Bochum 1998 – scientific programme

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HK: Hadronen und Kerne

HK 62: Hadronic Probes VI, Resonances, Reactions

HK 62.8: Talk

Thursday, March 19, 1998, 16:15–16:30, C

The π⁺–emission puzzle in _Λ⁴He decay — •R.G.E. Timmermans — KVI, University of Groningen, The Netherlands

The observed π⁺ emission from the weak decay of _Λ⁴He has been an intriguing puzzle for more than 30 years. Experimentally, the π⁺ to π⁻ ratio for _Λ⁴He decay is about 5%, whereas no unambiguous π⁺ events have been observed for other hypernuclei. Since the mesonic decay modes of the free Λ (→ p+π⁻, n+π⁰) produce only π⁻s and π⁰s, more complicated mechanisms must be responsible for the π⁺s in the decay of the bound Λ in _Λ⁴He.

In Ref. [1], we have proposed a solution to the puzzle involving the virtual Σ⁺ component of the _Λ⁴He wave function. It is the large probability for a virtual Σ⁺ which is unique to the _Λ⁴He hypernucleus: Because of its charge, _Λ⁴He does permit the Λ to make a virtual transition to a Σ⁺ without altering the structure of the “nuclear core” state, whereas one would anticipate only Σ⁻ (and Σ⁰) transitions in _Λ⁴H and other Λ hypernuclei.

In the spectrum of π⁺s, one “sees” the weak three–body decay of a highly–virtual Σ⁺ arising from the in–medium Λ↔Σ transition. This resolution of the π⁺ decay puzzle is consistent with the significant Σ⁺ probability unique to the _Λ⁴He hypernucleus, and also explains in a natural way the experimentally observed π⁺ energy spectrum and s–wave angular distribution.

[1] B.F. Gibson and R.G.E. Timmermans, Nucl. Phys. A, in press.