Observation of strongly deformed shapes in 154-152Dy nuclei at medium temperatures

The gamma-decay of the giant dipole resonance (GDR) built on excited states of 154-152Dy nuclei is studied. The selection of GDR decay from high spin states leading to specific exit channels was made possible by triggering on high spin isomers. The deduced energy splitting of the GDR implies large deformations (\beta\ congruent-to 0.4-0.5). The resonance widths of the components are comparable to those of the GDR built on the ground state indicating small shape fluctuations

GDR γ-ray decay in 156Dy∗ from regions selected on temperature and angular momentum

The strength distribution of the GDR built on highly excited states in a restricted temperature domain in Dy-156 and Dy-155 nuclei has been deduced by subtraction of gamma-ray spectra obtained for the decay of Dy-154* and Dy-156* from regions selected on angular momentum. The resulting difference spectra have been analyzed within the statistical model. The results show a large deformation (\beta\ similar to 0.51 +/- 0.29 and 0.35 +/- 0.14) for the angular-momentum regions with [J] similar to 32 ($) over bar h at T approximate to 1.8 +/- 0.2 MeV and [J] similar to 46 ($) over bar h at T approximate to 1.7 +/- 0.2 MeV respectively, in satisfactory agreement with calculations performed in the framework of Landau theory of shape transitions and statistical fluctuations. The deduced centroid energies are in agreement with the systematics of the GDR built on the ground state. The width of the GDR shows a systematic increase with increasing temperature

Quark helicity distributions in the nucleon for up, down, and strange quarks from semi--inclusive deep--inelastic scattering

Polarized deep--inelastic scattering data on longitudinally polarized hydrogen and deuterium targets have been used to determine double spin asymmetries of cross sections. Inclusive and semi--inclusive asymmetries for the production of positive and negative pions from hydrogen were obtained in a re--analysis of previously published data. Inclusive and semi--inclusive asymmetries for the production of negative and positive pions and kaons were measured on a polarized deuterium target. The separate helicity densities for the up and down quarks and the anti--up, anti--down, and strange sea quarks were computed from these asymmetries in a ``leading order'' QCD analysis. The polarization of the up--quark is positive and that of the down--quark is negative. All extracted sea quark polarizations are consistent with zero, and the light quark sea helicity densities are flavor symmetric within the experimental uncertainties. First and second moments of the extracted quark helicity densities in the measured range are consistent with fits of inclusive data

Exclusive Leptoproduction of rho^0 Mesons from Hydrogen at Intermediate Virtual Photon Energies

Measurements of the cross section for exclusive virtual-photoproduction of rho^0 mesons from hydrogen are reported. The data were collected by the HERMES experiment using 27.5 GeV positrons incident on a hydrogen gas target in the HERA storage ring. The invariant mass W of the photon-nucleon system ranges from 4.0 to 6.0 GeV, while the negative squared four-momentum Q^2 of the virtual photon varies from 0.7 to 5.0 GeV^2. The present data together with most of the previous data at W > 4 GeV are well described by a model that infers the W-dependence of the cross section from the dependence on the Bjorken scaling variable x of the unpolarized structure function for deep-inelastic scattering. In addition, a model calculation based on Off-Forward Parton Distributions gives a fairly good account of the longitudinal component of the rho^0 production cross section for Q^2 > 2 GeV^2.Comment: 10 pages, 6 embedded figures, LaTeX for SVJour(epj) document class. Revisions: curves added to Fig. 1, several clarifications added to tex

Evidence for a narrow |S|=1 baryon state at a mass of 1528 MeV in quasi-real photoproduction

Evidence for a narrow baryon state is found in quasi-real photoproduction on a deuterium target through the decay channel p K^0_S --> p pi^+ pi^-. A peak is observed in the p K^0_S invariant mass spectrum at 1528 +/- 2.6 (stat) +/-2.1 (syst) MeV. Depending on the background model,the naive statistical significance of the peak is 4--6 standard deviations and its width may be somewhat larger than the experimental resolution of sigma=4.3 -- 6.2 MeV. This state may be interpreted as the predicted S=+1 exotic Theta^{+}(uuddbar(s)) pentaquark baryon. No signal for an hypothetical Theta^{++} baryon was observed in the pK^+ invariant mass distribution. The absence of such a signal indicates that an isotensor Theta is excluded and an isovector Theta is unlikely.Comment: 8 pages, 4 figure

Isomer-selected decay of the GDR in 156Dy

The strength distribution of high-energy gamma-rays emitted in the statistical decay of the 156Dy compound nucleus, with E* = 92.5 MeV, has been measured inclusively and for selected decay pathways in which isomers in 149Dy, 151Dy and 152Dy were populated. For both spectra a deformation \beta\ = 0.3 is deduced and the centroid energies agree with those found from ground-state systematics. The high-energy gamma-ray spectrum triggered on isomeric decay shows an increase of the width of the GDR in comparison with the inclusive spectrum

NONCOLLECTIVE DEGREES OF FREEDOM IN PB-192

Two positive-parity and two negative-parity groups of states of non-collective character with excitation energies ranging from 3 to 7 MeV and spins as high as 21 HBAR have been established in 192Pb. The main features of each group are described phenomenologically as four-quasiparticle configurations, arising from coupling of the 2(1)+, 4(1)+ and 6(1)+ states to a specific aligned neutron two-quasiparticle configuration. Supporting evidence for this interpretation and the importance of proton excitations in this singly closed-shell nucleus is obtained from the predictions of the quasiparticle multi-step shell model (QMSM). In addition, a four-quasiparticle structure of predominant neutron character is proposed on the basis of comparisons with the experimental data on 194Pb and with the results of the QMSM calculations