1,188 research outputs found
Exchange Current Corrections to Neutrino--Nucleus Scattering
Relativistic exchange current corrections to neutrino--nucleus cross sections
are presented assuming non--vanishing strange quark form factors for the
constituent nucleons. For charged current processes the exchange current
corrections can lower the impulse approximation results by 10\% while these
corrections are found to be sensitive to both the nuclear density and the
strange quark axial form factor of the nucleon for neutral current processes.
Implications on the LSND experiment to determine this form factor are
discussed.Comment: 11 pages, 2 figures, revtex 3.0, full postscript version of the file
and figures available at
http://www.nikhefk.nikhef.nl/projects/Theory/preprints/preprints.html To
appear in Phys. Rev. Lett
The Solar hep Process in Effective Field Theory
Using effective field theory, we calculate the S-factor for the hep process
in a totally parameter-free formulation. The transition operators are organized
according to chiral counting, and their matrix elements are evaluated using the
realistic nuclear wave functions obtained in the
correlated-hyperspherical-harmonics method. Terms of up to
next-to-next-to-next-to-leading order in heavy-baryon chiral perturbation
theory are considered. Fixing the only parameter in the theory by fitting the
tritium \beta-decay rate, we predict the hep S-factor with accuracy better than
\sim 20 %.Comment: 4 pages, Revtex. Minor revision has been mad
Parameter-Free Calculation of the Solar Proton Fusion Rate in Effective Field Theory
Spurred by the recent complete determination of the weak currents in
two-nucleon systems up to in heavy-baryon chiral perturbation
theory, we carry out a parameter-free calculation of the solar proton fusion
rate in an effective field theory that combines the merits of the standard
nuclear physics method and systematic chiral expansion. Using the tritium
beta-decay rate as an input to fix the only unknown parameter in the effective
Lagrangian, we can evaluate with drastically improved precision the ratio of
the two-body contribution to the well established one-body contribution; the
ratio is determined to be (0.86\pm 0.05) %. This result is essentially
independent of the cutoff parameter for a wide range of its variation (500 MeV
\le \Lambda \le 800 MeV), a feature that substantiates the consistency of the
calculation.Comment: 10 pages. The argument is considerably more sharpened with a reduced
error ba
Magnetic properties of pure and Gd doped EuO probed by NMR
An Eu NMR study in the ferromagnetic phase of pure and Gd doped EuO was
performed. A complete description of the NMR lineshape of pure EuO allowed for
the influence of doping EuO with Gd impurities to be highlighted. The presence
of a temperature dependent static magnetic inhomogeneity in Gd doped EuO was
demonstrated by studying the temperature dependence of the lineshapes. The
results suggest that the inhomogeneity in 0.6% Gd doped EuO is linked to
colossal magnetoresistance. The measurement of the spin-lattice relaxation
times as a function of temperature led to the determination of the value of the
exchange integral J as a function of Gd doping. It was found that J is
temperature independent and spatially homogeneous for all the samples and that
its value increases abruptly with increasing Gd doping.Comment: 14 pages, 10 figures, to be published in Physical Review
X-rays and Protostars in the Trifid Nebula
The Trifid Nebula is a young HII region recently rediscovered as a
"pre-Orion" star forming region, containing protostars undergoing violent mass
ejections visible in optical jets as seen in images from the Infrared Space
Observatory and the Hubble Space Telescope. We report the first X-ray
observations of the Trifid nebula using ROSAT and ASCA. The ROSAT image shows a
dozen X-ray sources, with the brightest X-ray source being the O7 star, HD
164492, which provides most of the ionization in the nebula. We also identify
85 T Tauri star and young, massive star candidates from near-infrared colors
using the JHKs color-color diagram from the Two Micron All Sky Survey (2MASS).
Ten X-ray sources have counterpart near-infrared sources. The 2MASS stars and
X-ray sources suggest there are potentially numerous protostars in the young
HII region of the Trifid. ASCA moderate resolution spectroscopy of the
brightest source shows hard emission up to 10 keV with a clearly detected Fe K
line. The best model fit is a two-temperature (T = 1.2x10^6 K and 39x10^6 K)
thermal model with additional warm absorbing media. The hotter component has an
unusually high temperature for either an O star or an HII region; a typical
Galactic HII region could not be the primary source for such hot temperature
plasma and the Fe XXV line emission. We suggest that the hotter component
originates in either the interaction of the wind with another object (a
companion star or a dense region of the nebula) or from flares from deeply
embedded young stars.Comment: Accepted in ApJ (Oct, 20 issue, 2001
Parameter-free effective field theory calculation for the solar proton-fusion and processes
Spurred by the recent complete determination of the weak currents in two-nucleon systems up to in heavy-baryon chiral perturbation theory, we carry out a parameter-free calculation of the threshold -factors for the solar (proton-fusion) and processes in an effective field theory that {\it combines} the merits of the standard nuclear physics method and systematic chiral expansion. The power of the EFT adopted here is that one can correlate in a unified formalism the weak-current matrix elements of two-, three- and four-nucleon systems. Using the tritium -decay rate as an input to fix the only unknown parameter in the theory, we can evaluate the threshold factors with drastically improved precision; the results are and . The dependence of the calculated -factors on the momentum cutoff parameter has been examined for a physically reasonable range of . This dependence is found to be extremely small for the process, and to be within acceptable levels for the process, substantiating the consistency of our calculational scheme
Evolution of magnetic polarons and spin-carrier interactions through the metal-insulator transition in EuGdO
Raman scattering studies as functions of temperature, magnetic field, and
Gd-substitution are used to investigate the evolution of magnetic polarons and
spin-carrier interactions through the metal-insulator transition in
EuGdO. These studies reveal a greater richness of phase behavior
than have been previously observed using transport measurements: a
spin-fluctuation-dominated paramagnetic (PM) phase regime for T T
T, a two-phase regime for T T in which magnetic polarons
develop and coexist with a remnant of the PM phase, and an inhomogeneous
ferromagnetic phase regime for T T
1/N_c Expansion of the Heavy Baryon Isgur-Wise Functions
The 1/N_c expansion of the heavy baryon Isgur-Wise functions is discussed.
Because of the contracted SU(2N_f) light quark spin-flavor symmetry, the
universality relations among the Isgur-Wise functions of \Lambda_b to \Lambda_c
and \Sigma_b^{(*)} to \Sigma_c^{(*)} are valid up to the order of 1/N_c^2.Comment: 7 pages, latex, no figures, to appear in Phys. Rev.
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