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Using $(10.087pm0.044)times10^{9}$ $J/psi$ events collected by the BESIII detector at the BEPCII collider, we search for the hyperon semileptonic decay $Xi^{-} rightarrow Xi^{0} e^{-} bar{ u}_{e}$. No significant signal is observed and the upper limit on the branching fraction $mathcal B(Xi^{-} rightarrow Xi^{0} e^{-} bar{ u}_{e})$ is set to be $2.59times10^{-4}$ at 90 % confidence level. This result is one order of magnitude more strict than the previous best limit.
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Using $e^+e^-$ collision data at ten center-of-mass energies between 2.644 and 3.080 GeV collected with the BESIII detector at BEPCII and corresponding to an integrated luminosity of about 500 pb$^{-1}$, we measure the cross sections and effective form factors for the process $e^{+}e^{-}rightarrowXi^{0}bar{Xi}^{0}$ utilizing a single-tag method. A fit to the cross section of $e^{+}e^{-}rightarrowXi^{0}bar{Xi}^{0}$ with a pQCD-driven power function is performed, from which no significant resonance or threshold enhancement is observed. In addition, the ratio of cross sections for the processes $e^{+}e^{-}rightarrowXi^{-}bar{Xi}^{+}$ and $Xi^{0}bar{Xi}^{0}$ is calculated using recent BESIII measurement and is found to be compatible with expectation from isospin symmetry.
Using $(448.1pm2.9)times 10^{6}$ $psi(3686)$ events collected with the BESIII detector and a single-baryon tagging technique, we present the first observation of the decays $psi(3686)toXi(1530)^{0}bar{Xi}(1530)^{0}$ and $Xi(1530)^{0}bar{Xi}^0$. The branching fractions are measured to be ${cal{B}}(psi(3686)toXi(1530)^{0}bar{Xi}(1530)^{0}) = (6.77pm0.14pm0.39)times10^{-5}$ and ${cal{B}}(psi(3686)toXi(1530)^{0}bar{Xi}^{0}) = (0.53pm0.04pm0.03)times10^{-5}$. Here, the first and second uncertainties are statistical and systematic, respectively. In addition, the parameter associated with the angular distribution for the decay $psi(3686)toXi(1530)^{0}bar{Xi}(1530)^0$ is determined to be $alpha = 0.32pm0.19pm0.07$, in agreement with theoretical predictions within one standard deviation.
Using $2.93~mathrm{fb}^{-1}$ of $e^+e^-$ annihilation data collected at a center-of-mass energy $sqrt{s}=3.773$ GeV with the BESIII detector operating at the BEPCII collider, we search for the semileptonic $D^{0(+)}$ decays into a $b_1(1235)^{-(0)}$ axial-vector meson for the first time. No significant signal is observed for either charge combination. The upper limits on the product branching fractions are ${mathcal B}_{D^0to b_1(1235)^- e^+ u_e}cdot {mathcal B}_{b_1(1235)^-to omegapi^-}<1.12times 10^{-4}$ and ${mathcal B}_{D^+to b_1(1235)^0 e^+ u_e}cdot {mathcal B}_{b_1(1235)^0to omegapi^0}<1.75times 10^{-4}$ at the 90% confidence level.
The Born cross-sections and effective form factors for process $e^+e^-toXi^-bar{Xi}^+$ are measured at eight center-of-mass energies between 2.644 and 3.080 GeV, using a total integrated luminosity of 363.9 pb$^{-1}$ $e^+e^-$ collision data collected with the BESIII detector at BEPCII. After performing a fit to the Born cross-section of $e^+e^-toXi^-bar{Xi}^+$, no significant threshold effect is observed.
We present the first measurements of branching fractions of rare tau-lepton decays, $tau^- rightarrow pi^- u_{tau} ell^+ ell^-$ ($ell = e$ or $mu$), using a data sample corresponding to 562 fb$^{-1}$ collected at a center-of-mass energy of 10.58 GeV with the Belle detector at the KEKB asymmetric-energy $e^+ e^-$ collider. The $tau^- rightarrow pi^- u_tau e^+ e^-$ decay is observed for the first time with 7.0$sigma$ significance. The partial branching fraction determined by the structure-dependent mechanisms mediated by either a vector or an axial-vector current for the mass region $M_{pi e e}>1.05$ GeV/$c^2$ is measured to be $mathcal{B}(tau^-rightarrow pi^- u_tau e^+ e^-)[M_{pi^- e^+ e^-}>1.05~{rm GeV}/c^2] = (5.90 pm 0.53 pm 0.85 pm 0.11) times 10^{-6}$, where the first uncertainty is statistical, the second is systematic, and the third is due to the model dependence. In the full phase space, due to the different detection efficiencies for the structure-dependent mechanisms mediated by axial-vector and vector currents, the branching fraction varies from $mathcal{B}_{A}(tau^-rightarrow pi^- u_tau e^+ e^-) = (1.46 pm 0.13 pm 0.21) times 10^{-5}$ to $mathcal{B}_{V}(tau^-rightarrow pi^- u_tau e^+ e^-) = (3.01 pm 0.27 pm 0.43) times 10^{-5}$, respectively. An upper limit is set on the branching fraction of the $tau^- rightarrow pi^- u_tau mu^+ mu^-$ decay, $mathcal{B}(tau^-rightarrow pi^- u_tau mu^+ mu^-) < 1.14 times 10^{-5}$, at the 90% confidence level.
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