595 research outputs found
Nonsingular instantons for the creation of open universes
We show that the instability of the singular Vilenkin instanton describing
the creation of an open universe can be avoided using, instead of a minimally
coupled scalar field, an axionic massless scalar field which gives rise to the
Giddings-Strominger instanton. However, if we replace the singularity of the
Hawking Turok instanton for an axionic wormhole some interpretational and
technical difficulties would appear which can be overcome by introducing a
positive cosmological constant in the action. This would make the instanton
finite and free constant in the action. This would make the instanton finite
and free from any instabilities.Comment: 8 pages, RevTex. A new section on the instantonic global structure
and a figure have been added. To appear in Phys. Rev.
Effective Field Theories
Effective field theories encode the predictions of a quantum field theory at
low energy. The effective theory has a fairly low ultraviolet cutoff. As a
result, loop corrections are small, at least if the effective action contains a
term which is quadratic in the fields, and physical predictions can be read
straight from the effective Lagrangean.
Methods will be discussed how to compute an effective low energy action from
a given fundamental action, either analytically or numerically, or by a
combination of both methods. Basically,the idea is to integrate out the high
frequency components of fields. This requires the choice of a "blockspin",i.e.
the specification of a low frequency field as a function of the fundamental
fields. These blockspins will be the fields of the effective field theory. The
blockspin need not be a field of the same type as one of the fundamental
fields, and it may be composite. Special features of blockspins in nonabelian
gauge theories will be discussed in some detail.
In analytical work and in multigrid updating schemes one needs interpolation
kernels \A from coarse to fine grid in addition to the averaging kernels
which determines the blockspin. A neural net strategy for finding optimal
kernels is presented.
Numerical methods are applicable to obtain actions of effective theories on
lattices of finite volume. The constraint effective potential) is of particular
interest. In a Higgs model it yields the free energy, considered as a function
of a gauge covariant magnetization. Its shape determines the phase structure of
the theory. Its loop expansion with and without gauge fields can be used to
determine finite size corrections to numerical data.Comment: 45 pages, 9 figs., preprint DESY 92-070 (figs. 3-9 added in ps
format
Brane/Flux Annihilation and the String Dual of a Non-Supersymmetric Field Theory
We consider the dynamics of p anti-D3 branes inside the Klebanov-Strassler
geometry, the deformed conifold with M units of RR 3-form flux around the S^3.
We find that for p<<M the system relaxes to a nonsupersymmetric NS 5-brane
``giant graviton'' configuration, which is classically stable, but quantum
mechanically can tunnel to a nearby supersymmetric vacuum with M-p D3 branes.
This decay mode is exponentially suppressed and proceeds via the nucleation of
an NS 5-brane bubble wall. We propose a dual field theory interpretation of the
decay as the transition between a nonsupersymmetric ``baryonic'' branch and a
supersymmetric ``mesonic'' branch of the corresponding SU(2M-p)x SU(M-p) low
energy gauge theory. The NS 5-brane tunneling process also provides a simple
explanation of the geometric transition by which D3-branes can dissolve into
3-form flux.Comment: 27 pages, 4 figures, typo correcte
A Quantum Mechanical Model of Spherical Supermembranes
We present a quantum mechanical model of spherical supermembranes. Using
superfields to represent the cartesian coordinates of the membrane, we are able
to exactly determine its supersymmetric vacua. We find there are two classical
vacua, one corresponding to an extended membrane and one corresponding to a
point-like membrane. For the case, instanton effects then
lift these vacua to massive states. For the case, there is
no instanton tunneling, and the vacua remain massless. Similarities to
spherical supermembranes as giant gravitons and in Matrix theory on pp-waves is
discussed.Comment: 9 page
Failure due to fatigue in fiber bundles and solids
We consider first a homogeneous fiber bundle model where all the fibers have
got the same stress threshold beyond which all fail simultaneously in absence
of noise. At finite noise, the bundle acquires a fatigue behavior due to the
noise-induced failure probability at any stress. We solve this dynamics of
failure analytically and show that the average failure time of the bundle
decreases exponentially as the stress increases. We also determine the
avalanche size distribution during such failure and find a power law decay. We
compare this fatigue behavior with that obtained phenomenologically for the
nucleation of Griffith cracks. Next we study numerically the fatigue behavior
of random fiber bundles having simple distributions of individual fiber
strengths, at stress less than the bundle's strength (beyond which it fails
instantly). The average failure time is again seen to decrease exponentially as
the stress increases and the avalanche size distribution shows similar power
law decay. These results are also in broad agreement with experimental
observations on fatigue in solids. We believe, these observations regarding the
failure time are useful for quantum breakdown phenomena in disordered systems.Comment: 13 pages, 4 figures, figures added and the text is revise
Ab initio calculation of the neutron-proton mass difference
The existence and stability of atoms rely on the fact that neutrons are more
massive than protons. The measured mass difference is only 0.14\% of the
average of the two masses. A slightly smaller or larger value would have led to
a dramatically different universe. Here, we show that this difference results
from the competition between electromagnetic and mass isospin breaking effects.
