Hunting for Direct Cp Violation in Strange - Baryon Decays

Although CP-symmetry breaking has only been seen in neutral kaon decays, this mysterious phenomenon is expected to occur elsewhere. In this talk, how CP violation can arise in strange-baryon decays is briefly reviewed. The current status and prospects for searching for such an effect will be presented. Strange baryons were first discovered in cosmic-ray experiments in the late 1940s as particles heavier than the nucleons; 1 hence, they are also known as hyperons. When Lee and Yang proposed parity (P) nonconservation in 1956 to explain the famous θ-τ puzzle, they also suggested studying decays of hyperons as further tests of their idea. 2 Explicitly, they predicted that the distribution of protons in the weak decay Λ 0 → pπ − would have a forward-backward asymmetry, quantified by the decay parameter α Λ , with respect to the polarization of Λ 0. Just a few months after the classic 60 Co experiment performed by Madame C.S. Wu et al 3 was announced, the predicted forward-backward asymmetry in Λ 0 decay was indeed observed. 4 With the fall of parity conservation, invariance of the other discrete symmetries , charge conjugation (C) and time reversal (T), was also questioned. As early as 1957, Lee, Oehme and Yang suggested tests that were sensitive to T violation in K 0 decays. 5 Within a year, Okubo concluded that breaking of T symmetry would lead to a difference in the partial decay rates between Σ + and Σ − , 6 followed by Pais who proposed a test of T violation by comparing the magnitude of the α decay parameters of the Λ 0 and Λ 0 decays.