232 research outputs found
Polarization observables in deuteron photodisintegration below 360 MeV
High precision measurements of induced and transferred recoil proton polarization in d((gamma) over right arrow, (p) over right arrow )n have been performed for photon energies of 277-357 MeV and theta(cm) = 20 degrees-120 degrees. The measurements were motivated by a longstanding discrepancy between meson-baryon model calculations and data at higher energies. At the low energies of this experiment, theory continues to fail to reproduce the data, indicating that either something is missing in the calculations and/or there is a problem with the accuracy of the nucleon-nucleon potential being used. (C) 2011 Elsevier B.V. All rights reserved
Nuclear dependence of the coherent eta photoproduction reaction in a relativistic approach
We study the nuclear (or A) dependence of the coherent eta photoproduction
reaction in a relativistic impulse approximation approach. We use a standard
relativistic parameterization of the elementary amplitude, based on a set of
four Lorentz- and gauge-invariant amplitudes, to calculate the coherent
production cross section from He-4, C-12, and Ca-40. In contrast to
nonrelativistic treatments, our approach maintains the full relativistic
structure of the process. The nuclear structure affects the process through the
ground-state tensor density. This density is sensitive to relativistic effects
and depends on A in a different manner than the vector density used in
nonrelativistic approaches. This peculiar dependence results in He-4 having a
cross section significantly smaller than that of C-12---in contrast to existent
nonrelativistic calculations. Distortion effects are incorporated through an
eta-nucleus optical potential that is computed in a simple ``t rho''
approximation.Comment: 4 pages, 3 postscript figures. The replace is due to a misspelling in
the Authors' lis
Electroexcitation of the Δ+ (1232) at Low Momentum Transfer
We report on new p(e, e\u27 p)π°. measurements at the Δ+(1232) resonance at the low momentum transfer region, where the mesonic cloud dynamics is predicted to be dominant and rapidly changing, offering a test bed for chiral effective field theory calculations. The new data explore the Q2 dependence of the resonant quadrupole amplitudes and for the first time indicate that the Electric and the Coulomb quadrupole amplitudes converge as Q2 -\u3e 0. The measurements of the Coulomb quadrupole amplitude have been extended to the lowest momentum transfer ever reached, and suggest that more than half of its magnitude is attributed to the mesonic cloud in this region. The new data disagree with predictions of constituent quark models and are in reasonable agreement with dynamical calculations that include pion cloud effects, chiral effective field theory and lattice calculations. The measurements indicate that improvement is required to the theoretical calculations and provide valuable input that will allow their refinements
Three-Nucleon Photodisintegration of 3He
The three-nucleon photodisintegration of 3He has been calculated in the whole
phase space using consistent Faddeev equations for the three-nucleon bound and
scattering states. Modern nucleon-nucleon and 3N forces have been applied as
well as different approaches to nuclear currents. Phase space regions are
localized where 3N force effects are especially large. In addition
semi-exclusive cross sections for 3He(gamma,N) have been predicted which carry
interesting peak structures. Finally some data for the exclusive 3N breakup
process of 3He and its total breakup cross section have been compared to
theory.Comment: 28 pages, 6 png figures, 11 ps figures, modified version with changed
figures, conclusions unchanged, to appear in Phys.Rev.
Measurement of the Partial Cross Sections s(TT), s(LT) and [s(T)+epsilon*s(L)] of the p(e,e' pi+)n Reaction in the Delta(1232) Resonance
We report new precision p(e,e' pi+})n measurements in the Delta(1232)
resonance at Q2 = 0.127(GeV/c)2 obtained at the MIT-Bates Out-Of-Plane
scattering facility. These are the lowest, but non-zero, Q2 measurements in the
pi+ channel. The data offer new tests of the theoretical calculations,
particularly of the background amplitude contributions. The chiral effective
field theory and Sato-Lee model calculations are not in agreement with this
experiment
Investigation of the conjectured nucleon deformation at low momentum transfer
We report new precise H measurements at the
resonance at (GeV/c) using the MIT/Bates
out-of-plane scattering (OOPS) facility. The data reported here are
particularly sensitive to the transverse electric amplitude () of the
transition. Analyzed together with previous data yield
precise quadrupole to dipole amplitude ratios and and
for . They give credence to the conjecture of
deformation in hadronic systems favoring, at low , the dominance of
mesonic effects.Comment: 4 pages, 1figur
HD 181068: A Red Giant in a Triply-Eclipsing Compact Hierarchical Triple System
Hierarchical triple systems comprise a close binary and a more distant
component. They are important for testing theories of star formation and of
stellar evolution in the presence of nearby companions. We obtained 218 days of
Kepler photometry of HD 181068 (magnitude of 7.1), supplemented by groundbased
spectroscopy and interferometry, which show it to be a hierarchical triple with
two types of mutual eclipses. The primary is a red giant that is in a 45-day
orbit with a pair of red dwarfs in a close 0.9-day orbit. The red giant shows
evidence for tidally-induced oscillations that are driven by the orbital motion
of the close pair. HD 181068 is an ideal target for studies of dynamical
evolution and testing tidal friction theories in hierarchical triple systems.Comment: 22 pages, including supporting on-line material. This is the author's
version of the work. It is posted here by permission of the AAAS for personal
use, not for redistribution. The definitive version was published in Science
Vol. 332 no. 6026 pp. 216-218 (8 April 2011), doi:10.1126/science.1201762.
http://www.sciencemag.org/content/332/6026/216.ful
A Comparison of Polarization Observables in Electron Scattering from the Proton and Deuteron
Recoil proton polarization observables were measured for both the p(,e) and d(,en reactions at two values of Q using a newly commissioned proton
Focal Plane Polarimeter at the M.I.T.-Bates Linear Accelerator Center. The
hydrogen and deuterium spin-dependent observables and
, the induced polarization and the form factor ratio
were measured under identical kinematics. The deuterium and
hydrogen results are in good agreement with each other and with the plane-wave
impulse approximation (PWIA).Comment: 9 pages, 1 figure; accepted by Phys. Rev. Let
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