Chun-Qiu Zhao | Searcharxiv

arXiv2026

Disentangling isospin violation in charmonia decaying into $Ξ\overline Ξ$ pairs

Based on a model-independent isospin decomposition, we perform a data-driven analysis of $J/ψ$ and $ψ(2S)$ decays into $Ξ\barΞ$ pairs by leveraging the full set of observables from high-statistics BESIII data. We find that the isospin-violating amplitudes are predominantly driven by the electric couplings, with magnitudes reaching approximately 10%, contrasting with the smaller magnetic contributions of about 3%. A partial-wave analysis further reveals that $S$-wave isospin violation is comparable to the $D$-wave contribution.

hep-ph↗

arXiv2026

Spin density matrix of baryon-antibaryon pairs in electron-positron annihilation with $P$ and $CP$ violation including electron mass

In the center-of-mass frame, the spin density matrix for octet baryon-antibaryon pairs produced in polarized electron-positron annihilation is unambiguously determined within the one-photon-exchange approximation. The formalism simultaneously incorporates parity ($P$) and combined parity-charge conjugation ($CP$) violation arising from the coupling of the intermediate virtual photon to the baryon pair. The finite electron mass is explicitly accounted for as a correction to these $P$ and $CP$ violating effects. Built on the spin projection operator, the derived full and compact spin density matrix provides a natural starting point for evaluating other quantities of physical interest -- particularly angular distributions of sequential decays and the quantum entanglement of hyperon pairs.

hep-ph↗