Search

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.

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.

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.

Reaction K^- p --> pi^0 pi^0 Sigma^0 was measured with the Crystal Ball multiphoton spectrometer at eight K^- momenta from 514 to 750 MeV/c. Dynamics and the energy dependence of the reaction are shown by Dalitz plots, invariant mass spectra, production angular distributions, and total cross sections. The dipion invariant-mass spectra for K^- p --> pi^0 pi^0 Sigma^0 show an enhancement in the low mass region ; the opposite is the case for the K^- p --> pi^0 pi^0 Lambda and pi- p --> pi^0 pi^0 n reactions. There is no direct evidence for a f_0(600)-meson contribution to pi^0pi^0 production. Everywhere sigma_t(K^- p --> pi^0 pi^0 Sigma^0)< pi^0 pi^0 Lambda).

Precise new data for the reaction K-p->pi^0 Lambda are presented in the c.m. energy range 1565 to 1600 MeV. Our analysis of these data sheds new light on claims for the Sigma(1580) 3/2^- resonance, which (if it exists with the specified quantum numbers) must be an exotic baryon because of its very low mass. Our results show no evidence for this state.

Reaction K^-p -> pi^0pi^0 Lambda was measured at eight incident K- momenta between 514 and 750 MeV/c using the Crystal Ball multiphoton spectrometer. The reaction dynamics are displayed in total cross sections, Dalitz plots, invariant-mass spectra, production angular distributions, and the L polarization. The pi^0pi^0Lambda production is dominated by the pi^0Sigma^0(1385) intermediate state ; no trace of other light Sigma^* states is observed, and the role of the f_0(600) meson appears to be insignificant. A striking similarity is seen between K^-p -> pi^0pi^0 Lambda and pi^-p-> pi^0pi^0n ; this can be understood as a consequence of dynamical flavor symmetry.

Data for pion-single-charge exchange reactions at 750 MeV/c on complex nuclei are analyzed for y-scaling, or single scattering quasifree, responses. The angular dependence of the data is used to separate the spin and nonspin isovector responses, with comparisons to electron scattering results.

Results are presented for pi- and K- -induced reactions on a proton target leading to all-enutral final states. also shown are new data on rare and forbidden eta-emson decays. The detector is the Crystal Ball multi-photon spectrometer, which until recently was located in the C-6 secondary beam line at the AGS. Special emphasis is given to 2pi0 production.

We report a new measurement of the pi(-)p-->3pi(0)n total cross section from threshold to p(pi)=0.75 GeV/c. The cross section near the N(1535)1/2(-) resonance is only a few mub after subtracting the large eta-->3pi(0) background associated with pi(-)p-->etan. A simple analysis of our data results in the estimated branching fraction B[S-11-->piN(1440)1/2(+)]=(8+/-2)%. This is the first such estimate obtained with a three-pion production reaction.

Data are presented for the reaction pi^-p->pi^0pi^0n in the range from threshold to p_pi^-= 750 MeV/c. The systematics of the data and multipole analyses are examined for sensitivity to a f_0(600) ("sigma") meson. A one-pion-exchange mechanism is found to be very weak, or absent. The reaction appears to become dominated by sequential pi^0 decays through the Delta(1232) resonance as the beam momentum increases, along with substantial interference effects from several competing mechanisms.

The results of a search for K^- p->pi^0pi^0pi^0 Lambda(where the 3pi^0's are not from eta-meson decay) are presented. The data were obtained with the Crystal Ball spectrometer at eight beam momenta from 514 to 750 MeV/c. For the six beam momenta below p_K^- = 714 MeV/c, no signal was found the 90% C.L. upper limit obtained for the K^- p->3pi^0 Lambda total cross section sigma_t varies between 2 and 7 &micro ; ; b. This small upper limit is indicative that spontaneous pi^0 emission is insignificant, since the K^- p->pi^0pi^0pi^0 Lambda threshold is at p_K^- = 397 MeV/c. A signal was observed only at p_K^- = 750 MeV/c, with sigma_t = 25&plusmn ; ; 7 &micro ; ; b. These results can be explained only if triple pi^0 production goes predominantly by hyperon resonance deexcitation, K^- p->Sigma*->pi^0Lambda^*. There are several candidates for the Sigma^* but only one for the Lambda^*, namely, the Lambda(1520)3/2^-, as the threshold for K^- p->pi^0 Lambda(1520) is at p_K^- = 704 MeV/c.

Measurement of differential cross-sectuibs if tge reaction Pi- p-->eta n using the Crystal Ball detector

A polarized proton beam extracted from SATURNE II and the Saclay polarized proton target were used to determine the spin correlation parameter Aoosk and the rescattering observablesK os″ so; Dos″ok, Nos″sn, andN onsk at 1.80 and 2.10 GeV. The beam polarization was oriented perpendicular to the beam direction in the horizontal scattering plane and the target polarization was directed either along the vertical axis or longitudinally. Left-right and up-down asymmetries in the second scattering were measured. A check for the beam optimization with the beam and target polarizations oriented vertically provided other observables, of which results forD onon andK onno at 1.80, 1.85, 2.04, and 2.10 GeV are listed here. The new data at 2.10 GeV suggest a smooth energy dependence of spin triplet scattering amplitudes at fixed angles in the vicinity of this energy.