6,506 research outputs found
Mass and width of the sigma
I report on recent work done in collaboration with Irinel Caprini and
Gilberto Colangelo. We observe that the Roy equations lead to a representation
of the pion pion scattering amplitude that exclusively involves observable
quantities, but is valid for complex values of s. At low energies, this
representation is dominated by the contributions from the two subtraction
constants, which are known to remarkable precision from the low energy theorems
of chiral perturbation theory. Evaluating the remaining contributions on the
basis of the available data, we demonstrate that the lowest resonance carries
the quantum numbers of the vacuum and occurs in the vicinity of the threshold.
Although the uncertainties in the data are substantial, the pole position can
be calculated quite accurately, because it occurs in the region where the
amplitude is dominated by the subtractions. The calculation neatly illustrates
the fact that the dynamics of the Goldstone bosons is governed by the
symmetries of QCD.Comment: Contribution to the proceedings of MESON 2006 (Krakow
Low relaxation rate in a low-Z alloy of iron
The longest relaxation time and sharpest frequency content in ferromagnetic
precession is determined by the intrinsic (Gilbert) relaxation rate \emph{}.
For many years, pure iron (Fe) has had the lowest known value of for all pure ferromagnetic metals or binary alloys. We show that an
epitaxial iron alloy with vanadium (V) possesses values of which are
significantly reduced, to 355 Mhz at 27% V. The result can be understood
as the role of spin-orbit coupling in generating relaxation, reduced through
the atomic number .Comment: 14 pages, 4 figure
Strong spin-orbit induced Gilbert damping and g-shift in iron-platinum nanoparticles
The shape of ferromagnetic resonance spectra of highly dispersed, chemically
disordered Fe_{0.2}Pt_{0.8} nanospheres is perfectly described by the solution
of the Landau-Lifshitz-Gilbert (LLG) equation excluding effects by crystalline
anisotropy and superparamagnetic fluctuations. Upon decreasing temperature, the
LLG damping and a negative g-shift, g(T)-g_0, increase proportional
to the particle magnetic moments determined from the Langevin analysis of the
magnetization isotherms. These novel features are explained by the scattering
of the magnon from an electron-hole (e/h) pair mediated by the
spin-orbit coupling, while the sd-exchange can be ruled out. The large
saturation values, and , indicate the
dominance of an overdamped 1 meV e/h-pair which seems to originate from the
discrete levels of the itinerant electrons in the d_p=3 nm nanoparticles.Comment: 8 pages, 4 figures, accepted for publication in Phys. Rev. B
(http://prb.aps.org/
NNLO QCD corrections to event shape variables in electron positron annihilation
Precision studies of QCD at electron-positron colliders are based on
measurements of event shapes and jet rates. To match the high experimental
accuracy, theoretical predictions to next-to-next-to-leading order (NNLO) in
QCD are needed for a reliable interpretation of the data. We report the first
calculation of NNLO corrections O(alpha_s^3) to three-jet production and
related event shapes, and discuss their phenomenological impact.Comment: Contributed to 2007 Europhysics Conference on High Energy Physics,
Manchester, England 19-25 July 200
Photon - Jet Correlations and Constraints on Fragmentation Functions
We study the production of a large-pT photon in association with a jet in
proton-proton collisions. We examine the sensitivity of the jet rapidity
distribution to the gluon distribution function in the proton. We then assess
the sensitivity of various photon + jet correlation observables to the photon
fragmentation functions. We argue that RHIC data on photon-jet correlations can
be used to constrain the photon fragmentation functions in a region which was
barely accessible in LEP experiments.Comment: 23 pages, 9 figure
The Role of Grass Feeding in Improving Oxidative Stability and Increasing Vitamin B\u3csub\u3e12\u3c/sub\u3e Content of Beef and Veal
The objective of this study was to investigate the influence of grass feeding of cattle on the decontamination capacity against free radical (DCAFR) and vitamin B12 content of beef and veal. The meat samples from intensively (26 cattle, final weight 485±9 kg, concentrate feeding) and ecologically (25 heifers, final weight 472±32 kg, and 28 calves, final weight 260±24 kg, pasture grazing) managed cattle groups were investigated. The DCAFR was colorimetric assessed by examining damage to 1.4 benzoquinone. Vitamin B12 was analyzed by high performance liquid chromatography. Data indicate that the oxidative stability of meat samples from ecological management were significant (p \u3c 0.05) higher (1.39 mg/ ml sec) compared to samples from intensive management (1.09 mg/ ml sec). Meat from pasture grazed animals proves to be an especially good source of vitamin B12, which was present in the high amount in beef (3.24 mg/100g) and veal (3.32 mg/100g). This paper establishes the importance of grass feeding at the levels of the relevant essential nutrients supplied by meat
Spin Reorientations Induced by Morphology Changes in Fe/Ag(001)
By means of magneto-optical Kerr effect we observe spin reorientations from
in-plane to out-of-plane and vice versa upon annealing thin Fe films on Ag(001)
at increasing temperatures. Scanning tunneling microscopy images of the
different Fe films are used to quantify the surface roughness. The observed
spin reorientations can be explained with the experimentally acquired roughness
parameters by taking into account the effect of roughness on both the magnetic
dipolar and the magnetocrystalline anisotropy.Comment: 4 pages with 3 EPS figure
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