We performed lattice quantum-chromodynamics and quantum-electrodynamics
computations with four nondegenerate Wilson fermion flavors and computed the
neutron-proton mass-splitting with an accuracy of kilo-electron volts,
which is greater than by standard deviations. We also determine the
splittings in the , , and isospin multiplets,
exceeding in some cases the precision of experimental measurements.Comment: 57 pages, 15 figures, 6 tables, revised versio
D0 Matrix Mechanics: New Fuzzy Solutions at Large N
We wish to consider in this report the large N limit of a particular matrix
model introduced by Myers describing D-brane physics in the presence of an RR
flux background. At finite N, fuzzy spheres appear naturally as non-trivial
solutions to this matrix model and have been extensively studied. In this
report, we wish to demonstrate several new classes of solutions which appear in
the large N limit, corresponding to the fuzzy cylinder,the fuzzy plane and a
warped fuzzy plane. The latter two solutions arise from a possible "central
extension" to our model that arises after we account for non-trivial issues
involved in the large N limit. As is the case for finite N, these new solutions
are to be interpreted as constituent D0-branes forming D2 bound states
describing new fuzzy geometries.Comment: revised version: references added, derivation of "central extensions"
improved upon. To appear in JHE
D-Theory: Field Theory via Dimensional Reduction of Discrete Variables
A new non-perturbative approach to quantum field theory --- D-theory --- is
proposed, in which continuous classical fields are replaced by discrete
quantized variables which undergo dimensional reduction. The 2-d classical O(3)
model emerges from the (2+1)-d quantum Heisenberg model formulated in terms of
quantum spins. Dimensional reduction is demonstrated explicitly by simulating
correlation lengths up to 350,000 lattice spacings using a loop cluster
algorithm. In the framework of D-theory, gauge theories are formulated in terms
of quantum links --- the gauge analogs of quantum spins. Quantum links are
parallel transporter matrices whose elements are non-commuting operators. They
can be expressed as bilinears of anticommuting fermion constituents. In quantum
link models dimensional reduction to four dimensions occurs, due to the
presence of a 5-d Coulomb phase, whose existence is confirmed by detailed
simulations using standard lattice gauge theory. Using Shamir's variant of
Kaplan's fermion proposal, in quantum link QCD quarks appear as edge states of
a 5-d slab. This naturally protects their chiral symmetries without
fine-tuning. The first efficient cluster algorithm for a gauge theory with a
continuous gauge group is formulated for the U(1) quantum link model. Improved
estimators for Wilson loops are constructed, and dimensional reduction to
ordinary lattice QED is verified numerically.Comment: 15 pages, LaTeX, including 9 encapsulated postscript figures.
Contribution to Lattice 97 by 5 authors, to appear in Nuclear Physics B
(Proceeding Supplements). Requires psfig.tex and espcrc2.st
Time evolution of damage under variable ranges of load transfer
We study the time evolution of damage in a fiber bundle model in which the
range of interaction of fibers varies through an adjustable stress transfer
function recently introduced. We find that the lifetime of the material
exhibits a crossover from mean field to short range behavior as in the static
case. Numerical calculations showed that the value at which the transition
takes place depends on the system's disorder. Finally, we have performed a
microscopic analysis of the failure process. Our results confirm that the
growth dynamics of the largest crack is radically different in the two limiting
regimes of load transfer during the first stages of breaking.Comment: 8 pages, 7 figures, revtex4 styl
Trivial Vacua, High Orders in Perturbation Theory and Nontrivial Condensates
In the limit of an infinite number of colors, an analytic expression for the
quark condensate in is derived as a function of the quark mass and
the gauge coupling constant. For zero quark mass, a nonvanishing quark
condensate is obtained. Nevertheless, it is shown that there is no phase
transition as a function of the quark mass. It is furthermore shown that the
expansion of in the gauge coupling
has zero radius of convergence but that the perturbation series is Borel
summable with finite radius of convergence. The nonanalytic behavior can only be obtained by summing the perturbation series to infinite
order. The sum-rule calculation is based on masses and coupling constants
calculated from 't Hooft's solution to which employs LF
quantization and is thus based on a trivial vacuum. Nevertheless the chiral
condensate remains nonvanishing in the chiral limit which is yet another
example that seemingly trivial LF vacua are {\it not} in conflict with QCD
sum-rule results.Comment: REVTEX, 6 pages, 1 postscript figure available from the author by
emai
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