112 research outputs found
Instantons and SL(2,R) Symmetry in Type IIB Supergravity
We discuss the relation between the dual formulations of Type IIB
supergravity emphasizing the differences between Lorentz and Euclidean
signature. We demonstrate how the SL(2,R) symmetry of the usual action is
manifested in the solution of the equations of motion with Euclidean signature
for the dual theory.Comment: 4 pages, no figure
Instantons of Type IIB Supergravity in Ten Dimensions
A family of SO(10) symmetric instanton solutions in Type IIB supergravity is
developed. The instanton of least action is a candidate for the low-energy,
semiclassical approximation to the {D=--1} brane. Unlike a previously published
solution,[GGP] this admits an interpretation as a tunneling amplitude between
perturbatively degenerate asymptotic states, but with action twice that found
previously. A number of associated issues are discussed such as the relation
between the magnetic and electric pictures, an inversion symmetry of the
dilaton and the metric, the topology of the background, and some
properties of the solution in an "instanton frame" corresponding to a
Lagrangian in which the dilaton's kinetic energy vanishes.Comment: 15 pages, no figures; Version 2 has revised sections IV and V.
Earlier equations are essentially unchanged, but interpretation changed, on
advice of counse
The Effective Potential, the Renormalisation Group and Vacuum Stability
We review the calculation of the the effective potential with particular
emphasis on cases when the tree potential or the
renormalisation-group-improved, radiatively corrected potential exhibits
non-convex behaviour. We illustrate this in a simple Yukawa model which
exhibits a novel kind of dimensional transmutation. We also review briefly
earlier work on the Standard Model. We conclude that, despite some recent
claims to the contrary, it can be possible to infer reliably that the tree
vacuum does not represent the true ground state of the theory.Comment: 23 pages; 5 figures; v2 includes minor changes in text and additional
reference
The effective potential and the renormalisation group
We discuss renormalisation group improvement of the effective potential both
in general and in the context of scalar \p^4 and the Standard Model.
In the latter case we find that absolute stability of the electroweak vacuum
implies that , for \as (M_Z) = 0.11. We point out
that the lower bound on {\it decreases\/} if \as (M_Z) is increased.Comment: 22 pages plus three PostScript figures (appended), Liverpool preprint
LTH 288, University of Michigan preprint UM-TH-92-2
Extended QCD(2) from dimensional projection of QCD(4)
We study an extended QCD model in (1+1) dimensions obtained from QCD in 4D by
compactifying two spatial dimensions and projecting onto the zero-mode
subspace. We work out this model in the large limit and using light cone
gauge but keeping the equal-time quantization. This system is found to induce a
dynamical mass for transverse gluons -- adjoint scalars in QCD(2), and to
undergo a chiral symmetry breaking with the full quark propagators yielding
non-tachyonic, dynamical quark masses, even in the chiral limit. We study
quark-antiquark bound states which can be classified in this model by their
properties under Lorentz transformations inherited from 4D. The scalar and
pseudoscalar sectors of the theory are examined and in the chiral limit a
massless ground state for pseudoscalars is revealed with a wave function
generalizing the so called 't Hooft pion solution.Comment: JHEP class, 16 pages, 3 figures. Change in the title, some
improvements in section 2, minors changes and comments added in introduction
and conclusions. References added. Version appearing in JHE
Rigid invariance as derived from BRS invariance: The abelian Higgs model
Consequences of a symmetry, e.g.\ relations amongst Green functions, are
renormalization scheme independently expressed in terms of a rigid Ward
identity. The corresponding local version yields information on the respective
current. In the case of spontaneous breakdown one has to define the theory via
the BRS invariance and thus to construct rigid and current Ward identity
non-trivially in accordance with it. We performed this construction to all
orders of perturbation theory in the abelian Higgs model as a prelude to the
standard model. A technical tool of interest in itself is the use of a doublet
of external scalar ``background'' fields. The Callan-Symanzik equation has an
interesting form and follows easily once the rigid invariance is established.Comment: 33 pages, Plain Te
Non--decoupling, triviality and the parameter
The dependence of the parameter on the mass of the Higgs scalar and
the top quark is computed non--perturbatively using the expansion in
the standard model. We find an explicit expression for the parameter
that requires the presence of a physical cutoff. This should come as no
surprise since the theory is presumably trivial. By taking this cutoff into
account, we find that the parameter can take values only within a
limited range and has finite ambiguities that are suppressed by inverse powers
of the cutoff scale, the so called ``scaling--violations". We find that large
deviations from the perturbative results are possible, but only when the cutoff
effects are also large.Comment: 16pp, Figures NOT included, harvmac, minor modifications incl.
wording, refs., UCLA/92/TEP/23,OHSTPY-HEP-T-92-00
Defect Production in Slow First Order Phase Transitions
We study the formation of vortices in a U(1) gauge theory following a
first-order transition proceeding by bubble nucleation, in particular the
effect of a low velocity of expansion of the bubble walls. To do this, we use a
two-dimensional model in which bubbles are nucleated at random points in a
plane and at random times and then expand at some velocity .
Within each bubble, the phase angle is assigned one of three discrete values.
When bubbles collide, magnetic `fluxons' appear: if the phases are different, a
fluxon--anti-fluxon pair is formed. These fluxons are eventually trapped in
three-bubble collisions when they may annihilate or form quantized vortices. We
study in particular the effect of changing the bubble expansion speed on the
vortex density and the extent of vortex--anti-vortex correlation.Comment: 13 pages, RevTeX, 15 uuencoded postscript figure
About the realization of chiral symmetry in QCD2
Two dimensional massless Quantum Chromodynamics presents many features which
resemble those of the true theory. In particular the spectrum consists of
mesons and baryons arranged in flavor multiplets without parity doubling. We
analyze the implications of chiral symmetry, which is not spontaneously broken
in two dimensions, in the spectrum and in the quark condensate. We study how
parity doubling, an awaited consequence of Coleman's theorem, is avoided due to
the dimensionality of space-time and confinement. We prove that a chiral phase
transition is not possible in the theory.Comment: 9 pages, latex, ftuv/92-
Entanglement and Nonunitary Evolution
We consider a collapsing relativistic spherical shell for a free quantum
field. Once the center of the wavefunction of the shell passes a certain radius
R, the degrees of freedom inside R are traced over. We show that an observer
outside this region will determine that the evolution of the system is
nonunitary. We argue that this phenomenon is generic to entangled systems, and
discuss a possible relation to black hole physics.Comment: 14 pages, 1 figure; Added a clarification regarding the relation with
black hole physic
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