5,453 research outputs found
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
Recent Results from Jefferson Lab
Recent results on studies of the structure of nucleons and nuclei in the
regime of strong interaction QCD are discussed. Use of high current polarized
electron beams, polarized targets, and recoil polarimeters, in conjunction with
modern spectrometers and detector instrumentation allow much more detailed
studies of nucleon and nuclear structure than has been possible in the past.
The CEBAF accelerator at Jefferson Lab was build to study the internal
structure of hadrons in a regime where confinement is important and strong
interaction QCD is the relevant theory. I discuss how the first experiments
already make significant contributions towards an improved understanding of
hadronic structure.Comment: Lecture presented at the International School of Nuclear Physics,
Erice, Sicily, Italy, September 17 - 25, 199
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
The geometry and origin of ultra-diffuse ghost galaxies
The geometry and intrinsic ellipticity distribution of ultra diffuse galaxies
(UDGs) is determined from the line-of-sight distribution of axial ratios q of a
large sample of UDGs, detected by Koda et al. (2015) in the Coma cluster. With
high significance the data rules out an oblate, disk-like geometry,
characterised by major axi a=b>c. The data is however in good agreement with
prolate shapes, corresponding to a=b<c. This indicates that UDGs are not
thickened, rotating, axisymmetric disks, puffed up by violent processes.
Instead they are anisotropic elongated cigar- or bar-like structures, similar
to the prolate dwarf spheroidal galaxy population of the Local Group. The
intrinsic distribution of axial ratios of the Coma UDGs is flat in the range of
0.4 <= a/c <= 0.9 which will provide important constraints for theoretical
models of their origin. Formation scenarios that could explain the extended
prolate nature of UDGs are discussed.Comment: 13 pages, 4 figures, ApJ, in pres
Evidence of new nucleon resonances from electromagnetic meson production
The study of nucleon resonances in electromagnetic meson production with the
CLAS detector is discussed. The electromagnetic interaction is complementary to
pion scattering in the exploration of the nucleon excitation spectrum. Higher
mass states often decouple from the channel and are not seen in . Photoproduction of mesons, such as , and
may be more sensitive to many of these states. The CLAS
detector, combined with the use of energy-tagged polarized photons and
polarized electrons, as well as polarized targets and the measurement of recoil
polarization, are the tools needed for a comprehensive nucleon resonance
program. Some of the recently published high statistics data sets had
significant impact on further clarifying the nucleon excitation spectrum.Comment: 6 pages, 4 figures, presented at MESON 2012, Krako
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
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