598 research outputs found
Hydrodynamic fluctuations and the minimum shear viscosity of the dilute Fermi gas at unitarity
We study hydrodynamic fluctuations in a non-relativistic fluid. We show that
in three dimensions fluctuations lead to a minimum in the shear viscosity to
entropy density ratio as a function of the temperature. The minimum
provides a bound on which is independent of the conjectured bound in
string theory, , where is the entropy
density. For the dilute Fermi gas at unitarity we find \eta/s\gsim 0.2\hbar.
This bound is not universal -- it depends on thermodynamic properties of the
unitary Fermi gas, and on empirical information about the range of validity of
hydrodynamics. We also find that the viscous relaxation time of a hydrodynamic
mode with frequency diverges as , and that the shear
viscosity in two dimensions diverges as .Comment: 26 pages, 5 figures; final version to appear in Phys Rev
Ramond Equations of Motion in Superstring Field Theory
We extend the recently constructed NS superstring field theories in the small
Hilbert space to give classical field equations for all superstring theories,
including Ramond sectors. We also comment on the realization of supersymmetry
in this framework.Comment: 43 pages, 5 figure
Supersymmetric NambuJona-Lasinio Model on four-dimensional Non(anti)commutative Superspace
We construct the Lagrangian of the four-dimensional generalized
supersymmetric NambuJona-Lasinio (SNJL) model, which has
supersymmetry (SUSY) on non(anti)commutative superspace. A special attention is
paid to the examination on the nonperturbative quantum dynamics: The phenomenon
of dynamical-symmetry-breaking/mass-generation on the deformed superspace is
investigated. The model Lagrangian and the method of SUSY auxiliary fields of
composites are examined in terms of component fields. We derive the effective
action, examine it, and solve the gap equation for self-consistent mass
parameters.Comment: 16 pages, TeX mistakes corrected, accepted for publication in JHEP,
25 Jan. 200
Evidence for a gravitational Myers effect
An indication for the existence of a collective Myers solution in the
non-abelian D0-brane Born-Infeld action is the presence of a tachyonic mode in
fluctuations around the standard diagonal background. We show that this
computation for non-abelian D0-branes in curved space has the geometric
interpretation of computing the eigenvalues of the geodesic deviation operator
for U(N)-valued coordinates. On general grounds one therefore expects a
geometric Myers effect in regions of sufficiently negative curvature. We
confirm this by explicit computations for non-abelian D0-branes on a sphere and
a hyperboloid. For the former the diagonal solution is stable, but not so for
the latter. We conclude by showing that near the horizon of a Schwarzschild
black hole one also finds a tachyonic mode in the fluctuation spectrum,
signaling the possibility of a near-horizon gravitationally induced Myers
effect.Comment: LaTeX, 23 page
Non Abelian Tachyon Kinks
Starting from the action of two coincident non-BPS D9-branes, we investigate
kink configurations of the U(2) matrix tachyon field. We consider both Str and
Tr prescriptions for the trace over gauge indices of the non-BPS action.
Non-abelian tachyon condensation in the theory with Tr prescription, and the
resulting fluctuations about the kink profile, are shown to give rise to a
theory of two coincident BPS D8-branes. This is a natural non-abelian
generalization of Sen's mechanism of tachyon condensation on a single non-BPS
Dp-brane yielding a single BPS brane of codimesion one. By contrast, starting
with the Str gauge trace prescription of the coincident non-BPS D9-brane
action, such a generalization of Sen's mechanism appears problematic.Comment: 18 pages, references added, version to appear in JHE
Strong coupling in massive gravity by direct calculation
We consider four-dimensional massive gravity with the Fierz-Pauli mass term.
The analysis of the scalar sector has revealed recently that this theory
becomes strongly coupled above the energy scale \Lambda = (M_{Pl}^2 m^4)^{1/5}
where m is the mass of the graviton. We confirm this scale by explicit
calculations of the four-graviton scattering amplitude and of the loop
correction to the interaction between conserved sources.Comment: 9 pages, 3 figures, some clarifications adde
D-brane effective field theory from string field theory
Open string field theory is considered as a tool for deriving the effective
action for the massless or tachyonic fields living on D-branes. Some simple
calculations are performed in open bosonic string field theory which validate
this approach. The level truncation method is used to calculate successive
approximations to the quartic terms \phi^4, (A^\mu A_\mu)^2 and [A_\mu,
A_\nu]^2 for the zero momentum tachyon and gauge field on one or many bosonic
D-branes. We find that the level truncation method converges for these terms
within 2-4% when all massive fields up to level 20 are integrated out, although
the convergence is slower than exponential. We discuss the possibility of
extending this work to determine the structure of the nonabelian Born-Infeld
theory describing the gauge field on a system of many parallel bosonic or
supersymmetric D-branes. We also describe a brane configuration in which
tachyon condensation arises in both the gauge theory and string field theory
pictures. This provides a natural connection between recent work of Sen and
Zwiebach on tachyon condensation in string field theory and unstable vacua in
super Yang-Mills and Born-Infeld field theory.Comment: 23 pages, 4 figures, LaTeX; v3: sign error corrected, references
added, discussion of bosonic NBI extended. v4: bug in N coefficient table
fixed; qualitative results unchange
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