71,988 research outputs found
Instantons and the singlet-coupling in the chiral quark model
Chiral quark model with a broken-U(3) flavor symmetry can be interpreted as
the effective theory of the instanton-dominated non-perturbative QCD. This
naturally suggests the possibility of a negative singlet/octet coupling ratio,
which has been found, in a previous publication, to be compatible with the
phenomenological description of the nucleon spin-flavor structure.Comment: 9 page
Light pseudo-Goldstone bosons without explicit symmetry breaking
A mechanism is discussed to obtain light scalar fields from a spontaneously
broken continuous symmetry without explicitly breaking it. If there is a
continuous manifold of classical vacua in orbit space, its tangent directions
describe classically massless fields that may acquire mass from perturbations
of the potential that do not break the symmetry. We consider the simplest
possible example, involving a scalar field in the adjoint representation of
SU(N). We study the scalar mass spectrum and its RG running at one-loop level
including scalar and pseudoscalar Yukawa couplings to a massive Dirac fermion.Comment: minor typographical changes, 12 pages, 4 figure
Recent advancement of turbulent flow measurement techniques
Advancements of the fluctuating density gradient cross beam laser Schlieren technique, the fluctuating line-reversal temperature measurement and the development of the two-dimensional drag-sensing probe to a three-dimensional drag-sensing probe are discussed. The three-dimensionality of the instantaneous momentum vector can shed some light on the nature of turbulence especially with swirling flow. All three measured fluctuating quantities (density, temperature, and momentum) can provide valuable information for theoreticians
General Conditions for Lepton Flavour Violation at Tree- and 1-Loop Level
In this work, we compile the necessary and sufficient conditions a theory has
to fulfill in order to ensure general lepton flavour conservation, in the
spirit of the Glashow-Weinberg criteria for the absence of flavour-changing
neutral currents. At tree-level, interactions involving electrically neutral
and doubly charged bosons are investigated. We also investigate flavour changes
at 1-loop level. In all cases we find that the essential theoretical
requirements can be reduced to a few basic conditions on the particle content
and the coupling matrices. For 1-loop diagrams, we also investigate how exactly
a GIM-suppression can occur that will strongly reduce the rates of lepton
flavour violating effects even if they are in principle present in a certain
theory. In all chapters, we apply our criteria to several models which can in
general induce lepton flavour violation, e.g. LR-symmetric models or the MSSM.
In the end we give a summarizing table of the obtained results, thereby
demonstrating the applicability of our criteria to a large class of models
beyond the Standard Model.Comment: 31 pages, 2 figure
Flavor and Spin Contents of the Nucleon in the Quark Model with Chiral Symmetry
A simple calculation in the framework of the chiral quark theory of Manohar
and Georgi yields results that can account for many of the ''failures'' of the
naive quark model: significant strange quark content in the nucleon as
indicated by the value of the -
asymmetry in the nucleon as measured by the deviation from Gottfried sum rule
and by the Drell-Yan process, as well as the various quark contributions to the
nucleon spin as measured by the deep inelastic polarized lepton-nucleon
scatterings.Comment: figure has been separated from tex file. No other changes. Preprint
CMU-HEP94-3
One-loop quantum corrections to cosmological scalar field potentials
We study the loop corrections to potentials of complex or coupled real scalar
fields used in cosmology to account for dark energy, dark matter or dark fluid.
We show that the SUGRA quintessence and dark matter scalar field potentials are
stable against the quantum fluctuations, and we propose solutions to the
instability of the potentials of coupled quintessence and dark fluid scalar
fields. We also find that a coupling to fermions is very restricted, unless
this coupling has a structure which already exists in the scalar field
potential or which can be compensated by higher order corrections. Finally, we
study the influence of the curvature and kinetic term corrections.Comment: 11 pages, 1 figure, accepted for publication in Phys. Rev.
Structure of the electrospheres of bare strange stars
We consider a thin ( fm) layer of electrons (the
electrosphere) at the quark surface of a bare strange star, taking into account
the surface effects at the boundary with the vacuum. The quark surface holds
the electron layer by an extremely strong electric field, generated in the
electrosphere to prevent the electrons from escaping to infinity by
counterbalancing the degeneracy and thermal pressure. Because of the surface
tension and depletion of quarks a very thin (a few fm) charged layer of
quarks forms at the surface of the star. The formation of this layer modifies
the structure of the electrosphere, by significantly changing the electric
field and the density of the electrons, in comparison with the case when the
surface effects are ignored. Some consequences of the modification of the
electrosphere structure on the properties of strange stars are briefly
discussed.Comment: 23 pages, 6 figures, to appear in Ap
Chirally symmetric but confining dense and cold matter
The folklore tradition about the QCD phase diagram is that at the chiral
restoration phase transition at finite density hadrons are deconfined and there
appears the quark matter. We address this question within the only known
exactly solvable confining and chirally symmetric model. It is postulated
within this model that there exists linear Coulomb-like confining interaction.
The chiral symmetry breaking and the quark Green function are obtained from the
Schwinger-Dyson (gap) equation while the color-singlet meson spectrum results
from the Bethe-Salpeter equation. We solve this model at T=0 and finite
chemical potential and obtain a clear chiral restoration phase transition
at the critical value \mu_{cr}. Below this value the spectrum is similar to the
previously obtained one at \mu = 0. At \mu > \mu_{cr} the quarks are still
confined and the physical spectrum consists of bound states which are arranged
into a complete set of exact chiral multiplets. This explicitly demonstrates
that a chirally symmetric matter consisting of confined but chirally symmetric
hadrons at finite chemical potential is also possible in QCD. If so, there must
be nontrivial implications for astrophysics.Comment: 7 pp; the paper has been expanded to make some technical details more
clear; 3 new figures have been added. To appear in PR
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