Modification of the ω\omega-Meson Lifetime in Nuclear Matter

The photo production of $\omega$ mesons on the nuclei C, Ca, Nb and Pb has been measured using the Crystal Barrel/TAPS detector at the ELSA tagged photon facility in Bonn. The dependence of the $\omega$ meson cross section on the nuclear mass number has been compared with three different types of models, a Glauber analysis, a BUU analysis of the Giessen theory group and a calculation by the Valencia theory group. In all three cases, the inelastic $\omega$ width is found to be $130-150 \rm{MeV/c^2}$ at normal nuclear matter density for an average 3-momentum of 1.1 GeV/c. In the restframe of the $\omega$ meson, this inelastic $\omega$ width corresponds to a reduction of the $\omega$ lifetime by a factor $\approx 30$. For the first time, the momentum dependent $\omega$N cross section has been extracted from the experiment and is in the range of 70 mb.Comment: 5 pages, 4 figure

In-medium ω\omega mass from the γ+Nb→π0γ+X\gamma + Nb \to \pi^{0}\gamma + X reaction

Data on the photoproduction of $\omega$ mesons on nuclei have been re-analyzed in a search for in-medium modifications. The data were taken with the Crystal Barrel(CB)/TAPS detector system at the ELSA accelerator facility in Bonn. First results from the analysis of the data set were published by D. Trnka et al. in Phys. Rev. Lett 94 (2005) 192303 \cite{david}, claiming a lowering of the $\omega$ mass in the nuclear medium by 14$$ at normal nuclear matter density. The extracted $\omega$ line shape was found to be sensitive to the background subtraction. For this reason a re-analysis of the same data set has been initiated and a new method has been developed to reduce the background and to determine the shape and absolute magnitude of the background directly from the data. Details of the re-analysis and of the background determination are described. The $\omega$ signal on the $Nb$ target, extracted in the re-analysis, does not show a deviation from the corresponding line shape on a $LH_2$ target, measured as reference. The earlier claim of an in-medium mass shift is thus not confirmed. The sensitivity of the $\omega$ line shape to different in-medium modification scenarios is discussed.Comment: 13 pages and 11 figures, submitted for publicatio

Schwinger boson theory of anisotropic ferromagnetic ultrathin films

Ferromagnetic thin films with magnetic single-ion anisotropies are studied within the framework of Schwinger bosonization of a quantum Heisenberg model. Two alternative bosonizations are discussed. We show that qualitatively correct results are obtained even at the mean-field level of the theory, similar to Schwinger boson results for other magnetic systems. In particular, the Mermin-Wagner theorem is satisfied: a spontaneous magnetization at finite temperatures is not found if the ground state of the anisotropic system exhibits a continuous degeneracy. We calculate the magnetization and effective anisotropies as functions of exchange interaction, magnetic anisotropies, external magnetic field, and temperature for arbitrary values of the spin quantum number. Magnetic reorientation transitions and effective anisotropies are discussed. The results obtained by Schwinger boson mean-field theory are compared with the many-body Green's function technique.Comment: 14 pages, including 7 EPS figures, minor changes, final version as publishe

Characterization and intracellular localization of putative Chlamydia pneumoniae effector proteins

We here describe four proteins of Chlamydia pneumoniae, which might play a role in host-pathogen interaction. The hypothetical bacterial proteins CPn0708 and CPn0712 were detected in Chlamydia pneumoniae-infected host cells by indirect immunofluorescence tests with polyclonal antisera raised against the respective proteins. While CPn0708 was localized within the inclusion body, CPn0712 was identified in the inclusion membrane and in the surrounding host cell cytosol. CPn0712 colocalizes with actin, indicating its possible interaction with components of the cytoskeleton. Investigations on CPn0809 and CPn1020, two Chlamydia pneumoniae proteins previously described to be secreted into the host cell cytosol, revealed colocalization with calnexin, a marker for the ER. Neither CPn0712, CPn0809 nor CPn1020 were able to inhibit host cell apoptosis. Furthermore, transient expression of CPn0712, CPn0809 and CPn1020 by the host cell itself had no effect on subsequent infection with Chlamydia pneumoniae. However, microarray analysis of CPn0712-expressing host cells revealed six host cell genes which were regulated as in host cells infected with Chlamydia pneumoniae, indicating the principal usefulness of heterologous expression to study the effect of Chlamydia pneumoniae proteins on host cell modulation

Measurement of the Beam Asymmetry Σ\Sigma in the Forward Direction for pi0 Photoproduction

Photoproduction of neutral pions has been studied with the CBELSA/TAPS detector for photon energies between 0.92 and 1.68~GeV at the electron accelerator ELSA. The beam asymmetry~$\Sigma$ has been extracted for $115^\circ < \theta_{\rm c.m.} < 155^\circ$ of the $\pi^0$~meson and for $\theta_{\rm c.m.} < 60^\circ$. The new beam asymmetry data improve the world database for photon energies above 1.5~GeV and, by covering the very forward region, extend previously published data for the same reaction by our collaboration. The angular dependence of $\Sigma$ shows overall good agreement with the SAID parameterization.Comment: 11 pages, 10 figures, published in Phys. Rev. C, included LEPS data and MAID 2007 predictions for comparison with our dat

Human hippocampal neurogenesis drops sharply in children to undetectable levels in adults.

