11,932 research outputs found
HI Observations Towards the Sagittarius Dwarf Spheroidal Galaxy
We have measured the 21-cm line of Galactic HI over more than 50 square
degrees in the direction of the Sagittarius dwarf spheroidal galaxy. The data
show no evidence of HI associated with the dwarf spheroidal which might be
consider analogous to the Magellanic Stream as it is associated in both
position and velocity with the Large Magellanic Cloud. Nor do the HI data show
evidence for any disturbance in the Milky Way disk gas that can be
unambiguously assigned to interaction with the dwarf galaxy. The data shown
here limit the HI mass at the velocity of the Sagittarius dwarf to <7000 solar
masses over some 18 square degrees between Galactic latitudes -13 degrees and
-18 degrees.Comment: 5 pages, 4 figures; accepted for publication in Astronomy &
Astrophysic
A stochastic dominance approach to program evaluation with an application to child nutritional status in arid and semi-arid Kenya
Food Consumption/Nutrition/Food Safety,
Integrating out the Dirac sea in the Walecka model
We derive a purely fermionic no-sea effective theory, featuring
positive-energy states only for the Walecka model. In contrast to the so-called
mean-field theory approach with the no-sea approximation, where the Dirac sea
is simply omitted from the outset, we turn to the relativistic Hartree
approximation and explicitly construct a no-sea effective theory from the
underlying quantum field theory. Several results obtained within these two
approaches are confronted with each other. This sheds new light on the
reliability of the mean-field theory with the no-sea approximation as well as
the role of the Dirac sea. Restricting to 1+1 dimensions, we obtain new
analytical insights into nonuniform nuclear matter.Comment: 15 pages, 8 figures, several points clarified, Fig.7 replaced,
references adde
Radiative spin polarization in an ultrastrong magnetic field
We calculate the spin flip rates for an electron in a homogeneous magnetic
field for low excitations (). Our results apply for all field strengths
including those beyond the critical field strength at which the spin
contributes as much to the electron's energy as its rest mass. Existing
approximations either assume that the electron is in a sufficiently highly
excited state such that its orbit can be assumed to be classical or the
magnetic field be weak compared to the critical field. The regime of high
magnetic field strength and low excitations is therefore poorly covered by
them. By comparing our calculations to different approximations, we find that
in the high field, low excitation regime the spin flip rates are lower and the
equilibrium spin polarization is less pure then one would get by naively
applying existing approximations in this regime.Comment: 9 pages, 6 figure
Integrating out the Dirac sea: Effective field theory approach to exactly solvable four-fermion models
We use 1+1 dimensional large N Gross-Neveu models as a laboratory to derive
microscopically effective Lagrangians for positive energy fermions only. When
applied to baryons, the Euler-Lagrange equation for these effective theories
assumes the form of a non-linear Dirac equation. Its solution reproduces the
full semi-classical results including the Dirac sea to any desired accuracy.
Dynamical effects from the Dirac sea are encoded in higher order derivative
terms and multi-fermion interactions with perturbatively calculable, finite
coefficients. Characteristic differences between models with discrete and
continuous chiral symmetry are observed and clarified.Comment: 13 pages, 11 figures; v2: typos corrected (Eqs. 4 and 44
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