3600 two-pronged events, obtained in p-p interactions at 2 Bev in the BNL 20-in. hydrogen bubble chamber, have been analyzed. Cross sections have been measured for elastic scattering, for the two modes of single-pion production, p-\-p -» p-3 rn-\-ir + , p+p -> p-\-p-\-n°, and for strange-particle production. The branching ratio for the two one-pion production reactions is c-(£wr + )/a-(^7r°)=4.l7=fc:0.25. Momentum distributions and Q values indicate that single-pion production proceeds almost entirely through the (f,f) resonant state. The data have been considered in terms of the extended isobar model and also a one-pion exchange model for production. The branching ratio and momentum distributions can be explained by including a small effect from the /=£ resonant state in addition to the dominant I-\ resonance. The cm. angular distribution of the nucleons in single-pion production shows very marked backward-forward peaking indicating a one-pion exchange mechanism. Absolute differential cross sections as a function of laboratory kinetic energy have been calculated from Selleri's equation for the pmr + reaction. There is good agreement with the data for low four-momentum transfers [# 2 <0.15(Bev/c) 2 ], but for higher momentum transfers the theoretical cross sections are larger than the experimental cross sections.

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...