830 research outputs found
Energy Dependence of the Delta Resonance: Chiral Dynamics in Action
There is an important connection between the low energy theorems of QCD and
the energy dependence of the Delta resonance in pi-N scattering, as well as the
closely related gamma^{*} N -> pi N reaction. The resonance shape is due not
only to the strong pi-N interaction in the p wave but the small interaction in
the s wave; the latter is due to spontaneous chiral symmetry breaking in QCD
(i.e. the Nambu-Goldstone nature of the pion). A brief overview of experimental
tests of chiral perturbation theory and chiral based models is presentedComment: 11 pages, 6 figures, Festschrift for S.N. yan
Measurements of the Generalized Electric and Magnetic Polarizabilities of the Proton at Low Q2 Using the VCS Reaction
The mean square polarizability radii of the proton have been measured for the
first time in a virtual Compton scattering experiment performed at the
MIT-Bates out-of-plane scattering facility. Response functions and
polarizabilities obtained from a dispersion analysis of the data at Q2=0.06
GeV2/c2 are in agreement with O(p3) heavy baryon chiral perturbation theory.
The data support the dominance of mesonic effects in the polarizabilities, and
the increase of beta with increasing Q2 is evidence for the cancellation of
long-range diamagnetism by short-range paramagnetism from the pion cloud
Measurements of the \gamma * p --> \Delta(1232) reaction at low Q2
We report new p measurements in the
resonance at the low momentum transfer region utilizing the
magnetic spectrometers of the A1 Collaboration at MAMI. The mesonic cloud
dynamics are predicted to be dominant and appreciably changing in this region
while the momentum transfer is sufficiently low to be able to test chiral
effective calculations. The results disagree with predictions of constituent
quark models and are in reasonable agreement with dynamical calculations with
pion cloud effects, chiral effective field theory and lattice calculations. The
reported measurements suggest that improvement is required to the theoretical
calculations and provide valuable input that will allow their refinements
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
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
Refractive elastic scattering of carbon and oxygen nuclei: The mean field analysis and Airy structures
The experimental data on the OC and OC elastic
scatterings and their optical model analysis are presented. Detailed and
complete elastic angular distributions have been measured at the Strasbourg
Vivitron accelerator at several energies covering the energy range between 5
and 10 MeV per nucleon. The elastic scattering angular distributions show the
usual diffraction pattern and also, at larger angles, refractive effects in the
form of nuclear rainbow and associated Airy structures. The optical model
analysis unambiguously shows the evolution of the refractive scattering
pattern. The observed structure, namely the Airy minima, can be consistently
described by a nucleus-nucleus potential with a deep real part and a weakly
absorptive imaginary part. The difference in absorption in the two systems is
explained by an increased imaginary (mostly surface) part of the potential in
the OC system. The relation between the obtained potentials and
those reported for the symmetrical OO and CC
systems is drawn.Comment: 10 pages, 9 figures, Phys. rev. C in pres
Lowest Q^2 Measurement of the gamma*p -> Delta Reaction: Probing the Pionic Contribution
To determine nonspherical angular momentum amplitudes in hadrons at long
ranges (low Q^2), data were taken for the p(\vec{e},e'p)\pi^0 reaction in the
Delta region at Q^2=0.060 (GeV/c)^2 utilizing the magnetic spectrometers of the
A1 Collaboration at MAMI. The results for the dominant transition magnetic
dipole amplitude and the quadrupole to dipole ratios at W=1232 MeV are:
M_{1+}^{3/2} = (40.33 +/- 0.63_{stat+syst} +/- 0.61_{model})
(10^{-3}/m_{\pi^+}),Re(E_{1+}^{3/2}/M_{1+}^{3/2}) = (-2.28 +/- 0.29_{stat+syst}
+/- 0.20_{model})%, and Re(S_{1+}^{3/2}/M_{1+}^{3/2}) = (-4.81 +/-
0.27_{stat+syst} +/- 0.26_{model})%. These disagree with predictions of
constituent quark models but are in reasonable agreement with lattice
calculations with non-linear (chiral) pion mass extrapolations, with chiral
effective field theory, and with dynamical models with pion cloud effects.
These results confirm the dominance, and general Q^2 variation, of the pionic
contribution at large distances.Comment: 6 pages, 3 figures, 1 tabl
Measurement of the Transverse-Longitudinal Cross Sections in the p (e,e'p)pi0 Reaction in the Delta Region
Accurate measurements of the p(e,e?p)pi0 reaction were performed at
Q^2=0.127(GeV/c)^2 in the Delta resonance energy region. The experiments at the
MIT-Bates Linear Accelerator used an 820 MeV polarized electron beam with the
out of plane magnetic spectrometer system (OOPS). In this paper we report the
first simultaneous determination of both the TL and TL? (``fifth" or polarized)
cross sections at low Q^{2} where the pion cloud contribution dominates the
quadrupole amplitudes (E2 and C2). The real and imaginary parts of the
transverse-longitudinal cross section provide both a sensitive determination of
the Coulomb quadrupole amplitude and a test of reaction calculations.
Comparisons with model calculations are presented. The empirical MAID
calculation gives the best overall agreement with this accurate data. The
parameters of this model for the values of the resonant multipoles are
|M_{1+}(I=3/2)|= (40.9 \pm 0.3)10^{-3}/m_pi, CMR= C2/M1= -6.5 \pm 0.3%,
EMR=E2/M1=-2.2 \pm 0.9%, where the errors are due to the experimental
uncertainties.Comment: 10 pages, 3 figures, minor corrections and addition
Parity nonconservation in deuteron photoreactions
We calculate the asymmetries in parity nonconserving deuteron
photodisintegration due to circularly polarized photons gamma+d to n+p with the
photon laboratory energy ranging from the threshold up to 10 MeV and the
radiative capture of thermal polarized neutrons by protons n+p to gamma+d. We
use the leading order electromagnetic Hamiltonian neglecting the smaller
nuclear exchange currents. Comparative calculations are done by using the
Reid93 and Argonne v18 potentials for the strong interaction and the DDH and
FCDH "best" values for the weak couplings in a weak one-meson exchange
potential. A weak NDelta transition potential is used to incorporate also the
Delta(1232)-isobar excitation in the coupled-channels formalism.Comment: 14 pages, 13 figures (18 eps files), LaTeX2
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