701 research outputs found
A family of triaxial modified Hubble mass models: effects of the additional radial functions
The projected properties of triaxial generalization of the modified Hubble
mass models are studied. These models are constructed by adding the additional
radial functions, each multiplied by a low-order spherical harmonic, to the
models of \citet{ct00}. The projected surface density of mass models can be
calculated analytically which allows us to derive the analytic expressions of
axial ratio and position angles of major axis of constant density elliptical
contours at asymptotic radii. The models are more general than those studied
earlier in the sense that the inclusions of additional terms in density
distribution, allows one to produce varieties of the radial profile of axial
ratio and position angle, in particular, their small scale variations at inner
radii. Strong correlations are found to exist between the observed axial ratio
evaluated at and at which occupy well-separated regions
in the parameter space for different choices of the intrinsic axial ratios.
These correlations can be exploited to predict the intrinsic shape of the mass
model, independent of the viewing angles. Using Bayesian statistics, the result
of a test case launched for an estimation of the shape of a model galaxy is
found to be satisfactory.Comment: 23 pages, 8 figures, accepted for publication in New Astronom
Glueballs and k-strings in SU(N) gauge theories : calculations with improved operators
We test a variety of blocking and smearing algorithms for constructing
glueball and string wave-functionals, and find some with much improved overlaps
onto the lightest states. We use these algorithms to obtain improved results on
the tensions of k-strings in SU(4), SU(6), and SU(8) gauge theories. We
emphasise the major systematic errors that still need to be controlled in
calculations of heavier k-strings, and perform calculations in SU(4) on an
anisotropic lattice in a bid to minimise one of these. All these results point
to the k-string tensions lying part-way between the `MQCD' and `Casimir
Scaling' conjectures, with the power in 1/N of the leading correction lying in
[1,2]. We also obtain some evidence for the presence of quasi-stable strings in
calculations that do not use sources, and observe some near-degeneracies
between (excited) strings in different representations. We also calculate the
lightest glueball masses for N=2, ...,8, and extrapolate to N=infinity,
obtaining results compatible with earlier work. We show that the N=infinity
factorisation of the Euclidean correlators that are used in such mass
calculations does not make the masses any less calculable at large N.Comment: 49 pages, 15 figure
Baryogenesis in Cosmological Model with Superstring-Inspired E_6 Unification
We have developed a concept of parallel existence of the ordinary (O) and
hidden (H) worlds with a superstring-inspired E_6 unification, broken at the
early stage of the Universe into SO(10) X U(1) - in the O-world, and SU(6)' X
SU(2)' - in the H-world. As a result, we have obtained in the hidden world the
low energy symmetry group G'_SM X SU(2)'_\theta, instead of the Standard Model
group G_SM. The additional non-Abelian SU(2)'_\theta group with massless gauge
fields, "thetons", is responsible for the dark energy. We present a
baryogenesis mechanism with the B-L asymmetry produced by the conversion of
ordinary leptons into particles of the hidden sector.Comment: 15 pages, 2 figure
Random Exchange Disorder in the Spin-1/2 XXZ Chain
The one-dimensional XXZ model is studied in the presence of disorder in the
Heisenberg Exchange Integral. Recent predictions obtained from renormalization
group calculations are investigated numerically using a Lanczos algorithm on
chains of up to 18 sites. It is found that in the presence of strong
X-Y-symmetric random exchange couplings, a ``random singlet'' phase with
quasi-long-range order in the spin-spin correlations persists. As the planar
anisotropy is varied, the full zero-temperature phase diagram is obtained and
compared with predictions of Doty and Fisher [Phys. Rev. B {\bf 45 }, 2167
(1992)].Comment: 9 pages + 8 plots appended, RevTex, FSU-SCRI-93-98 and
ORNL/CCIP/93/1
The S-matrix of the Faddeev-Reshetikhin Model, Diagonalizability and PT Symmetry
We study the question of diagonalizability of the Hamiltonian for the
Faddeev-Reshetikhin (FR) model in the two particle sector. Although the two
particle S-matrix element for the FR model, which may be relevant for the
quantization of strings on , has been calculated recently
using field theoretic methods, we find that the Hamiltonian for the system in
this sector is not diagonalizable. We trace the difficulty to the fact that the
interaction term in the Hamiltonian violating Lorentz invariance leads to
discontinuity conditions (matching conditions) that cannot be satisfied. We
determine the most general quartic interaction Hamiltonian that can be
diagonalized. This includes the bosonic Thirring model as well as the bosonic
chiral Gross-Neveu model which we find share the same S-matrix. We explain this
by showing, through a Fierz transformation, that these two models are in fact
equivalent. In addition, we find a general quartic interaction Hamiltonian,
violating Lorentz invariance, that can be diagonalized with the same two
particle S-matrix element as calculated by Klose and Zarembo for the FR model.
