Precision hyperon physics at J/$\psi$ and $\psi'$ factories

Abstract

A novel multidimensional method to study hyperon and antihyperon decays and to search for a direct CP-violation signal is presented. The method uses spin polarization and entanglement of baryon-antibaryon pairs produced in electron-positron annihilation at the / and resonances. The method was applied in a series of proof-of-concept analyses at BESIII. The helicity amplitudes describing electron-positron annihilation into ΛΛ, Σ + Σ − , Ξ − Ξ + and Ω − Ω + pairs at / or were determined. A significant transverse polarization of the (anti)hyperons in / and decays was observed and spin of the Ω − baryon was determined in a model independent way using the reaction properties. The asymmetry parameters of the main (anti)hyperon decay modes, the nonleptonic two-body decays, were precisely measured. The result for the asymmetry parameter Λ in Λ → − revises by 17% the value used for nearly 50 years in all analyses of Λ polarization.Additionally, in the sequential weak decays Ξ − → Λ − and Ω − → Λ − the spin rotation parameter was measured.A comparison of the hyperon and antihyperon decay parameters allows for tests of direct CPsymmetry violation, that are complementary to the / measurement in kaon decays. Of special importance are the new results on + − → / → Ξ − Ξ + , where the final-state interaction and weak-phase difference are disentangled. Prospects of future measurements at Super Charm-Tau Factories are discussed with the emphasis on the impact of the longitudinal polarization of the electron beams on the precision of the CP-violation tests.

Hyperon physics at BESIII

“…The BESIII experiment has now provided results on J/ψ → Λ Λ using the full dataset of 10 10 J/ψ events but for other channels, the statistical uncertainty on the CP-tests can still be improved by to a factor of about 2.7. In the future, more than 10 12 J/ψ events may be produced at planned Super Charm-Tau factories [28,29], and with a polarized electron beam these facilities could reach a level of precision where the SM predictions could be directly tested [30].…”

Light hyperon physics at BESIII

“…With the 10 10 / events already collected at BESIII, the statistical precision on the CP-tests could potentially be reduced by a factor of about 2.7. Future Super Charm-Tau factories currently in planning [25,26] may produce more than 10 12 / events, and if a polarized electron beam could be used, the statistical precision at such facilities could be pushed down to the level of the SM predictions [27].…”