New neurons continue to be generated in the subgranular zone of the dentate gyrus of the adult mammalian hippocampus. This process has been linked to learning and memory, stress and exercise, and is thought to be altered in neurological disease. In humans, some studies have suggested that hundreds of new neurons are added to the adult dentate gyrus every day, whereas other studies find many fewer putative new neurons. Despite these discrepancies, it is generally believed that the adult human hippocampus continues to generate new neurons. Here we show that a defined population of progenitor cells does not coalesce in the subgranular zone during human fetal or postnatal development. We also find that the number of proliferating progenitors and young neurons in the dentate gyrus declines sharply during the first year of life and only a few isolated young neurons are observed by 7 and 13 years of age. In adult patients with epilepsy and healthy adults (18-77 years; n = 17 post-mortem samples from controls; n = 12 surgical resection samples from patients with epilepsy), young neurons were not detected in the dentate gyrus. In the monkey (Macaca mulatta) hippocampus, proliferation of neurons in the subgranular zone was found in early postnatal life, but this diminished during juvenile development as neurogenesis decreased. We conclude that recruitment of young neurons to the primate hippocampus decreases rapidly during the first years of life, and that neurogenesis in the dentate gyrus does not continue, or is extremely rare, in adult humans. The early decline in hippocampal neurogenesis raises questions about how the function of the dentate gyrus differs between humans and other species in which adult hippocampal neurogenesis is preserved

Neutron skin of 208^{208}Pb from Coherent Pion Photoproduction

Information on the size and shape of the neutron skin on $^{208}$Pb has been extracted from coherent pion photoproduction cross sections measured using the Crystal Ball together with the Glasgow tagger at the MAMI electron beam facility. On exploitation of an interpolated fit of a theoretical model to the measured cross sections the half-height radius and diffuseness of the neutron distribution are found to be 6.70$\pm 0.03(stat)$ fm and 0.55$\pm 0.01(stat)$$^{+0.02}_{-0.03}(sys)$ fm respectively, corresponding to a neutron skin thickness $\Delta r_{np}$=0.15$\pm 0.03(stat)$$^{+0.01}_{-0.03}(sys)$ fm. The results give the first successful extraction of a neutron skin with an electromagnetic probe and indicate the skin of $^{208}$Pb has a halo character. The measurement provides valuable new constraints on both the structure of nuclei and the equation of state for neutron-rich matter.Comment: 4 figures 5 pages. Version submitted to journal. Includes additional studies of systematic effects in the extracted diffuseness, which led to a small increase in the quoted systematic error. These additional studies are discussed in the revised manuscript. Also includes minor editorial improvements to the tex

Photoproduction of Neutral Pions off Protons

Photoproduction of neutral pions has been studied with the CBELSA/TAPS detector in the reaction $\gamma p\to p\pi^0$ for photon energies between 0.85 and 2.50 GeV. The $\pi^0$ mesons are observed in their dominant neutral decay mode: $\pi^0\to\gamma\gamma$. For the first time, the differential cross sections cover the very forward region, $\theta_{\rm c.m.}<60^\circ$. A partial-wave analysis of these data within the Bonn-Gatchina framework observes the high-mass resonances $G_{17}$(2190), $D_{13}$(2080), and $D_{15}$(2070).Comment: 12 pages, 9 figures, minor modifications to PWA sectio

Photoproduction of meson pairs: First measurement of the polarization observable I^s

The polarization observable I^s, a feature exclusive to the acoplanar kinematics of multi-meson final states produced via linearly polarized photons, has been measured for the first time. Results for the reaction g p -> p pi0 eta are presented for incoming photon energies between 970 MeV and 1650 MeV along with the beam asymmetry I^c. The comparably large asymmetries demonstrate a high sensitivity of I^s to the dynamics of the reaction. Fits using Bonn-Gatchina partial wave analysis demonstrate that the new polarization observables carry significant information on the contributing partial waves.Comment: 11 pages, 6 figures, v2 to appear in Phys. Lett.

Photoproduction of {\omega} Mesons off the Proton

The differential cross sections and unpolarized spin-density matrix elements for the reaction $\gamma p\to p\omega$ were measured using the CBELSA/TAPS experiment for initial photon energies ranging from the reaction threshold to 2.5 GeV. These observables were measured from the radiative decay of the $\omega$ meson, $\omega\to\pi^0\gamma$. The cross sections cover the full angular range and show the full extent of the $t$-channel forward rise. The overall shape of the angular distributions in the differential cross sections and unpolarized spin-density matrix elements are in fair agreement with previous data. In addition, for the first time, a beam of linearly-polarized tagged photons in the energy range from 1150 MeV to 1650 MeV was used to extract polarized spin-density matrix elements. These data were included in the Bonn-Gatchina partial wave analysis (PWA). The dominant contribution to $\omega$ photoproduction near threshold was found to be the $3/2^+$ partial wave, which is primarily due to the sub-threshold $N(1720)\,3/2^+$ resonance. At higher energies, pomeron-exchange was found to dominate whereas $\pi$-exchange remained small. These $t$-channel contributions as well as further contributions from nucleon resonances were necessary to describe the entire dataset: the $1/2^-$, $3/2^-$, and $5/2^+$ partial waves were also found to contribute significantly.Comment: 8 pages, 5 figures, accepted for publication in Physics Letters