39,828 research outputs found
How non-linear scaling relations unify dwarf and giant elliptical galaxies
Dwarf elliptical galaxies are frequently excluded from bright galaxy samples
because they do not follow the same linear relations in diagrams involving
effective half light radii R_e or mean effective surface brightnesses _e.
However, using two linear relations which unite dwarf and bright elliptical
galaxies we explain how these lead to curved relations when one introduces
either the half light radius or the associated surface brightness. In
particular, the curved _e - R_e relation is derived here. This and other
previously misunderstood curved relations, once heralded as evidence for a
discontinuity between faint and bright elliptical galaxies at M_B ~ -18 mag,
actually support the unification of such galaxies as a single population whose
structure (i.e. stellar concentration) varies continuously with stellar
luminosity and mass.Comment: 4 pages including 2 figures, to appear in "A Universe of dwarf
galaxies", Conf. Proc. (Lyon, June 14-18, 2010
The Casimir Effect for Fermions in One Dimension
We study the Casimir problem for a fermion coupled to a static background
field in one space dimension. We examine the relationship between interactions
and boundary conditions for the Dirac field. In the limit that the background
becomes concentrated at a point (a ``Dirac spike'') and couples strongly, it
implements a confining boundary condition. We compute the Casimir energy for a
masslike background and show that it is finite for a stepwise continuous
background field. However the total Casimir energy diverges for the Dirac
spike. The divergence cannot be removed by standard renormalization methods. We
compute the Casimir energy density of configurations where the background field
consists of one or two sharp spikes and show that the energy density is finite
except at the spikes. Finally we define and compute an interaction energy
density and the force between two Dirac spikes as a function of the strength
and separation of the spikes.Comment: 18 pages, 6 figure
Subleading corrections to the Double Coset Ansatz preserve integrability
In this article we compute the anomalous dimensions for a class of operators,
belonging to the SU(3) sector of the theory, that have a bare dimension of
order N. For these operators the large N limit and the planar limit are
distinct and summing only the planar diagrams will not capture the large N
dynamics. Although the spectrum of anomalous dimensions has been computed for
this class of operators, previous studies have neglected certain terms which
were argued to be small. After dropping these terms diagonalizing the
dilatation operator reduces to diagonalizing a set of decoupled oscillators. In
this article we explicitly compute the terms which were neglected previously
and show that diagonalizing the dilatation operator still reduces to
diagonalizing a set of decoupled oscillators.Comment: 1 + 39 pages; v2: references adde
Electroweak Baryogenesis and the Standard Model Effective Field Theory
We investigate electroweak baryogenesis within the framework of the Standard
Model Effective Field Theory. The Standard Model Lagrangian is supplemented by
dimension-six operators that facilitate a strong first-order electroweak phase
transition and provide sufficient CP violation. Two explicit scenarios are
studied that are related via the classical equations of motion and are
therefore identical at leading order in the effective field theory expansion.
We demonstrate that formally higher-order dimension-eight corrections lead to
large modifications of the matter-antimatter asymmetry. The effective field
theory expansion breaks down in the modified Higgs sector due to the
requirement of a first-order phase transition. We investigate the source of the
breakdown in detail and show how it is transferred to the CP-violating sector.
We briefly discuss possible modifications of the effective field theory
framework.Comment: 21 pages + appendices. V2: Corrected a factor-2 mistake which has
changed the results for the baryon asymmetry quantitatively. Main conclusions
of the v1 still hol
Updated Mass Scaling Relations for Nuclear Star Clusters and a Comparison to Supermassive Black Holes
We investigate whether nuclear star clusters and supermassive black holes
follow a common set of mass scaling relations with their host galaxy's
properties, and hence can be considered to form a single class of central
massive object. We have compiled a large sample of galaxies with measured
nuclear star cluster masses and host galaxy properties from the literature and
fit log-linear scaling relations. We find that nuclear star cluster mass,
M_{NC}, correlates most tightly with the host galaxy's velocity dispersion: log
M_{NC} = (2.11 \pm 0.31) log (\sigma/54) + (6.63 \pm 0.09), but has a slope
dramatically shallower than the relation defined by supermassive black holes.
We find that the nuclear star cluster mass relations involving host galaxy (and
spheroid) luminosity and stellar and dynamical mass, intercept with but are in
general shallower than the corresponding black hole scaling relations. In
particular M_{NC} \propto {M}_{Gal,dyn}^{0.55 \pm 0.15}; the nuclear cluster
mass is not a constant fraction of its host galaxy or spheroid mass. We
conclude that nuclear stellar clusters and supermassive black holes do not form
a single family of central massive objects.Comment: 8 pages, 3 figure
Higher Loop Nonplanar Anomalous Dimensions from Symmetry
In this article we study the action of the one loop dilatation operator on
operators with a classical dimension of order N. These operators belong to the
su(2) sector and are constructed using two complex fields Y and Z. For these
operators non-planar diagrams contribute already at the leading order in N and
the planar and large N limits are distinct. The action of the one loop and the
two loop dilatation operator reduces to a set of decoupled oscillators and
factorizes into an action on the Z fields and an action on the Y fields. Direct
computation has shown that the action on the Y fields is the same at one and
two loops. In this article, using the su(2) symmetry algebra as well as
structural features of field theory, we give compelling evidence that the
factor in the dilatation operator that acts on the Ys is given by the one loop
expression, at any loop order.Comment: 1+40 page
Quasi-degenerate neutrinos and tri-bi-maximal mixing
We consider how, for quasi-degenerate neutrinos with tri-bi-maximal mixing at
a high-energy scale, the mixing angles are affected by radiative running from
high to low-energy scales in a supersymmetric theory. The limits on the
high-energy scale that follow from consistency with the observed mixing are
determined. We construct a model in which a non-Abelian discrete family
symmetry leads both to a quasi-degenerate neutrino mass spectrum and to near
tri-bi-maximal mixing.Comment: 8 pages, 3 figure
Smart Tea Project
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