[en] The SELEX experiment (E781) at Fermilab has observed two statistically compelling high mass states near 3.6 GeV/c², decaying to Λ_c⁺ K^- π⁺ and Λ_c⁺ K^- π⁺π⁺. These final states are Cabibbo-allowed decay modes of doubly charmed baryons Ξ_cc⁺ and Ξ_cc⁺⁺, respectively. The masses are in the range expected from theoretical considerations, but the spectroscopy is surprising. SELEX also has weaker preliminary evidence for a state near 3.8 GeV/c², a high mass state decaying to Λ_c⁺ K^- π⁺π⁺, possibly an excited Ξ_cc⁺⁺ (ccu*). Data are presented and discussed

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No abstract available

[en] Test results are described on three inexpensive 5-inch photomultiplier tubes of Venetian-blind construction to be used for collection of Cherenkov light produced by electromagnetic showers in lead glass. Parameters evaluated include photoelectron collection efficiency, pulse-height resolution, amplitude and rate stability, and linearity

[en] We studied the ³He(π⁺,pp)p and ³He(π^-,pn)n reactions at 37 MeV at TRIUMF. The outgoing protons were detected with NaI(Tl) detectors and the coincident protons (neutrons) with an array of plastic scintillator. The angular distributions for the absorption of a pion on two nucleons were determined. For the ³He(π⁺,pp)p reaction, the cross section shape is similar to that of d(π⁺,pp), while the magnitude is about 1.5 times larger (7.9 ± 0.7 mb). For the ³He(π^-, pn)n reaction, the differential cross sections (with respect to the detected proton) are asymmetric about 90 degrees in the π-pp center-of-mass system, with the probability higher for proton emission at backward angles. The magnitude of the cross section is 0.85 ± 0.15mb. The present data are at the lowest energy compared to previous pion absorption on ³He. One expects from recent theoretical studies that low energy absorption is of particular interest for probing short-range nucleon- nucleon correlations. Our cross section results for π⁺ and π^- are similar to those obtained at higher energies

[en] Fermilab E781 plans measurements of γ-Sigma and γ-pion interactions using a 600 GeV beam of Sigmas and pions, and a virtual photon target. Pion polarizabilities and radiative transitions will be measured in this experiment. The former can test a precise prediction of chiral symmetry; the latter for α₁ (1260) → πγ is important for understanding the polarizability. The experiment also measures polarizabilities and radiative transitions for Sigma hyperons. The polarizabilities can test predictions of baryon chiral perturbation theory. The radiative transitions to the Σ^* (1385) provide a measure of the magnetic moment of the s-quark. Previous experimental and theoretical results for γπ and γΣ interactions are given. The E781 experiment is described. (orig.)

[en] Doubly differential continuum cross sections from the ⁹⁰Zr(p, n) ⁹⁰Nb reaction have been analyzed via a multipole-decomposition technique. No quasi-free charge-exchange background has been subtracted, following the assumption that the observed cross sections are primarily due to one-step charge-exchange leading to 1p-1h states of all multipolarities to all excitations. The theoretical shapes of the differential cross sections for each J^π multipole have been taken from random-phase approximation (RPA) - distorted-wave impulse approximation (DWIA) calculations. the experimental dσ / dΩ for each 1 MeV excitation-energy bin have been decomposed into different multipole components by a least squares fit. This RPA-based analysis should determine the J^π = 1⁺ cross sections with different, and also fewer, assumptions than usual for describing the underlying background. It can be of general importance in determining the extent of possible quenching of Gamow-Teller (GT) strength. The present decomposition accounts for all the theoretically predicted GT strength. The purpose of the present report is to illustrate an analysis based on RPA-DWIA shapes rather than to present final-decomposition results

[en] Measurements have been made of the absolute yield of the positron emitters ¹¹C and ¹³N produced by 76-MeV negative pions stopped in thick stacked-foil targets of C (as graphite), N (as boron nitride), O (as water), and a tissue-equivalent mixture. The depth distribution of radioactivity exhibits a distinct peak in the water, boron nitride, and tissue-equivalent targets which correlates well with the position and width of the pion stopping distribution and with the high-LET component of dose. There are 0.0186 ¹¹C nuclei and 0.0085 ¹³N nuclei produced per pion stopped in water, and 0.0484 ¹¹C nuclei produced per pion stopped in nitrogen. These yields are compred to predictions of an intranuclear cascade calculation. The yields of ¹¹C and ¹³N in the stopping region from secondary particle-induced reactions are low, and do not significantly limit the ability to localize the pion star region. The activities measured for the tissue-equivalent target agree with the predictons based on the measurements for carbon, nitrogen, and oxygen targets and are similar to the yields for water. The utilizaton of a positron camera to image the radioactivity distributon in the pion stopping region is discussed

[en] Possible experimental searches of doubly charmed baryons and tetraquarks at fixed target experiments with high energy hadron beams and a high intensity spectrometer are considered here. The baryons are: Ξ_cc⁺(ccd), Ξ_cc⁺⁺(ccu), and Ω_cc⁺(ccs); and the tetraquark is T (cc anti u anti d). Estimates are given of masses, lifetimes, internal structure, production cross sections, decay modes, branching ratios, and yields. Experimental requirements are given for optimizing the signal and minimizing the backgrounds. This paper is designed as an experimental and theoretical review. It may therefore be of assistance in the planning for a future state-of-the-art high statistics charm experiment, in the spirit of the aims of the recent CHARM2000 workshop. (orig.)

[en] Angular distributions of differential cross sections were measured for the first time for pion absorption on a T = 1, S = 0 nucleon pair and for absorption on a T = 0, S = 1 pair in the ³He nucleus. A large isospin dependence is observed in the differential cross sections. The ratio of cross sections sigma(³He(π⁺,2p))/sigma( ³He(π^-,pn)) is 15.3 +- 1.2. The results show evidence of an isoscalar component of the final state in the reaction ³He(π^-,pn)n, which cannot be mediated by Δ resonance formation