The quantum entangled $J/\psi \to {\mathrm{\Sigma }}^{+}{\overline{\mathrm{\Sigma }}}^{-}$ pairs from $(1.0087\pm 0.0044)\times {10}^{10}J/\psi$ events taken by the BESIII detector are used to study the nonleptonic two-body weak decays ${\mathrm{\Sigma }}^{+}\to n{\pi }^{+}$ and ${\overline{\mathrm{\Sigma }}}^{-}\to \overline{n}{\pi }^{-}$. The $CP$-odd weak decay parameters of the decays ${\mathrm{\Sigma }}^{+}\to n{\pi }^{+}$ (${\alpha }_{+}$) and ${\overline{\mathrm{\Sigma }}}^{-}\to \overline{n}{\pi }^{-}$ (${\overline{\alpha }}_{-}$) are determined to be $0.0481\pm 0.0031_{\mathrm{stat}}\pm 0.0019_{\mathrm{syst}}$ and $-0.0565\pm 0.0047_{\mathrm{stat}}\pm 0.0022_{\mathrm{syst}}$, respectively. The decay parameter ${\overline{\alpha }}_{-}$ is measured for the first time, and the accuracy of ${\alpha }_{+}$ is improved by a factor of 4 compared to the previous results. The simultaneously determined decay parameters allow the first precision $CP$ symmetry test for any hyperon decay with a neutron in the final state with the measurement of $A_{CP}=({\alpha }_{+}+{\overline{\alpha }}_{-})/({\alpha }_{+}-{\overline{\alpha }}_{-})=-0.080\pm 0.052_{\mathrm{stat}}\pm 0.028_{\mathrm{syst}}$. Assuming $CP$ conservation, the average decay parameter is determined as $⟨{\alpha }_{+}⟩=({\alpha }_{+}-{\overline{\alpha }}_{-})/2=-0.0506\pm 0.0026_{\mathrm{stat}}\pm 0.0019_{\mathrm{syst}}$, while the ratios ${\alpha }_{+}/{\alpha }_0$ and ${\overline{\alpha }}_{-}/{\overline{\alpha }}_0$ are $-0.0490\pm 0.0032_{\mathrm{stat}}\pm 0.0021_{\mathrm{syst}}$ and $-0.0571\pm 0.0053_{\mathrm{stat}}\pm 0.0032_{\mathrm{syst}}$, where ${\alpha }_0$ and ${\overline{\alpha }}_0$ are the decay parameters of the decays ${\mathrm{\Sigma }}^{+}\to p{\pi }^0$ and ${\overline{\mathrm{\Sigma }}}^{-}\to \overline{p}{\pi }^0$, respectively.

Exclusive hyperon-antihyperon production provides a unique insight for understanding of the intrinsic dynamics when strangeness is involved. In this paper, we review the results of $\Lambda \overline{\Lambda }$ production via different reactions from various experiments, e.g., via $\overline{p}p$ annihilation from the LEAR experiment PS185, via electron-positron annihilation using the energy scan method at the CLEO-c and BESIII experiments and the initial-state-radiation approach utilized at the BaBar experiment. The production cross section of $\Lambda \overline{\Lambda }$ near the threshold is sensitive to QCD based prediction. Experimental high precision data for $\overline{p}p\to \overline{\Lambda }\Lambda$ close to the threshold region is obtained. The cross section of $e^{+}{e}^{-}\to \Lambda \overline{\Lambda }$ is measured from its production threshold to high energy. A non-zero cross section for $e^{+}{e}^{-}\to \Lambda \overline{\Lambda }$ near threshold is observed at BaBar and BESIII, which is in disagreement with the pQCD prediction. However, more precise data is needed to confirm this observation. Future experiments, utilizing $\overline{p}p$ reaction such as PANDA experiment or electron-positron annihilation such as the BESIII and BelleII experiments, are needed to extend the experimental data and to understand the $\Lambda \overline{\Lambda }$ production.

