1,668 research outputs found
Non-Douglas-Kazakov phase transition of two-dimensional generalized Yang-Mills theories
In two-dimensional Yang-Mills and generalized Yang-Mills theories for large
gauge groups, there is a dominant representation determining the thermodynamic
limit of the system. This representation is characterized by a density the
value of which should everywhere be between zero and one. This density itself
is determined through a saddle-point analysis. For some values of the parameter
space, this density exceeds one in some places. So one should modify it to
obtain an acceptable density. This leads to the well-known Douglas-Kazakov
phase transition. In generalized Yang-Mills theories, there are also regions in
the parameter space where somewhere this density becomes negative. Here too,
one should modify the density so that it remains nonnegative. This leads to
another phase transition, different from the Douglas-Kazakov one. Here the
general structure of this phase transition is studied, and it is shown that the
order of this transition is typically three. Using carefully-chosen parameters,
however, it is possible to construct models with phase-transition orders not
equal to three. A class of these non-typical models are also studied.Comment: 11 pages, accepted for publication in Eur. Phys. J.
Dissolving D0-brane into D2-brane with background B-field
D0-branes on a D2-brane with a constant background B-field are unstable due
to the presence of a tachyonic mode and expected to dissolve into the D2-brane
to formulate a constant D0-charge density. In this paper we study such a
dissolution process in terms of a noncommutative gauge theory. Our results show
that the localized D0-brane spreads out over all of space on the D2-brane as
the tachyon rolls down into a stable vacuum. D0-branes on a D2-brane can be
described as unstable solitons in a noncommutative gauge theory in 2+1
dimensions in the Seiberg-Witten limit. In contrast to the case of annihilation
of a non-BPS D-brane, we are free from difficulty of disappearance of DOF,
since there exist open strings after the tachyon condensation. We solve an
equation of motion of the gauge field numerically, and our results show that
the localized soliton smears over all of noncommutative space. In addition, we
evaluate distributions of D-brane charge, F-string charge, and energy density
via formulas derived in Matrix theory. Our results show that the initial
singularities of D0-charge and energy density are resolved by turning on the
tachyon, and they disperse over the whole space on the D2-brane during the
tachyon condensation process.Comment: 42 pages, 20 figures, JHEP style; references added, clarifications
added in section 3.1; references adde
Anomalous Dimension of Non-Singlet Wilson Operators at O(1/N_f) in Deep Inelastic Scattering
We use the large N_f self consistency formalism to compute the
critical exponent corresponding to the renormalization of the flavour
non-singlet twist two Wilson operators which arise in the operator product
expansion of currents in deep inelastic processes. Expanding the
-dimensional expression in powers of the
coefficients of agree with the known two loop structure of the
corresponding renormalization group function and we deduce analytic expressions
for all moments, , at three and higher orders in perturbation theory in the
\overline{\mbox{MS}} scheme at .Comment: 13 Latex pages, 1 figure (available from author on request), LTH-32
A relativistic dynamical model for pi-N scattering
We present a unitary relativistic quasi-potential model for describing the
low-energy pion-nucleon interaction, based on the equal time Bethe-Salpeter
equation. It preserves the covariant structure of a relativistic spin 1/2
particle for the nucleon propagator, to be contrasted to other quasi-potential
approximations.Comment: 4 pages, Latex2e. To appear in the Proceedings of XV Int. Conf. on
Few-Body Problems in Physics (Groningen, July 1997
Non-Commutative Instantons and the Seiberg-Witten Map
We present several results concerning non-commutative instantons and the
Seiberg-Witten map. Using a simple ansatz we find a large new class of
instanton solutions in arbitrary even dimensional non-commutative Yang-Mills
theory. These include the two dimensional ``shift operator'' solutions and the
four dimensional Nekrasov-Schwarz instantons as special cases. We also study
how the Seiberg-Witten map acts on these instanton solutions. The infinitesimal
Seiberg-Witten map is shown to take a very simple form in operator language,
and this result is used to give a commutative description of non-commutative
instantons. The instanton is found to be singular in commutative variables.Comment: 26 pages, AMS-LaTeX. v2: the formula for the commutative description
of the Nekrasov-Schwarz instanton corrected (sec. 4). v3: minor correction
The Coupling of Yang-Mills to Extended Objects
The coupling of Yang-Mills fields to the heterotic string in bosonic
formulation is generalized to extended objects of higher dimension (p-branes).
