461 research outputs found
Eta electroproduction on nuclei in the nucleon resonance region
We investigate eta electroproduction on nuclei for Q^2=2.4 and 3.6 GeV^2 in
the framework of a coupled-channel BUU transport model. We analyze the
importance of final state interactions and side feeding and compare with
findings drawn from eta photoproduction. It is shown that in contrast to
photoproduction the influence of etas stemming from secondary processes becomes
important at high Q^2.Comment: 5 pages, 5 figure
Resonances and higher twist in polarized lepton-nucleon scattering
We present a detailed analysis of resonance contributions in the context of
higher twist effects in the moments of the proton spin structure function g_1.
For each of these moments, it is found that there exists a characteristic Q^2
region in which (perturbative) higher twist corrections coexist with
(non-perturbative) resonance contribution of comparable magnitude.Comment: 17 pages LaTe
Nucleon Polarizibilities for Virtual Photons
We generalize the sum rules for the nucleon electric plus magnetic
polarizability and for the nucleon spin-polarizability
, to virtual photons with . The dominant low energy cross
sections are represented in our calculation by one-pion-loop graphs of
relativistic baryon chiral perturbation theory and the -resonance
excitation. For the proton we find good agreement of the calculated
with empirical values obtained from integrating up
electroproduction data for . The proton spin-polarizability
switches sign around and it joins smoothly the
"partonic" curve, extracted from polarized deep-inelastic scattering, around
. For the neutron our predictions of and
agree reasonably well at with existing determinations.
Upcoming (polarized) electroproduction experiments will be able to test the
generalized polarizability sum rules investigated here.Comment: 12 pages, 5 figures, submittes to Nuclear Physics
THE EXTERNAL ELECTRON BEAM OF THE CAMBRIDGE ELECTRON ACCELERATOR
The resonant beam extraction system of Cambridge Synchrotron is described, and measurements of the extraction efficiency and extracted beam properties are compared with values predicted by calculations. (auth
Quark model approach to the meson electroproduction on the proton
A recently developed quark model approach to pseudoscalar meson
photoproduction is extended to electroproduction process for the meson
in the kinematics of momentum transfer 4 (GeV/c) and total
center of mass energy 1.6 GeV. Existing data are well reproduced and
the roles of the and resonances are closely
investigated. In the study of the longitudinal excitation of the
resonance, a reliable constraint on the properties is obtained
by cleanly removing the electromagnetic transition from the amplitude. Thus, the fitted quantities can be
determined with an uncertainty of about 15%. This could be the first direct
constraint on the properties in theory.Comment: 31 pages, 9 figures; Substantially modified version, to appear on
Jour. of Phys.
Proton polarizability effect in the Lamb shift of the hydrogen atom
The proton polarizability correction to the Lamb shift of electronic and
muonic hydrogen is calculated on the basis of isobar model and experimental
data on the structure functions of deep inelastic lepton-nucleon scattering.
The contributions of the Born terms, vector-meson exchanges and nucleon
resonances are taken into account in the construction of the photoabsorption
cross sections for transversely and longitudinally polarized virtual photons
sigma_{T,L}.Comment: 11 pages, 3 figure
Deep lithospheric structures along the southern central Chile Margin from wide-angle P-wave modellilng
Crustal- and upper-mantle structures of the subduction zone in south central Chile, between 42 degrees S and 46 degrees S, are determined from seismic wide-angle reflection and refraction data, using the seismic ray tracing method to calculate minimum parameter models. Three profiles along differently aged segments of the subducting Nazca Plate were analysed in order to study subduction zone structure dependencies related to the age, that is, thermal state, of the incoming plate. The age of the oceanic crust at the trench ranges from 3 Ma on the southernmost profile, immediately north of the Chile triple junction, to 6.5 Ma old about 100 km to the north, and to 14.5 Ma old another 200 km further north, off the Island of Chiloe. Remarkable similarities appear in the structures of both the incoming as well as the overriding plate. The oceanic Nazca Plate is around 5 km thick, with a slightly increasing thickness northward, reflecting temperature changes at the time of crustal generation. The trench basin is about 2 km thick except in the south where the Chile Ridge is close to the deformation front and only a small, 800-m-thick trench infill could develop. In south central Chile, typically three quarters (1.5 km) of the trench sediments subduct below the decollement in the subduction channel. To the north and south of the study area, only about one quarter to one third of the sediments subducts, the rest is accreted above. Similarities in the overriding plate are the width of the active accretionary prism, 35-50 km, and a strong lateral crustal velocity gradient zone about 75-80 km landward from the deformation front, where landward upper-crustal velocities of over 5.0-5.4 km s<SU-1</SU decrease seaward to around 4.5 km s<SU-1</SU within about 10 km, which possibly represents a palaeo-backstop. This zone is also accompanied by strong intraplate seismicity. Differences in the subduction zone structures exist in the outer rise region, where the northern profile exhibits a clear bulge of uplifted oceanic lithosphere prior to subduction whereas the younger structures have a less developed outer rise. This plate bending is accompanied by strongly reduced rock velocities on the northern profile due to fracturing and possible hydration of the crust and upper mantle. The southern profiles do not exhibit such a strong alteration of the lithosphere, although this effect may be counteracted by plate cooling effects, which are reflected in increasing rock velocities away from the spreading centre. Overall there appears little influence of incoming plate age on the subduction zone structure which may explain why the M-w = 9.5 great Chile earthquake from 1960 ruptured through all these differing age segments. The rupture area, however, appears to coincide with a relatively thick subduction channel
Parity-Violating Excitation of the \Delta(1232): Hadron Structure and New Physics
We consider prospects for studying the parity-violating (PV) electroweak
excitation of the \Delta(1232) resonance with polarized electron scattering.
Given present knowledge of Standard Model parameters, such PV experiments could
allow a determination of the N -> \Delta electroweak helicity amplitudes. We
discuss the experimental feasibility and theoretical interpretability of such a
determination as well as the prospective implications for hadron structure
theory. We also analyze the extent to which a PV N -> \Delta measurement could
constrain various extensions of the Standard Model.Comment: 43 pages, RevTex, 8 PS figures, uses epsf.sty, rotate.sty, version to
appear in Nucl. Phys. A, main points emphasized, some typos correcte
Hadronic properties of the S_{11}(1535) studied by electroproduction off the deuteron
Properties of excited baryonic states are investigated in the context of
electroproduction of baryon resonances off the deuteron. In particular, the
hadronic radii and the compositeness of baryon resonances are studied for
kinematic situations in which their hadronic reinteraction is the dominant
contribution. Specifically, we study the reaction at for kinematics in which the produced hadronic state reinteracts
predominantly with the spectator nucleon. A comparison of constituent quark
model and effective chiral Lagrangian calculations of the shows
substantial sensitivity to the structure of the produced resonance.Comment: 24 pages, 5 figure
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