A search for C P violation with an entangled system of Ξ − Ξ ¯ + pairs is performed, using ( 10 , 087 ± 44 ) × 10 6 J / ψ events collected with the BESIII experiment. A nine-dimensional helicity amplitude is used to fit e + e − → J / ψ → Ξ − Ξ ¯ + and its subsequent decays. The Ξ − and Ξ ¯ + decay parameters are determined with higher precision compared to the best results reported so far. Furthermore, the strong phase difference, ( δ P − δ S ) = ( 0.3 ± 1.2 ± 0.2 ) × 10 − 2 rad , and the weak phase difference, ( ξ P − ξ S ) = ( − 0.2 ± 1.2 ± 0.1 ) × 10 − 2 rad , are directly determined. These are the most precise measurements to date. The C P asymmetry observables A C P Ξ = ( − 7.8 ± 4.8 ± 0.8 ) × 10 − 3 and Δ ϕ C P Ξ = ( 0.6 ± 5.1 ± 0.2 ) × 10 − 3 rad are determined, which are consistent with C P conservation. In addition, independent measurements of the Λ decay parameter and C P asymmetry A C P Λ = ( − 2.9 ± 4.3 ± 0.7 ) × 10 − 3 are also obtained, which are in agreement with the previous measurements, but with much improved precision.

Based on $4.5{\mathrm{fb}}^{-1}$ of $e^{+}{e}^{-}$ collision data accumulated at center-of-mass energies between 4599.53 and 4698.82 MeV with the BESIII detector, the decay ${\mathrm{\Lambda }}_c^{+}\to n{K}_S^0{\pi }^{+}{\pi }^0$ is observed for the first time with a significance of $9.2\sigma$. The branching fraction is measured to be $(0.85\pm 0.13\pm 0.03)\%$, where the first uncertainty is statistical and the second is systematic, which differs from the theoretical prediction based on isospin by $4.4\sigma$. This indicates that there may be resonant contributions or some unknown dynamics in this decay.

We report a direct search for a new gauge boson, X , with a mass of 17 MeV / c 2 , which could explain the anomalous excess of e + e − pairs observed in the Be 8 nuclear transitions. The search is conducted in the charmonium decays χ c J → X J / ψ ( J = 0 , 1 , 2 ) via the radiative transition ψ ( 3686 ) → γ χ c J using ( 2712.4 ± 14.3 ) × 10 6 ψ ( 3686 ) events collected with the BESIII detector at the BEPCII collider. No significant signal is observed, and the updated upper limit on the coupling strength of charm quark and the new gauge boson, ε c , is set to be | ε c | < 1.2 × 10 − 2 at 90% confidence level. We also report new constraints on the mixing strength ε between the Standard Model photon and dark photon γ ′ in the mass range from 5 to 300 MeV / c 2 . The upper limits at 90% confidence level vary within ( 2.5 – 17.5 ) × 10 − 3 depending on the γ ′ mass.

A bstract: Using e ⁺ e ⁻ collision data corresponding to a total integrated luminosity of 20 fb ^{− 1} collected with the BESIII detector at the BEPCII collider, we present a measurement of the Born cross section for the process $$ {e}^{+}{e}^{-}\to p{K}^{-}{K}^{-}{\overline{\Xi}}^{+} $$ e + e − → p K − K − Ξ ¯ + at 39 center-of-mass energies between 3.5 and 4.9 GeV with a partial reconstruction technique. By performing a fit to the dressed cross section of $$ {e}^{+}{e}^{-}\to p{K}^{-}{K}^{-}{\overline{\Xi}}^{+} $$ e + e − → p K − K − Ξ ¯ + with a power law function for continuum production and one resonance at a time for the ψ (3770), ψ (4040), ψ (4160), ψ (4230), ψ (4360), ψ (4415) or ψ (4660), respectively, the upper limits for the product of partial electronic width and branching fraction into the final state $$ p{K}^{-}{K}^{-}{\overline{\Xi}}^{+} $$ p K − K − Ξ ¯ + for these resonances are determined at the 90% confidence level.