Story of sigma hypernuclei: a modern fable

摘要

The reality of (Sigma) hypernuclei has been the subject of intense concern among211u001eexperimenters and theoreticians for more than 20 years. The possible existence of 211u001e(Sigma) hypernuclei was first suggested by a pioneering experiment on a (sup 9)Be 211u001etarget at the CERN PS. There were reported to be two narrow ((Gamma) < 8 211u001eMeV) peaks in the continuum region. This finding was quite unexpected since the 211u001ewidths of (Sigma) states were believed to be large due to the strong conversion 211u001eprocess. It is obvious that if such relatively long-lived systems were confirmed 211u001eunambiguously by experiment, their masses and widths provide important 211u001econstraints on the (Sigma)N effective interaction and its relation to the 211u001e(Lambda)N and NN interactions. Since the (Sigma) carries isospin, the role of 211u001eisospin and isospin conservation in hadronic reactions could be explored. This 211u001ereport stimulated a number of subsequent experiments at the BNL-AGS and KEK, 211u001ealong with further experiments with a specially created short kaon beam at the 211u001eCERN PS. Experimental data were reported for different targets at different 211u001emomenta and at different conditions. Various tagging techniques were employed to 211u001esuppress backgrounds, but always at the expense of a reduction in statistical 211u001equality. Because of problems with resolution and statistics, contradictions among 211u001ethe different sets of data resulted more often in clouding the issues than 211u001eclarifying them. Thus, up until a few years ago, there was no statistically clear 211u001econfirmation of this surprising finding. In the last few years, however, a series 211u001eof definitive experiments has been performed at the BNL-AGS in an effort to 211u001eresolve the discrepancies and settle the controversy. Besides repeating the 211u001einitial experiment on Be, a target of (sup 6)Li was run. The earlier stopped kaon 211u001edata on (sup 4)He was repeated in an in-flight experiment, and the suggestion of 211u001ea (Sigma) bound state, with virtually pure isospin, confirmed. We now have a 211u001ebetter understanding of the role of isospin in the nucleon-hyperon interaction 211u001eand the importance of three body forces in hyper-nuclei. According to that 211u001eunderstanding, it is unlikely that (Sigma) states for A > 5 will ever be 211u001eseen. In this review, a critique of the past BNL experiments is presented and 211u001econclusions on the status of the (Sigma) database drawn.