Peter D. Barnes

Experimental observation of the Σ hypernuclei, Σ6H and Σ16C

Abstract The hypernuclei Σ 6 H and Σ 16 C were observed by the (K − , π + ) reaction on targets of 6 Li and liquid O, respectively, at 713 MeV/ c incident K − momentum. Structure is seen in Σ 6 H which may be interpreted in terms of particle-hole excitations similar to the previously observed states in Λ 6 Li. The inablitity to resolve individual Σ hypernuclear levels in Σ 16 C, due in part to the excitation of non-coherent states as a result of the large momentum transfer of about 130 MeV/ c , precludes the extraction of the Σ-nucleus spin-orbit potential strength. The Σ-nucleus well depth appears to be 7 to 10 MeV less than that for the Λ.

States of 12ΛC formed in the reaction 12C(K−, π−)☆

Abstract States of 12 Λ C formed in the (K − , π − ) reaction have been studied for momentum transfers up to 260 MeV/ c , using an incident K − beam of 800 MeV/ c momentum. The angular distributions for the g.s. and for a peak at 11 meV have been measured between 0° and 19° in the laboratory. Limits on the splitting of the 11 MeV peak and on the formation of low-lying excited states are given.

States of

Abstract States of 12 Λ C formed in the (K − , π − ) reaction have been studied for momentum transfers up to 260 MeV/ c , using an incident K − beam of 800 MeV/ c momentum. The angular distributions for the g.s. and for a peak at 11 meV have been measured between 0° and 19° in the laboratory. Limits on the splitting of the 11 MeV peak and on the formation of low-lying excited states are given.

^{12}

Abstract Results are presented from a study of the reaction p p→ Λ Λ near threshold. Over 3000 events recorded at s values 14.6 and 25.5 MeV above the Λ Λ threshold (2231.2 MeV) have been analysed. Results for the production cross section, differential cross section, and the Λ and Λ polarization are given at both energies and are compared with recent theoretical calculations of this process.

C (

Abstract The PS185 experiment at the CERN Low Energy Antiproton Ring (LEAR) has studied the reaction p p → \ gLΛ at several momenta. In this paper results from two runs with high statistics at 1.546 GeV/c and 1.695 GeV/c are described. Based on 4063 and 11362 analysed events, respectively, differential and integrated cross sections, polarizations and spin correlations are presented. The singlet fraction, extracted from the spin correlations, is consistent with zero at both momenta, showing that the \ gLΛ pairs are produced in a pure triplet state. A comparison of the decay asymmetry parameters of Λ and \ gL reduces the upper limits for the violation of the CP invariance for this system.

\Lambda

Abstract The excitation function of the reaction p p→ Λ Λ in the threshold region has been measured at LEAR. Sixteen measurements of the total cross section, in the energy range between 0.85 MeV below threshold and 4.05 MeV above, are presented. The shapes of the measured differential cross sections indicate a remarkably strong p-wave contribution even down to the reaction threshold. We also report here the measurement of significant polarizations in the threshold region; these are compared with previous higher-energy data.

) Formed in the Reaction

Abstract In a study of hyperon production with antiprotons at the CERN Low-Energy Antiproton Ring (LEAR), we have measured the ratio A=(α + α )/(α− α ) . Here α and α are the decay asymmetry parameters of the Λ and Λ non-leptonic charged decays. The hyperons were produced with identical polarization in the reaction p p→ Λ Λ at an incident p momentum of 1.546 GeV/c. The result, A=− 0.07 ± 0.09, is consistent with zero, as required from CP invariance.

^{12}

Abstract Experimental evidence is presented for a source of unusually high strangeness content located at mid-rapidity in 200 GeV/ c per nucleon collisions of 32 S projectiles with a Pb target. The enhancement is not seen in p+Pb reactions measured in the same apparatus at the same energy. This source becomes dominant for central collisions.

C (

Abstract The 210 Po(t, α) 209 Bi reaction has been studied at 20 MeV triton energy with an overall resolution of 20 keV FWHM. Absolute cross sections were established by a comparison of the (t, α) intensities to elastic triton scattering. The three lowest single-proton states in 209 Bi were observed and spectroscopic factors were extracted from the measured cross sections by DWBA analysis. Starting at 2.43 MeV, six levels were strongly excited. These states have large 2p-1h proton components. The spectroscopic factors were extracted by a comparison of 210 Po(t, α) cross sections to 208 Pb(t, α) 207 T1 single-proton-hole cross sections measured previously at the same bombarding energy. The observed states and their properties were analyzed within the framework of the particle- (or hole) vibration coupling model. Surface vibrational states in 208 Pb coupled to single protons as well as the proton pair addition modes (states in 210 Po) coupled to a proton-hole were included in the analysis.

K^-

Abstract A search for collective states in 210 Pb has been conducted utilizing inelastic scattering of 20 MeV tritons and 20.5 MeV protons. In the triton scattering, cross sections were obtained for levels at 0.795, 1.87 and 2.83 MeV. The 0.795 level is a known 2 + level. It is excited with a cross section of one-half of that observed for the first 2 + level of 206 Pb under identical conditions. The two other states are assigned as 3 − states. The cross sections obtained for these two levels are approximately 2 3 and case1 3 of the cross section found in the (t, t′) reaction for the first 3 − state of 208 Pb. In the proton scattering, the above results are confirmed and several other levels observed. The results are compared with DWBA calculations.