The corresponding propagators are easily found to be ($ r fa)= [24w 2 (^2-m 2 )]~1{ (yp+m) ti'l(yp+m)T s (yp~m)T r 2+2(yp+m)l\ (yp-m)r r (yp+m)} +I72m^~1{yp tr[r s r^]+2 tr[ T^rs r-]+27^r 5 r--2r sT^r -+2r s r^}+[8w]-1 {tr[r ,r']+2r j*}. Similarly, for the spin-(^) field we may write and find for the propagators (rp r $ s ) + = [\2m 2 (p 2~m2 )']~l{ (yp+m) tr[(7^+w)r s (7^-w)r r ]-(yp+m)T s (yp-m)T r (yp+m)} -[36W 2 ]-1 {T^ tr[r s r^]-4 tr[ 7^rs r^+ 7^rs r'*+5r s 7^r'-+r s r^}+[4m]-1 {tr[r s r^-r s r'*}.The explicit evaluation of these propagators is straightforward but the result is not particularly illuminating, and we omit it. We note that the residues at the pole p 2 =m 2 are identical with those of SDS, but that the contact terms are different. The asymptotic behavior for large p is no worse than linear.Low-energy iT~-meson interactions in hydrogen are studied in the following channels: K~+p-*K-+p, K-+p-*2-+T + ,K~+p -• S+-T-7T-, and cross sections, as a function of momentum, are presented in the region of 60-300 MeV/c K~ laboratory momentum. These cross sections, combined with existing data, are used to fit the zero-effective-range theory of Dalitz and Tuan. Two possible solutions are obtained; the preferred one agrees with previous higher energy data. The favored solution also suggests an S-wave bound state at 1410 MeV, which could be associated with the Fo* at 1405 MeV whose spin is still undetermined. Various properties of the two solutions are presented for K~p interactions and K 2°p interactions.

The photon spectrum from K " stopping in liquid hydrogen has been measured with a high-resolution (1.5% FWHM at 300 MeV) Nal(Tl) detector. The branching ratios for K~p-+ Ay (E y -281.4 MeV) and K~p-*X°y (£,-219.5 MeV) were obtained. The results are i? Ay aas (0.86±0.07±8:Ag)xlO" 3 and R L o y -(1.44 ± 0.20-8:18) x 10 ~3, where the first error is statistical and the second systematic. These results are not in good agreement with published predictions or with previous Ay measurements. PACS numbers: 14.20.Jn, 13.40.Hq, 13.75JzThe low-energy K~p system has been the object of study for some years. Early in these investigations 1 it became evident that there might be a Y* resonance below the 1432-MeV K ~p threshold. This J p = j~ state is now known as the A(1405).At first this resonance was identified as a K~p bound state. With the advent of nonrelativistic constituentquark models, such as the Isgur-Karl model, 2 it was postulated to be a three-quark configuration. However, its mass was not well predicted. The MIT bag model 3 predicts two low-lying J p ™ j~ states which might be identified with the A(1405); however, a recent calculation by Umino and Myhrer 4 in a chiral bag model with broken SU(2) xSU(2) symmetry shows that one of these states moves up substantially in energy compared with the prediction of Ref. 3. The low-energy K ~p system has also been studied theoretically with a chiral SU(3)/?xSU(3)z, extension of the cloudy bag model. 5 In that model, calculations of the s-wave KN scattering cross sections indicate that the A(1405) is dominantly a KN bound state. The recent K~p data of Hemingway 6 show that the resonance shape is not well described by a Breit-Wigner line shape, but it is in agreement with the shape predicted by the cloudy bag model or A'-matrix theory. After more than 25 years of investigation, the structure of the A(1405) is still rather uncertain.In both atomic and nuclear physics, studies of radiative decays of excited states have provided the key to our understanding of their structure. While radiative widths of a number of excited baryon states with S^O have been measured, 7 only the radiative decay of the A(1520) excited state has been observed for S = -1 baryons. The published experiment of Mast et al. 8 suffered from low statistics as well as a substantial background, and the unpublished experiment of Bertini et al. 9 had rather poor photon-energy resolution and a large background. Consistent results were not obtained by the two groups. The experiment reported below was performed to provide information on the radiative widths of the A(1405) for the decays A(1405)-Ay and A(1405)-^ Z°y-The branching ratios R\ Y and R Ii o Y for the stopped-A' ~ reactions K~p-•Ayand K~p-+I, 0 y were measured. To the extent that the participation of the intermediate A(1405) in the radiative capture is understood, the radiative widths can be determined from the branching ratios.Two theoretical calculations 10,11 of these branching ratios were carried out in anticipation of the results from this experiment. The...