, we determineσ(e⁺e⁻→$$ {B}_s^0{\overline{B}}_s^0 $$ $B_s^0{\overline{B}}_s^0$X) andσ(e⁺e⁻→$$ B\overline{B} $$ $B\overline{B}$X) in the same energy range. We measure the fraction of$$ {B}_s^0 $$ $B_s^0$events at Υ(10860) to bef_s= ($$ {22.0}_{-2.1}^{+2.0} $$ ${22.0}_{-2.1}^{+2.0}$)%. 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/$$ {\overline{D}}^0 $$ ${\overline{D}}^0$X)/$$ \mathcal{B} $$ $B$($$ {B}_s^0 $$ $B_s^0$→$$ {D}_s^{\pm } $$ $D_s^{\pm }$X) = 0.416 ± 0.018 ± 0.092. The results are obtained using the data collected with the Belle detector at the KEKB asymmetric-energye⁺e⁻collider. 

We report on a search for a resonance $X$decaying to a pair of muons in $e^{+}{e}^{-}\to {\mu }^{+}{\mu }^{-}X$events in the $0.212–9.000\mathrm{GeV}/c^2$mass range, using $178{\mathrm{fb}}^{-1}$of data collected by the Belle II experiment at the SuperKEKB collider at a center of mass energy of 10.58 GeV. The analysis probes two different models of $X$beyond the standard model: a $Z^{\prime }$vector boson in the $L_{\mu }-L_{\tau }$model and a muonphilic scalar. We observe no evidence for a signal and set exclusion limits at the 90% confidence level on the products of cross section and branching fraction for these processes, ranging from 0.046 fb to 0.97 fb for the $L_{\mu }-L_{\tau }$model and from 0.055 fb to 1.3 fb for the muonphilic scalar model. For masses below $6\mathrm{GeV}/c^2$, the corresponding constraints on the couplings of these processes to the standard model range from 0.0008 to 0.039 for the $L_{\mu }-L_{\tau }$model and from 0.0018 to 0.040 for the muonphilic scalar model. These are the first constraints on the muonphilic scalar from a dedicated search. Published by the American Physical Society2024 

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 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 $B^{+}\to K^{+}\nu \overline{\nu }$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 Society2024 

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 $\mathrm{\Upsilon }\left(4S\right)$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 Society2024