Helicity amplitudes and electromagnetic decays of strange baryon resonances

We present results for the helicity amplitudes of the lowest-lying hyperon resonances Y*, computed within the framework of the Bonn constituent-quark model, which is based on the Bethe-Salpeter approach. The seven parameters entering the model are fitted against the best known baryon masses. Accordingly, the results for the helicity amplitudes are genuine predictions. Some hyperon resonances are seen to couple more strongly to a virtual photon with finite Q^2 than to a real photon. Other Y*'s, such as the S_{01}(1670) Lambda resonance or the S_{11}(1620) Sigma resonance, have large electromagnetic decay widths and couple very strongly to real photons. The negatively-charged and neutral members of a Sigma* triplet may couple only moderately to the Sigma(1193), while the positively-charged member of the same Sigma* triplet displays a relatively large coupling to the Sigma^+(1193) state. This illustrates the necessity of investigating all isospin channels in order to obtain a complete picture of the hyperon spectrum.Comment: 4 pages, 1 figure, 1 table, Proceedings of the Conference "International Workshop on the Physics of Excited Baryons NSTAR 05", Tallahassee, Florida (USA), Oct. 2005, contributed tal

Static observables of relativistic three-fermion systems with instantaneous interactions

We show that static properties like the charge radius and the magnetic moment of relativistic three-fermion bound states with instantaneous interactions can be formulated as expectation values with respect to intrinsically defined wavefunctions. The resulting operators can be given a natural physical interpretation in accordance with relativistic covariance. We also indicate how the formalism may be generalized to arbitrary moments. The method is applied to the computation of static baryon properties with numerical results for the nucleon charge radii and the baryon octet magnetic moments. In addition we make predictions for the magnetic moments of some selected nucleon resonances and discuss the decomposition of the nucleon magnetic moments in contributions of spin and angular momentum, as well as the evolution of these contributions with decreasing quark mass.Comment: 13 pages, including 2 figures and 3 tables, submitted to Eur.Phys.J.

Electromagnetic form factors of hyperons in a relativistic quark model

The relativistically covariant constituent quark model developed by the Bonn group is used to compute the EM form factors of strange baryons. We present form-factor results for the ground-state and some excited hyperons. The computed magnetic moments agree well with the experimental values and the magnetic form factors follow a dipole $Q^2$ dependence.Comment: 4 pages, 1 figure, Proceedings for NSTAR '04 conference in Grenoble, France, March 24-27, 2004 (World Scientific

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A search for signatures of interactions of X-ray binary outflows with their environments with ALMA

We observed the X-ray binaries CirX-1, ScoX-1, GRS 1915+105, GX13+1, and CygX-1 with the Atacama Large Millimeter/submillimeter Array (ALMA). Unresolved continuum emission is found at the positions of all the sources at a frequency of 92 GHz, with flux densities ranging between 0.8 and 10 mJy/beam. In all cases the emission can be associated with jets that have been extensively observed at lower frequencies. We searched for line emission from H$\alpha$ recombination, SiO,H2O, and CH3OH at the positions of all the sources and, for CirX-1 and CygX-1, also at regions where shocks associated with an interaction between the jet and the interstellar medium had previously been observed. The search did not yield any significant detection, resulting in 3$\sigma$ upper limits between 0.65 and 3.7 K km s$^{-1}$ for the existence of line emission in these regions. In contrast, we detected spatially unresolved SiO emission in the field of view of GX13+1, and we tentatively associate this emission with a SiO maser in a potential young stellar object or evolved star. We also found spatially extended line emission at two additional sites in the field of view of GX13+1 that we tentatively associate with emission from SO and CH3OH; we speculate that it may be associated with a star-forming region, but again we cannot rule out alternative origins such as emission from evolved stars.Comment: Accepted for publication in A&

Bound q2qˉ2q^2\bar q^2 states in a constituent quark model

We consider the existence of bound systems consisting of two quarks and two antiquarks ($q^2\bar q^2$) within the framework of a constituent quark model. The underlying quark dynamics is described by a linear confinement potential and an effective $q^2\bar q^2$ interaction which has its origin in instanton effects of QCD. We calculate the spectra and examine the internal structure of the states found.Comment: 11 pages, needs epsf.st

Semileptonic decays of baryons in a relativistic quark model

We calculate semileptonic decays of light and heavy baryons in a relativistically covariant constituent quark model. The model is based on the Bethe-Salpeter-equation in instantaneous approximation. It generates satisfactory mass spectra for mesons and baryons up to the highest observable energies. Without introducing additional free parameters we compute on this basis helicity amplitudes of electronic and muonic semileptonic decays of baryons. We thus obtain form factor ratios and decay rates in good agreement with experiment.Comment: 8 pages, 10 figures, 2 tables, typos remove

A Study of Lyman-Alpha Quasar Absorbers in the Nearby Universe

Spectroscopy of ten quasars obtained with the Goddard High Resolution Spectrograph (GHRS) of the Hubble Space Telescope (HST) is presented. A clustering analysis reveals an excess of nearest neighbor line pairs on velocity scales of 250-750 km/s at a 95-98% confidence level. The hypothesis that the absorbers are randomly distributed in velocity space can be ruled out at the 99.8% confidence level. No two-point correlation power is detected (xi < 1 with 95% confidence). Lyman-alpha absorbers have correlation amplitudes on scales of 250-500 km/s at least 4-5 times smaller than the correlation amplitude of bright galaxies. A detailed comparison between absorbers in nearby galaxies is carried out on a limited subset of 11 Lyman- alpha absorbers where the galaxy sample in a large contiguous volume is complete to M_B = -16. Absorbers lie preferentially in regions of intermediate galaxy density but it is often not possible to uniquely assign a galaxy counterpart to an absorber. This sample provides no explicit support for the hypothesis that absorbers are preferentially associated with the halos of luminous galaxies. We have made a preliminary comparison of the absorption line properties and environments with the results of hydrodynamic simulations. The results suggest that the Lyman-alpha absorbers represent diffuse or shocked gas in the IGM that traces the cosmic web of large scale structure. (abridged)Comment: 36 pages of text, 15 figures, 4 tables, 36 file