Probing the structure of Nucleons in Electromagbetic Interactions

Open problems in the study of the nucleon structure using electromagnetic probes are discussed. The focus is on experimental aspects in the regime of strong interaction QCD. Significant progress in our understanding of the nucleon structure in this domain of QCD may be expected in the first decade of the next millenium. This is due to major experimental and theoretical efforts currently underway in this field.Comment: 9 pages, 6 figures, plenary talk at PANIC9

The Formation of the Milky Way in the Cosmological Context

The formation of the Milky Way is discussed within the context of the cold dark matter scenario. Several problems arise which can be solved if the Galaxy experienced an early phase of gas heating and decoupling from the dark matter substructure. This model combines the Eggen, Lynden-Bell and Sandage picture of a monolithic protogalactic collapses with the Searle and Zinn picture of an early merging phase of substructures into one consistent scenario of Galactic formation.Comment: 5 pages, conference proceeding. to appear in "Cosmic Evolution", eds. M. Lemoine and E. Vangioni-Fla

Thermal Instability and the Formation of Clumpy Gas Clouds

The radiative cooling of optically thin gaseous regions and the formation of a two-phase medium and of cold gas clouds with a clumpy substructure is investigated. In optically thin clouds, the growth rate of small isobaric density perturbations is independent of their length scale. However, the growth of a perturbation is limited by its transition from isobaric to isochoric cooling. The temperature at which this transition occurs decreases with the length scale of the perturbation. Consequently small scale perturbations have the potential to reach higher amplitudes than large scale perturbations. When the amplitude becomes nonlinear, advection overtakes the pressure gradient in promoting the compression resulting in an accelerated growth of the disturbance. The critical temperature for transition depends on the initial amplitude. The fluctuations which can first reach nonlinearity before their isobaric to isochoric transition will determine the characteristic size and mass of the cold dense clumps which would emerge from the cooling of an initially nearly homogeneous region of gas. Thermal conduction is in general very efficient in erasing isobaric, small-scale fluctuations, suppressing a cooling instability. A weak, tangled magnetic field can however reduce the conductive heat flux enough for low-amplitude fluctuations to grow isobarically and become non-linear if their length scales are of order 0.01 pc. Finally, we demonstrate how a 2-phase medium, with cold clumps being pressure confined in a diffuse hot residual background component, would be sustained if there is adequate heating to compensate the energy loss.Comment: 26 pages, Latex, 10 postscript figures, ApJ, in pres

Excitation of Nucleon Resonances

I discuss developments in the area of nucleon resonance excitations that are necessary to bring our understanding of nucleon structure in the regime of strong QCD to a qualitatively new level. They involve the collection of high quality data in various channels, a more rigorous approach in the search for "missing" quark model states, an effort to compute some critical quantities in nucleon resonance excitations from first principles, i.e. QCD, and a proposal aimed at obtaining an understanding of a fundamental quantity in nucleon structure.Comment: 9 pages, 2 figure

Probing the Structure of Nucleons in the Resonance Region with CLAS at Jefferson Lab

The physics of electromagnetic excitation of nucleon resonances and of their relevance in nucleon structure studies are discussed. Preliminary data from the CLAS detector on the N-Delta(1232) transition multipoles, the helicity amplitudes of the N*(1535), and the search for so-called "missing resonances" at Jefferson Lab are presented.Comment: Talk presented at Few Body Problems in Physics, Taipei, March 6-10, 2000, 10 pages, 8 figure

The Challenge of Modelling Galactic Disks

Detailed models of galactic disk formation and evolution require knowledge about the initial conditions under which disk galaxies form, the boundary conditions that affect their secular evolution and the micro-physical processes that drive the multi-phase interstellar medium and regulate the star formation history. Unfortunately, up to now, most of these ingredients are still poorly understood. The challenge therefore is to, despite this caveat, construct realistic models of galactic disks with predictive power. This short review will summarize some problems related to numerical simulations of galactic disk formation and evolution.Comment: 7 pages, 2 figures, invited contribution: IAU Symposium 254 on "The Galaxy Disk in Cosmological Context

Spin Physics in the Resonance Region

Recent results from Jefferson Lab on measurement of inclusive double polarization asymmetries in the nucleon resonance region are discussed. Preliminary results on the first moment of the structure function g_1(x,Q^2) for protons, and on the generalized Gerasimov-Drell-Hearn integral for neutrons are presented as well. The first moment for protons shows a strong Q^2 dependence below Q^2 = 1 GeV^2, and changes sign near Q^2 = 0.3 GeV^2. Strong double polarization asymmetries are observed in exclusive electroproduction of pions from polarized protons.Comment: 9 pages, 6 figures, Talk presented at the 3rd International Conference on Perspectives in Hadronic Physics, Trieste May 7-11, 200

Strangeness Physics with CLAS at JLab

A brief overview of strangeness physics with the CLAS detector at JLab is given, mainly covering the domain of nucleon resonances. Several excited states predicted by the symmetric constituent quark model may have significant couplings to the K-Lambda or K-Sigma channels. I will discuss data that are relevant in the search for such states in the strangeness channel, and give an outlook on the future prospects of the N* program at JLab with electromagnetic probes.Comment: 9 pages, 7 figures, Conference on Strangeness in Nuclei, SENDAI 200