R. Sawafta

Measurement of tensor polarization in elastic electron–deuteron scattering at large momentum transfer

Tensor polarization observables ( t(20), t(21), and t(22)) have been measured in elastic electron-deuteron scattering for six values of momentum transfer between 0.66 and 1.7 (GeV/c)(2). The experiment was performed at the Jefferson Laboratory in Hall C using the electron High Momentum Spectrometer, a specially designed deuteron magnetic channel and the recoil deuteron polarimeter POLDER. The new data determine to much larger Q2 the deuteron charge form factors G(C) and G(Q). They are in good agreement with relativistic calculations and disagree with perturbative QCD predictions.

The superconducting kaon spectrometer — SKS

A superconducting kaon spectrometer has been installed in the north experimental hall of the KEK 12-GeV proton synchrotron. The spectrometer was designed to serve for nuclear physics experiments with meson beams in the 1 GeV/c region, particular emphasis being laid on study of Λ-hypernuclei via (π+, K+) reactions. In order to obtain Λ-hypernuclear data with better statistics and energy resolution, it was designed to have a good momentum resolution of 0.1% FWHM and a large acceptance of 100 msr. It consists of a large superconducting dipole magnet, tracking chambers, and trigger counters that can efficiently select kaons from large background of pions and protons. The overall energy resolution for scattering is realized together with a beam-line spectrometer in the K6 beam line, the momentum resolution of which was also designed to be better than 0.1% FWHM. A good energy resolution of better than 2 MeV FWHM has been confirmed in π−-12C elastic scattering and in the (π+,K+) reaction on 12C.

Elastic and inelastic scattering of K+ from 6Li and 12C

Abstract Cross sections for inelastic scattering of 635 and 715 MeV/ c K + from 12 C are compared with DWIA calculations in order to check for possible density-dependent effects on K + -nucleon cross sections within nuclei. New fits to 715 MeV/ c K + scattering spectra from 6 Li give elastic and inelastic cross sections and permit an evaluation of the density dependence of these effects. The same DWIA methods are also used to estimate quadrupole non-central contributions to elastic K + scattering from the 1 + ground state of 6 Li, and these are found to be very small.

The influence of the nuclear medium on K+ total cross sections

Abstract The study of the K+ total cross sections on a wide range of nuclei has revealed important modifications of the free-space K+-nucleon interaction when the nucleon is embedded in a nucleus. Data indicating such an effect were first obtained in a comparison of the total cross sections on carbon and deuterium. In this letter we report new measurements on 6Li, silicon, and calcium, and demonstrate that the previously reported modifications occur quite generally.

Search for double-Λ hypernuclei and the H-dibaryon in the (K−,K+) reaction on 12C

Abstract A search for double- Λ hypernuclei ( 12 ΛΛ Be) and H -dibaryons using the 12 C( K − , K + ) reaction was performed at the BNL-AGS using a high-intensity 1.8 GeV/ c K − beam. A missing-mass analysis below the end point of the quasi-free Ξ − production was used to investigate these S =−2 systems. The upper limit obtained for the forward-angle cross section of 12 ΛΛ Be production is 6 to 10 nb/sr. This is the first search for the direct production of double- Λ hypernuclei to reach the sensitivity required to observe the signal predicted by theoretical calculations. For the H -production cross section, we have obtained an upper limit in the range of a few nb/sr to 10 nb/sr for the H mass below 2100 MeV/ c 2 . This upper limit is the most sensitive H search result to date for a tightly bound H .

K+ elastic scattering from C and 6Li at 715 MeV/c

Abstract Elastic differential cross sections for K + mesons scattered from nat C and 6 Li targets have been measured at an incident momentum of 715 MeV/c and at angles of 7° to 42° in the laboratory frame. The experimental cross sections agree, within errors, with two different parameter-free impulse approximation calculations. To reduce the effects of the systematic errors, the ratio of the experimental cross sections for nat C to 6 Li is compared to the theoretical values, and these ratios do not agree with theory. This discrepancy suggests either a density-dependent alteration of K + -nucleon amplitudes or a failure of the optical potential calculations to describe these nuclides adequately.

Weak decays of He-4(Lambda)

We measured the lifetime and the mesonic and non-mesonic decay rates of the 4He-Lambda hypernucleus. The hypernuclei were created using a 750 MeV/c momentum K- beam on a liquid 4He target by the reaction 4He(K-,pi-)4He-Lambda. The 4He-Lambda lifetime was directly measured using protons from Lambda p -> n p non-mesonic decay (also referred to as proton-stimulated decay) and was found to have a value of tau = 245 +/- 24 ps. The mesonic decay rates were determined from the observed numbers of pi-s and pi0s as Gamma_pi-/Gamma_tot = 0.270 +/- 0.024 and Gamma_pi0/Gamma_tot = 0.564 +/- 0.036, respectively, and the values of the proton- and neutron-stimulated decay rates were extracted as Gamma_p/Gamma_tot = 0.169 +/- 0.019 and Gamma_n/Gamma_tot <= 0.032 (95% CL), respectively. The effects of final-state interactions and possible 3-body Lambda N N decay contributions were studied in the context of a simple model of nucleon-stimulated decay. Nucleon-nucleon coincidence events were observed and were used in the determination of the non-mesonic branching fractions. The implications of the results of this analysis were considered for the empirical Delta I = 1/2 rule and the decay rates of the 4H-Lambda hypernucleus.

Do narrow Σ hypernuclear states exist

Abstract The Σ hypernuclear problem has been the subject of intense experimental and theoretical debate for more than 15 years. Despite these efforts, the Σ-nucleus interaction is not well understood. Experimental data have been taken on different targets at different momenta and at different conditions. These data suffer from poor resolution and insufficient statistics. Contradictions among the different sets of data often resulted more in clouding the issues than clarifying them. This paper aims at clearing up this long-standing problem. Hypernuclear mass spectra for in-flight ( K − , π ± ) reactions from 1 H, 2 H, 6 Li and 9 Be have been measured. Preliminary results based on analysis of the ( K − , π ± ) spectra from 9 Be are discussed.

Electroproduction of kaons on light nuclei

The A(e,e{prime}K{sup +})YX reaction on H, D, {sup 3}He, and {sup 4}He was investigated in Hall C at CEBAF. Data were obtained for Q{sup 2} {approx} 0.35 and 0.5 GeV{sup 2} at 3.245 GeV. The missing mass spectra for both H and D are fitted with Monte-Carlo simulations incorporating peaks corresponding to {Lambda} production on the proton and {Sigma} production on both the proton and neutron. For D, the cross section ratio {Sigma}{sup 0}/{Sigma}{sup {minus}} {approx} 2, and excess yield close to the thresholds for {Lambda} and {Sigma} production can be attributed to final-state interactions that are compared to the data. The analysis of the data for the He targets is in a more preliminary state with broader quasi-free peaks resulting from the higher Fermi momenta. Evidence for bound {Lambda}-hypernuclear states is seen and other structure may be present.

Searching for the H-Dibaryon at Brookhaven

At the Brookhaven AGS several experiments are searching for the unique strangeness S = −2 H-dibaryon with the quark composition (uuddss). The E813/E836 collaboration, in particular, is using a high-intensity, separated 1.8 GeV/c K − beam and two different target configurations. In E836 the reaction K − + 3He→K + + H + n is used to search for a relatively deeply-bound H. Complementary to E836 the reactions K − + p → Ξ − + K +, followed by (Ξ −, d)atom → H + n are used to search near twice the A mass. The status of these two experiments is summarized, and other H-dibaryon searches are briefly reviewed.