For odd p, the Bianchi identities obeyed by the field strengths of the
(p+1)-forms receive Chern-Simons corrections which, in the case of the 5-brane,
are consistent with an earlier conjecture based on string/5-brane duality.Comment: 14 Page
The Nambu-Jona-Lasinio Model at O(1/N^2)
We write down the anomalous dimensions of the fields of the
Nambu--Jona-Lasinio model or chiral Gross Neveu model with a continuous global
chiral symmetry for the two cases and
at in a expansion.Comment: 9 latex pages, 4 figures (available on request from the author),
LTH-308, (2 eqns corrected
Ab initio Quantum and ab initio Molecular Dynamics of the Dissociative Adsorption of Hydrogen on Pd(100)
The dissociative adsorption of hydrogen on Pd(100) has been studied by ab
initio quantum dynamics and ab initio molecular dynamics calculations. Treating
all hydrogen degrees of freedom as dynamical coordinates implies a high
dimensionality and requires statistical averages over thousands of
trajectories. An efficient and accurate treatment of such extensive statistics
is achieved in two steps: In a first step we evaluate the ab initio potential
energy surface (PES) and determine an analytical representation. Then, in an
independent second step dynamical calculations are performed on the analytical
representation of the PES. Thus the dissociation dynamics is investigated
without any crucial assumption except for the Born-Oppenheimer approximation
which is anyhow employed when density-functional theory calculations are
performed. The ab initio molecular dynamics is compared to detailed quantum
dynamical calculations on exactly the same ab initio PES. The occurence of
quantum oscillations in the sticking probability as a function of kinetic
energy is addressed. They turn out to be very sensitive to the symmetry of the
initial conditions. At low kinetic energies sticking is dominated by the
steering effect which is illustrated using classical trajectories. The steering
effects depends on the kinetic energy, but not on the mass of the molecules.
Zero-point effects lead to strong differences between quantum and classical
calculations of the sticking probability. The dependence of the sticking
probability on the angle of incidence is analysed; it is found to be in good
agreement with experimental data. The results show that the determination of
the potential energy surface combined with high-dimensional dynamical
calculations, in which all relevant degrees of freedon are taken into account,
leads to a detailed understanding of the dissociation dynamics of hydrogen at a
transition metal surface.Comment: 15 pages, 9 figures, subm. to Phys. Rev.
Localized Tachyons and the g_cl conjecture
We consider C/Z_N and C^2/Z_N orbifolds of heterotic string theories and Z_N
orbifolds of AdS_3. We study theories with N=2 worldsheet superconformal
invariance and construct RG flows. Following Harvey, Kutasov, Martinec and
Moore, we compute g_cl and show that it decreases monotonically along RG flows-
as conjectured by them. For the heterotic string theories, the gauge degrees of
freedom do not contribute to the computation of g_cl.Comment: Corrections and clarifications made, 19 page
Puffed Noncommutative Nonabelian Vortices
We present new solutions of noncommutative gauge theories in which coincident
unstable vortices expand into unstable circular shells. As the theories are
noncommutative, the naive definition of the locations of the vortices and
shells is gauge-dependent, and so we define and calculate the profiles of these
solutions using the gauge-invariant noncommutative Wilson lines introduced by
Gross and Nekrasov. We find that charge 2 vortex solutions are characterized by
two positions and a single nonnegative real number, which we demonstrate is the
radius of the shell. We find that the radius is identically zero in all
2-dimensional solutions. If one considers solutions that depend on an
additional commutative direction, then there are time-dependent solutions in
which the radius oscillates, resembling a braneworld description of a cyclic
universe. There are also smooth BIon-like space-dependent solutions in which
the shell expands to infinity, describing a vortex ending on a domain wall.Comment: 21 pages, 3 eps figures. v2: published version, analytic solution
adde
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