This paper reports on the status of narrow C hypernuclear states below and above C threshold. We have measured (K-, n*) spectra from 4He, 6Li and gBe at pK = 600 MeV/c with reasonable energy resolution and high statistics. The (K-, 7r*) spectra from 'Li and gBe show no evidence of any narrow structure above the C threshold, and hence rule out such previously claimed narrow states in this region. The 4He(K-,s-) spectrum clearly shows a peak below the C+ production threshold, confirming the existence of a bound state of $He, which was reported in the 4He(Stopped K-,n-) reaction. As in the case of stopped kaons, no such peak was found in the 4He(K-,r+) spectrum. Quantitatively reliable parameters for this level have been established.
1. Introduction
Vigorous studies of A hypernuclei have been pursued for more than three decades. The AN interaction is reasonably well understood, and A hypernuclear spectra can be well described by theoretical models. However, it has long been debated whether there exist any narrow states of hypernuclei containing the C hyperon below or above threshold. Such narrow excitations were thought to be impossible because of the strong conversion C -+ A occurring in nuclear matter.
Although very little information concerning the CN interaction is known, it was believed that its magnitude was comparable to the AN interaction (11.
Quantitative estimates of the C-nucleus potential were made on the basis of an analysis of C--atom X-ray data by Batty [2]. He estimated the potential at the center of a nucleus to be -(25~30)
MeV for the real part and -(10~15) MeV for the imaginary part, with a form proportional to the nuclear density. Thus, it was believed that the conversion width would be too broad to obtain spectroscopic information.
Subsequent reanalyses [3,4] of the C--atom X-ray data were performed with a potential form non-linear in the nuclear density. In these, the real part has become very shallow or even repulsive [3], so that formation of any bound states is very unlikely. However, the C--atom X-ray data are only sensitive to the potential depth far from a nuclear surface, and do not probe the nuclear interior.
Various reports of structures both below and above C threshold were reported in 4He, 'jLi gBe i2C, and 160 [5-lo]. Based on these reports, various conclusions were drawn concerning the C spin-orbit splitting, and the nature of the CN effective interaction and its relation to AN and NN interactions.
The validity of these conclusions is in question