Particles directly produced at electron–positron colliders, such as the J/ψ meson, decay with relatively high probability into a baryon–antibaryon pair 1 . For spin-1/2 baryons, the pair can have the same or opposite helicites. A non-vanishing phase ΔΦ between the transition amplitudes to these helicity states results in a transverse polarization of the baryons 2$^{–}$4 . From the joint angular distribution of the decay products of the baryons, this phase as well as the parameters characterizing the baryon and the antibaryon decays can be determined. Here, we report the measurement of ΔΦ = 42.4 ± 0.6 ± 0.5° using Λ → pπ$^{−}$ and $\bar \Lambda \to \bar p\pi ^ + ,\bar n\pi ^0$ decays at BESIII. We find a value for the Λ → pπ$^{−}$ decay parameter of α$_{−}$ = 0.750 ± 0.009 ± 0.004, 17 ± 3% higher than the current world average, which has been used as input for all Λ polarization measurements since 1978 5$^{,}$6 . For $\bar \Lambda \to \bar p\pi ^ +$ we find α$_{+}$ = −0.758 ± 0.010 ± 0.007, giving A$_{CP}$ = (α$_{−}$ + α$_{+}$)/(α$_{−}$ − α$_{+}$) = −0.006 ± 0.012 ± 0.007, a precise direct test of charge–parity symmetry (CP) violation in Λ decays.

Polarization and Entanglement in Baryon-Antibaryon Pair Production in Electron-Positron Annihilation

The cross section of the process e+e-→ωχc0 is measured at center-of-mass energies from s=4.178 to 4.278 GeV using a data sample of 7  fb-1 collected with the BESIII detector operating at the BEPCII storage ring. The dependence of the cross section on s shows a resonant structure with mass of (4218.5±1.6(stat)±4.0(syst))  MeV/c2 and width of (28.2±3.9(stat)±1.6(syst))  MeV, respectively. This observation confirms and improves upon the result of a previous study. The angular distribution of the e+e-→ωχc0 process is extracted for the first time.

Cross section measurements of e + e − → ω χ c 0 from √ s = 4.178 to 4.278 GeV

By analyzing a 2.93 fb(-1) data sample of e(+)e(-) collisions, recorded at a center-of-mass energy of 3.773 GeV with the BESIII detector operated at the BEPCII collider, we report the first observa ...

Observation of the Semileptonic D + Decay into the K ̄ 1 (1270 ) 0 Axial-Vector Meson


 Using inclusive decays of the J/ψ, a precise determination of the number of J/ψ events collected with the BESIII detector is performed. For the two data sets taken in 2009 and 2012, the numbers of J/ψ events are recalculated to be (224.0±1.3)×10^6 and (1088.5±4.4)×10^6 respectively, which are in good agreement with the previous measurements. For the J/ψ sample taken in 2017–2019, the number of events is determined to be (8774.0±39.4)×10^6. The total number of J/ψ events collected with the BESIII detector is determined to be (10087±44)×10^6, where the uncertainty is dominated by systematic effects and the statistical uncertainty is negligible. Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Article funded by SCOAP3 and published under licence by Chinese Physical Society and the Institute of High Energy Physics of the Chinese Academy of Science and the Institute of Modern Physics of the Chinese Academy of Sciences and IOP Publishing Ltd.

https://iopscience.iop.org/article/10.1088/1674-1137/ac5c2e/pdf

Number of J/ψ events at BESIII

We report the first observation of D^{+}→τ^{+}ν_{τ} with a significance of 5.1σ. We measure B(D^{+}→τ^{+}ν_{τ})=(1.20±0.24_{stat}±0.12_{syst})×10^{-3}. Taking the world average B(D^{+}→μ^{+}ν_{μ})=(3.74±0.17)×10^{-4}, we obtain R_{τ/μ}=Γ(D^{+}→τ^{+}ν_{τ})/Γ(D^{+}→μ^{+}ν_{μ})=3.21±0.64_{stat}±0.43_{syst}., which is consistent with the standard model expectation of lepton flavor universality. Using external inputs, our results give values for the D^{+} decay constant f_{D^{+}} and the Cabibbo-Kobayashi-Maskawa matrix element |V_{cd}| that are consistent with, but less precise than, other determinations.

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Observation of the Leptonic Decay D+ →τ+ντ

Using J/ψ decays collected with the BES III detector, Lundcharm model parameters are optimized with J/ψ light hadron decays. The dependence of the response function on model parameters is approximated up to the quadratic term, and the model parameters are optimized by simultaneously fitting J/ψ inclusive charged track distributions and event shapes. The Monte Carlo simulations show that optimal parameters yield satisfactory MC distributions as compared to both the J/ψ and ψ(2S) data distributions. These optimal values are suggested for the Lundcharm model to produce J/ψ and ψ(2S) decays to light hadrons.

https://iopscience.iop.org/article/10.1088/0256-307X/31/6/061301/pdf

Tuning and Validation of the Lundcharm Model with J/ψ Decays

It is crucial to determine the spin and parity (JP) of Zc (3900)± for understanding its structure. We perform a helicity amplitude analysis on Zc (3900)± in the process e+e− → π+π− J/ψ with the hypotheses JP = 0−, 1+, 1−, 2+ and 2− . To estimate the significance of JP = 1+ over other hypotheses, we perform a Monte Carlo simulation study, and we also discuss the prospect of measuring the spin parity in the future experiment with a large data sample.

Rong-Gang Ping

Papers

Tuning and Validation of the Lundcharm Model with J/ψ Decays

Helicity Amplitude Analysis of Z(c)(3900)(+/-) in e(+)e(-) -> pi(+)pi(-) J/psi

Exclusive {{\rm{\varOmega }}}_{{ccc}}^{++}{\bar{{\rm{\varOmega }}}}_{{ccc}}^{--} Production from {Z}^{0}/{\gamma }^{* } Decays