This family of generalized interaction Hamiltonians is not Hermitian, but is
symmetric. We show that the wave functions for this system are also
symmetric. Thus, the theory is in a unbroken phase which guarantees the
reality of the energy spectrum as well as the unitarity of the S-matrix.Comment: 32 pages, 1 figure; references added, version published in JHE
Le Chatelier-Braun principle in cosmological physics
Assuming that dark energy may be treated as a fluid with a well defined
temperature, close to equilibrium, we argue that if nowadays there is a
transfer of energy between dark energy and dark matter, it must be such that
the latter gains energy from the former and not the other way around.Comment: 6 pages, revtex file, no figures; version accepted for publication in
General Relativity and Gravitatio
The gauge-string duality and heavy ion collisions
I review at a non-technical level the use of the gauge-string duality to
study aspects of heavy ion collisions, with special emphasis on the trailing
string calculation of heavy quark energy loss. I include some brief
speculations on how variants of the trailing string construction could provide
a toy model of black hole formation and evaporation. This essay is an invited
contribution to "Forty Years of String Theory" and is aimed at philosophers and
historians of science as well as physicists.Comment: 21 page
Spin injection into a ballistic semiconductor microstructure
A theory of spin injection across a ballistic
ferromagnet-semiconductor-ferromagnet junction is developed for the Boltzmann
regime. Spin injection coefficient is suppressed by the Sharvin
resistance of the semiconductor , where is the
Fermi-surface cross-section. It competes with the diffusion resistances of the
ferromagnets , and in the absence of contact
barriers. Efficient spin injection can be ensured by contact barriers. Explicit
formulae for the junction resistance and the spin-valve effect are presented.Comment: 5 pages, 2 column REVTeX. Explicit prescription relating the results
of the ballistic and diffusive theories of spin injection is added. To this
end, some notations are changed. Three references added, typos correcte
A scheme with two large extra dimensions confronted with neutrino physics
We investigate a particle physics model in a six-dimensional spacetime, where
two extra dimensions form a torus. Particles with Standard Model charges are
confined by interactions with a scalar field to four four-dimensional branes,
two vortices accommodating ordinary type fermions and two antivortices
accommodating mirror fermions. We investigate the phenomenological implications
of this multibrane structure by confronting the model with neutrino physics
data.Comment: LATEX, 24 pages, 9 figures, minor changes in the tex
Caustic Formation in Tachyon Effective Field Theories
Certain configurations of D-branes, for example wrong dimensional branes or
the brane-antibrane system, are unstable to decay. This instability is
described by the appearance of a tachyonic mode in the spectrum of open strings
ending on the brane(s). The decay of these unstable systems is described by the
rolling of the tachyon field from the unstable maximum to the minimum of its
potential. We analytically study the dynamics of the inhomogeneous tachyon
field as it rolls towards the true vacuum of the theory in the context of
several different tachyon effective actions. We find that the vacuum dynamics
of these theories is remarkably similar and in particular we show that in all
cases the tachyon field forms caustics where second and higher derivatives of
the field blow up. The formation of caustics signals a pathology in the
evolution since each of the effective actions considered is not reliable in the
vicinity of a caustic. We speculate that the formation of caustics is an
artifact of truncating the tachyon action, which should contain all orders of
derivatives acting on the field, to a finite number of derivatives. Finally, we
consider inhomogeneous solutions in p-adic string theory, a toy model of the
bosonic tachyon which contains derivatives of all orders acting on the field.
For a large class of initial conditions we conclusively show that the evolution
is well behaved in this case. It is unclear if these caustics are a genuine
prediction of string theory or not.Comment: 23 pages, 5 figures; accepted for publication in JHEP. Revised
derivation of eikonal equation for the DBI action. Added comments concerning
the relationship between p-adic string theory and tachyon matter. Added
second example of inhomogeneous evolution in p-adic string theory. Misleading
statements concerning caustic-free evolution removed, references adde
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