2,303,929 research outputs found
Cosmological evolution of a D-brane
We study the cosmological evolution of a single BPS D-brane in the absence of
potential, which is in the category of the Chaplygin gas cosmological model.
When such a D-brane coupled to gravity moves in the bulk with a non-vanishing
velocity, it tends to slow down to zero velocity via mechanisms like
gravitational waves leakage to the bulk, losing its kinetic energy to fuel the
expansion of the universe on the D-brane. If the initial velocity of the
D-brane is high enough, the universe on the D-brane undergoes a dust-like stage
at early times and an acceleration stage at late times, as observed in the
original Chaplygin gas model. When the D-brane velocity is initially zero, the
D-brane will always remain fixed at some position in the bulk, with the brane
tension over the Plank mass squared as a cosmological constant. Interestingly,
this kind of fixed brane universe can arise as defects from tachyon inflation
on a non-BPS D-brane with one dimension higherWe study the cosmological
evolution of a single BPS D-brane coupled to gravity in the absence of
potential. When such a D-brane moves in the bulk with non-vanishing velocity,
it tends to slow down to zero velocity via mechanisms like gravitational wave
leakage to the bulk, losing its kinetic energy to fuel the expansion of the
universe on the D-brane. If the initial velocity of the D-brane is high enough,
the universe on the D-brane undergoes a dust-like stage at early times and an
acceleration stage at late times, realising the original Chaplygin gas model.
When the D-brane velocity is initially zero, the D-brane will always remain
fixed at some position in the bulk, with the brane tension over the Plank mass
squared as a cosmological constant. It is further shown that this kind of fixed
brane universe can arise as defects from tachyon inflation on a non-BPS D-brane
with one dimension higher.Comment: 13 page
Gravitational clustering in a D-dimensional Universe
We consider the problem of gravitational clustering in a D-dimensional
expanding Universe and derive scaling relations connecting the exact mean
two-point correlation function with the linear mean correlation function, in
the quasi-linear and non-linear regimes, using the standard paradigms of
scale-invariant radial collapse and stable clustering. We show that the
existence of scaling laws is a generic feature of gravitational clustering in
an expanding background, in all dimensions except D=2 and comment on the
special nature of the 2-dimensional case. The D-dimensional scaling laws
derived here reduce, in the 3-dimensional case, to scaling relations obtained
earlier from N-body simulations. Finally, we consider the case of clustering of
2-dimensional particles in a 2-D expanding background, governed by a force
-GM/R, and show that the correlation function does not grow (to first order)
until much after the recollapse of any shell.Comment: 4 pages, no figures. Accepted by Physical Review
The Standard Model on a D-brane
We present a consistent string theory model which reproduces the Standard
Model, consisting of a D3-brane at a simple orbifold singularity. We study some
simple features of the phenomenology of the model. We find that the scale of
stringy physics must be in the multi-TeV range. There are natural hierarchies
in the fermion spectrum and there are several possible experimental signatures
of the model.Comment: 8 pages Latex, 1 fig. v2: discussion improved, added new reference
Impurity scattering in a d-wave superconductor
The influence of (non-magnetic and magnetic) impurities on the transition
temperature of a d-wave superconductor is studied anew within the framework of
BCS theory. Pairing interaction decreases linearly with the impurity
concentration. Accordingly suppression is proportional to the
(potential or exchange) scattering rate, , due to impurities. The
initial slope versus is found to depend on the superconductor contrary
to Abrikosov-Gor'kov type theory. Near the critical impurity concentration
drops abruptly to zero. Because the potential scattering rate is
generally much larger than the exchange scattering rate, magnetic impurities
will also act as non-magnetic impurities as far as the decrease is
concerned. The implication for the impurity doping effect in high
superconductors is also discussed.Comment: 12 pages and 1 figure, PlainTex, submitted to Mod. Phys. Lett. B, For
more information, please see "http://taesan.kaist.ac.kr/~yjkim
A-D-E Quivers and Baryonic Operators
We study baryonic operators of the gauge theory on multiple D3-branes at the
tip of the conifold orbifolded by a discrete subgroup Gamma of SU(2). The
string theory analysis predicts that the number and the order of the fixed
points of Gamma acting on S^2 are directly reflected in the spectrum of
baryonic operators on the corresponding quiver gauge theory constructed from
two Dynkin diagrams of the corresponding type. We confirm the prediction by
developing techniques to enumerate baryonic operators of the quiver gauge
theory which includes the gauge groups with different ranks. We also find that
the Seiberg dualities act on the baryonic operators in a non-Abelian fashion.Comment: 46 pages, 17 figures; v2: minor corrections, note added in section 1,
references adde
A D-Brane Alternative to the MSSM
The success of SU(5)-like gauge coupling unification boundary conditions
has biased most attempts to embed the SM interactions
into a unified structure. After discussing the limitations of the orthodox
approach, we propose an alternative that appears to be quite naturally implied
by recent developments based on D-brane physics. In this new alternative: 1)
The gauge group, above a scale of order 1 TeV, is the minimal left-right
symmetric extension of the
SM; 2) Quarks, leptons and Higgs fields come in three generations; 3) Couplings
unify at an intermediate string scale GeV with boundary
conditions . This corresponds to the natural
embedding of gauge interactions into D-branes and is different from the
standard SO(10) embedding which corresponds to . Unification only
works in the case of three generations;
4) Proton stability is automatic due to the presence of discrete
R-parity and lepton parities. A specific Type IIB string orientifold model with
the above characteristics is constructed. The existence of three generations is
directly related to the existence of three complex extra dimensions.
In this model the string scale can be identified with the intermediate scale
and SUSY is broken also at that scale due to the presence of anti-branes in the
vacuum.
We discuss a number of phenomenological issues in this model including Yukawa
couplings and a built-in axion solution to the strong-CP problem. The present
framework could be tested by future accelerators by finding the left-right
symmetric extension of the SM at a scale of order 1 TeV.Comment: 50 pages, 7 figures. References adde
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