Beam asymmetry Sigma for pi(+) and pi(0) photoproduction on the proton for photon energies from 1.102 to 1.862 GeV

Beam asymmetries for the reactions gamma p -\u3e p pi(0) and gamma p -\u3e n pi(+) have been measured with the CEBAF Large Acceptance Spectrometer (CLAS) and a tagged, linearly polarized photon beam with energies from 1.102-1.862 GeV. A Fourier moment technique for extracting beam asymmetries from experimental data is described. The results reported here possess greater precision and finer energy resolution than previous measurements. Our data for both pion reactions appear to favor the SAID and Bonn-Gatchina scattering analyses over the older Mainz MAID predictions. After incorporating the present set of beam asymmetries into the world database, exploratory fits made with the SAID analysis indicate that the largest changes from previous fits are for properties of the Delta(1700)3/2(-) and Delta(1905) 5/2(+) states

Probing the nucleon structure with CLAS

An overview of recent results with CLAS is presented with emphasis on nucleon resonance studies, nucleon spin structure, and generalized parton distributions.Comment: Plenary talk presented at NSTAR 2007, Bonn, German

On the cross section ratio sigma_n/sigma_p in eta photoproduction

The recently discovered enhancement of eta photoproduction on the quasi-free neutron at energies around sqrt(s)~1.67 GeV is addressed within a SU(3) coupled channel model. The quasi-free cross sections on proton and neutron, sigma_n and sigma_p, can be quantitatively explained. In this study, the main source for the peak in sigma_n/sigma_p is a coupled channel effect in S-wave that explains the dip-bump structure in gamma n --> eta n. In particular, the photon coupling to the intermediate meson-baryon states is important. The stability of the result is extensively tested and consistency with several pion- and photon-induced reactions is ensured.Comment: Version accepted by Phys. Lett.

Aging model for solid lubricants used in weapon stronglinks: Oxidation chemistry and hardware review

In support of efforts to model the performance of the MC2969 strong link for stockpile life extension, the kinetics of oxidation of the MoS{sub 2} based solid lubricant coating have been determined. The lubricant oxidation is primarily influenced by the extent of burnishing of the coating after application and curing. The activation energy for lubricant oxidation is low and agrees well with reported values for MoS{sub 2} coatings and particles. The type of substrate material and the amount of H{sub 2}O vapor present have little influence on the oxidation kinetics, but do affect the chemical species found on the surface, including sulfate species which enhance substrate corrosion. The analysis of field returned hardware shows oxidation levels within the range of those obtained throughout the oxidation study

Distributive justice with and without culture

Academic treatments of distributive justice normally adopt a static approach centred on resource allocation among a set of individual agents. The resulting models, expressed in mathematical language, make no allowance for culture, as they never engage with the society’s way of life or the moulding of individuals within society. This paper compares the static approach to distributive justice with a cultural one, arguing that a case for redistribution should rest upon its cultural effects in assisting well-being and social cohesion. Unless we recognise culture, we can have little understanding of why inequalities matter, where they come from, and how they might be reduced. Redistribution may be motivated by universal value judgements taken from external sources, but it also entails internal cultural changes that refashion social relations through cumulative causation. In practical terms, it has to penetrate beyond reallocating resource endowments to bring revised attitudes in a society less tolerant of unequal outcomes. Egalitarian reforms will flourish only if they generate and reflect an egalitarian culture

Backward-angle photoproduction of π0\pi^0 mesons on the proton at EγE_\gamma = 1.5--2.4 GeV

Differential cross sections and photon beam asymmetries for $\pi^0$ photoproduction have been measured at $E_\gamma$ = 1.5--2.4 GeV and at the $\pi^0$ scattering angles, --1 $<$ cos$\Theta_{c.m.} <$ --0.6. The energy-dependent slope of differential cross sections for $u$-channel $\pi^0$ production has been determined. An enhancement at backward angles is found above $E_\gamma$ = 2.0 GeV. This is inferred to be due to the $u$-channel contribution and/or resonances. Photon beam asymmetries have been obtained for the first time at backward angles. A strong angular dependence has been found at $E_\gamma >$ 2.0 GeV, which may be due to the unknown high-mass resonances.Comment: 12 pages, 4 figures, submitted to PL

Constraints on Decaying Dark Matter from Fermi Observations of Nearby Galaxies and Clusters

We analyze the impact of Fermi gamma-ray observations (primarily non-detections) of selected nearby galaxies, including dwarf spheroidals, and of clusters of galaxies on decaying dark matter models. We show that the fact that galaxy clusters do not shine in gamma rays puts the most stringent limits available to-date on the lifetime of dark matter particles for a wide range of particle masses and decay final states. In particular, our results put strong constraints on the possibility of ascribing to decaying dark matter both the increasing positron fraction reported by PAMELA and the high-energy feature in the electron-positron spectrum measured by Fermi. Observations of nearby dwarf galaxies and of the Andromeda Galaxy (M31) do not provide as strong limits as those from galaxy clusters, while still improving on previous constraints in some cases.Comment: 27 pages, 5 figures, submitted to JCAP, revised version with some additions and correction

Penta-quark states with hidden charm and beauty

More and more hadron states are found to be difficult to be accommodated by the quenched quark models which describe baryons as 3-quark states and mesons as antiquark-quark states. Dragging out an antiquark-quark pair from the gluon field in hadrons should be an important excitation mechanism for hadron spectroscopy. Our recent progress on the penta-quark states with hidden charm and beauty is reviewed.Comment: Plenary talk at the 5th Asia-Pacific Conference on Few-Body Problems in Physics 2011 (APFB2011), 22-26 Aug., 2011, Seoul, Kore

Photoproduction of pions and properties of baryon resonances from a Bonn-Gatchina partial wave analysis

Masses, widths and photocouplings of baryon resonances are determined in a coupled-channel partial wave analysis of a large variety of data. The Bonn-Gatchina partial wave formalism is extended to include a decomposition of t- and u-exchange amplitudes into individual partial waves. The multipole transition amplitudes for $\gamma p\to p\pi^0$ and $\gamma p\to n\pi^+$ are given and compared to results from other analyses.Comment: 18 pages, 14 figure