We observe a signal for the doubly charmed baryon Xi(+)(cc) in the charged decay mode Xi(+)(cc)-->Lambda(+)(c)K-pi(+) in data from SELEX, the charm hadroproduction experiment at Fermilab. We observe an excess of 15.9 events over an expected background of 6.1+/-0.5 events, a statistical significance of 6.3sigma. The observed mass of this state is 3519+/-1 MeV/c(2). The Gaussian mass width of this state is 3 MeV/c(2), consistent with resolution; its lifetime is less than 33 fs at 90% confidence.

We observe a signal for the doubly charmed baryon Ξ + cc in the decay mode Ξ + cc → pD + K − to complement the previous reported decay Ξ + cc → Λ + c K − π + in data from SELEX, the charm hadroproduction experiment at Fermilab. In this new decay mode we observe an excess of 5.62 events over a combinatoric background estimated by event mixing to be 1.38 ± 0.13 events. The mixed background has Gaussian statistics, giving a signal significance of 4.8σ. The Poisson probability that a background fluctuation can produce the apparent signal is less than 6.4 × 10 −4. The observed mass of this state is 3518 ± 3 MeV/c 2 , consistent with the published result. Averaging the two results gives a mass of 3518.7 ± 1.7 MeV/c 2. The observation of this new weak decay mode confirms the previous SELEX suggestion that this state is a double charm baryon. The relative branching ratio for these two modes is 0.36 ± 0.21.

We review the experimental and theoretical status of weak radiative hyperon decays. Our discussion centers on a controversy over the validity of Hara’s theorem, which was originally expected to be respected by these decays. After presenting the hadron level theorem we describe experiments that have provided convincing evidence against its applicability to these decays. In the theoretical part we juxtapose the hadron level and quark level approaches and discuss the violation of Hara’s theorem in the latter. We review quark model phenomenology, which offers a promising description of experimental data. Measurements that should be made to settle the theoretical controversy are pinpointed. The importance of radiative hyperon decays in understanding the nonlocal composite nature of hadrons is stressed.

We report the results of a polarized-2-beta-decay experiment carried out in the Fermilab Proton Center charged-hyperon beam. These results are based on 49 671 observed 2 -+ n e -V decays. The 2-beam had a nominal momentum of 250 GeV/c and was produced by 400-GeV/c protons impinging on a Cu target. At a production angle of 2.5 mrad, the polarization was (23.6f 4.31%. The decay asymmetries of the electron ( a , = -0.519+0.104), neutron ( a , = +0.509+0.102), and antineutrino (a,= -0.230f 0.061) were measured and used to establish sign and approximate magnitude of the axial-vector-to-vector form-factor ratio g , / f , . The form-factor ratios g , / f l / and f 2 / f were determined most sensitively from the neutron and electron center-of-mass spectra, respectively. We obtain 1 g , / f l -0.237g,/f1 =0.327i0.007i0.019 and f , ( O ) / f , ( 0 ) = -0.96 f0.07k0.13, where the stated errors are statistical and systematic, respectively. A general fit that includes the asymmetries and makes the conventional assumption g , =O gives the final value g , ( O ) / f , ( 0 ) = -0.328k0.019. The data are also compatible with positive values for g , / f , combined with corresponding reduced values for g , / f , / .