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Differential cross sections for K- radiative capture in flight on the proton, leading to the γΛ and γΣ0 final states, have been measured at eight K- momenta between 514 and 750 MeV/c. The data were obtained with the Crystal Ball multiphoton spectrometer installed at the separated K/π beam line C6 of the BNL Alternating Gradient Synchrotron. The results substantially improve the existing experimental data available for studying radiative decays of excited hyperon states. An exploratory theoretical analysis is performed within the Regge-plus-resonance approach. According to this analysis, the γΣ0 final state is dominated by hyperon-resonance exchange and hints at an important role for a resonance in the mass region of 1700 MeV. In the γΛ final state, in contrast, the resonant contributions account for only half the strength, and the data suggest the importance of a resonance in the mass region of 1550 MeV.

We report the first measurements of the total cross section of the reaction K-p-->Sigma 0 gamma at eight beam momenta from 514 to 750 MeV/c. The data were obtained at the Alternating Gradient Synchrotron (AGS) at Brookhaven National Laboratory using the Crystal Ball detector consisting of 672 NaI crystals. All charged particles were vetoed by a barrel of plastic scintillators, resulting in a study of only the neutral decays of the Sigma 0. The prompt photon and the photons from the decay products of Sigma 0 were detected for each reconstructed event ; the neutron was also detected in a small fraction of the events.

Measured values of the differential cross sections for pion-nucleon charge exchange, pi- p --> pi0 n, are presented for pi- momenta of 103, 112, 120, 130, 139, 152, and 178 MeV/c. Complete angular distributions were obtained by using the Crystal Ball detector at the Alternating Gradient Synchrotron at Brookhaven National Laboratory. Statistical uncertainties of the differential cross sections vary from 3% to 6% in the backward angle region, and from 6% to about 20% in the forward region with the exception of the two most forward angles. The systematic uncertainties are estimated to be about 3% for all momenta., 18 pages, 12 figures, submitted to Phys. Rev. C

The reaction K-p00 neutrals was studied with the Crystal Ball detector at the BNL Alternating Gradient Synchrotron for beam momenta between 514 and 750 MeV/c. The photons from 0 and 0 decays were detected, as well as the neutron in a small fraction of the events. The differential cross section and the 0 polarization are reported at eight momenta and nearly the full angular range. Total cross sections are derived from Legendre polynomial fits to the data. The measurements presented are of considerably higher precision than previous results.

New results on the rare, doubly radiative decay eta -->pi0 gama gamma have been obtained from a revised analysis of the Crystal Ball experiment performed at the AGS. The analysis yields the first information on the dependence of the decay width, Gamma (eta-->pi0 gamma gamma), on the two-photon invariant mass squared, m2(gamma gamma). A re-evaluation of the branching ratio is also made, BR(eta-->pi0 gamma gamma= (2.21± 0.24stat± 0.47syst)×10-4 ; it implies that the decay width is Gamma(eta-->pi0 gamma gamma) =0.285± 0.031stat± 0.061syst eV. These results are close to predictions based on chiral perturbation theory with vector-meson dominance.

We report a new measurement of the differential cross section for pi^-p-->pi^0 n from p_pi=649 to 752 MeV/c, which is around the opening of the eta channel (685 MeV/c). Our data support the main features of the pi^-p charge-exchange differential cross sections generated by the SAID piN partial-wave analysis. The opening of the eta channel has a clear effect on the shape of the excitation function for dsigma (pi^-p-->pi^0 n), which is most noticeable in the backward direction.

The differential cross section for eta production in reaction pi-p-->eta n has been measured over the full angular range at seven incident - beam momenta from threshold to p_pi-=747 MeV/c using the Crystal Ball multiphoton spectrometer. The angular distributions are S wave dominated. At 10 MeV/c above threshold, a small D-wave contribution appears that interferes with the main S wave. The total eta production cross section sigma^tot is obtained by integration of d sigma/d omega. Starting at threshold, sigma^tot rises rapidly, as expected for S-wave-dominated production. The features of the pi-p-->eta n cross section are strikingly similar to those of the SU(3) flavor-related process K-p-->eta Lambda. Comparison of the pi-p-->eta n reaction is made with eta photoproduction.

The branching ratio (BR) for the rare decay eta-->pi^0 gamma gamma was measured with the Crystal Ball multiphoton spectrometer. The result, BR(eta-->pi^0 gamma gamma)=(3.5&plusmn ; ; 0.7_stat&plusmn ; ; 0.6_syst)×10-4, is in agreement with calculations of chiral perturbation theory to third order. We have used a sample of 28 million eta mesons produced at the BNL Alternating Gradient Synchrotron using the pi-p-->eta n reaction close to threshold. We detail the intricate extraction of the signal, which has about 1.6 thousand eta-->pi^0 gamma gamma events, from the overwhelming background of eta -->3pi^0 decays and from the pi-p-->pi^0 pi^0 n reaction.

A new upper limit is presented for the branching ratio (BR) of eta -->3 gamma. This decay is not allowed by charge-conjugation invariance. We have determined that BR(eta -->3gamma)Pi^0 gamma-->3gamma)pi^0pi^0 gamma gamma)eta n reaction close to threshold.

Measured values of the differential cross section for pion-nucleon charge exchange, pi^– -->p pi^0n, are presented at pi^- momenta of 148, 174, 188, 212, 238, 271, 298, and 323 MeV/c, a region dominated by the Lambda (1232) resonance. Complete angular distributions were obtained using the Crystal Ball detector at the Alternating Gradient Synchrotron (AGS) at Brookhaven National Laboratory (BNL). Statistical uncertainties of the differential cross sections are typically 2 to 6%, exceptions being the results at the lowest momentum and at the most forward measurements at the five lowest momenta. We estimate the systematic uncertainties to be 3 to 6%.

Reaction pi^- p pi^0pi^0n has been measured with high statistics in the beam momentum range 270 - 750 MeV/c. The data were obtained using the Crystal Ball multiphoton spectrometer, which has 93% of 4pi solid angle coverage. The dynamics of the pi^- p pi^0pi^0n reaction and the dependence on the beam energy are displayed in total cross sections, Dalitz plots, invariant-mass spectra, and production angular distributions. Special attention is paid to the evaluation of the acceptance that is needed for the precision determination of the total cross section sigma_t(pi^- p pi^0pi^0n). The energy dependence of sigma_t(pi- p pi^0pi^0n) shows a shoulder at the Roper resonance [i.e., the N(1440)1/2^+], and there is also a maximum near the N(1520)3/2-. It illustrates the importance of these two resonances to the pi^0pi^0 production process. The Dalitz plots are highly nonuniform ; they indicate that the pi^0pi^0n final state is dominantly produced via the pi^0Delta^0(1232) intermediate state. The invariant-mass spectra differ much from the phase-space distributions. The production angular distributions are also different from the isotropic distribution, and their structure depends on the beam energy. For beam momenta above 550 MeV/c, the density distribution in the Dalitz plots strongly depends on the angle of the outgoing dipion system (or equivalently on the neutron angle). The role of the f_0(600) meson (also known as the sigma) in pi^0pi^0n production remains controversial.