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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, n0) produce only πs 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.

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