441 research outputs found
A Note on Interactions of (Non-Commutative) Instantons Via AdS/CFT
We consider the interaction between instantons and anti-instantons in
four-dimensional N=4 super-Yang-Mills theory at large N and large 't Hooft
coupling as described by D-instantons via AdS/CFT duality. We give an estimate
of the strength of the interaction in various regimes. We discuss also the case
of Non-Commutative super Yang-Mills theory where the interaction between
instantons and anti-instantons can be used as a way to probe the locality
properties of the theory in the supergravity picture, without explicit
reference to the definition of local operators.Comment: harvmac, 13 p
Stringy Fuzziness as the Custodian of Time-Space Noncommutativity
We study aspects of obtaining field theories with noncommuting time-space
coordinates as limits of open-string theories in constant electric-field
backgrounds. We find that, within the standard closed-string backgrounds, there
is an obstruction to decoupling the time-space noncommutativity scale from that
of the string fuzziness scale. We speculate that this censorship may be
string-theory's way of protecting the causality and unitarity structure. We
study the moduli space of the obstruction in terms of the open- and
closed-string backgrounds. Cases of both zero and infinite brane tensions as
well as zero string couplings are obtained. A decoupling can be achieved
formally by considering complex values of the dilaton and inverting the role of
space and time in the light cone. This is reminiscent of a black-hole horizon.
We study the corresponding supergravity solution in the large-N limit and find
that the geometry has a naked singularity at the physical scale of
noncommutativity.Comment: 13 pages, corrected typos, added references and comments on
thermodynamic
A Note on Softly Broken MQCD
We consider generic MQCD configurations with matter described by semi-infinte
D4-branes and softly broken supersymmetry. We show that the matter sector does
not introduce supersymmetry breaking parameters so that the most relevant
supersymmetry breaking operator at low energies is the gaugino mass term. By
studying the run-away properties of these models in the decoupling limit of the
adjoint matter, we argue that these softly broken MQCD configurations fail to
capture the infrared physics of QCD at scales below the gaugino mass scale.Comment: 10+1 pages, TeX. V2: small comments and acknowledgements added,
conclusions unchange
Remarks on Thermal Strings outside Black Holes
We discuss the semiclassical approximation to the level density of (super)
strings propagating in non-compact coset manifolds . We show that the WKB
ansatz agrees with heuristic red-shift arguments with respect to the ``exact"
sigma model metric, up to some deviations from minimal coupling, parametrized
by the dilaton. This approximation is used to study thermal ensembles of free
strings in black holes, with the ``brick wall" regularization of `t Hooft. In
two dimensions the entropy diverges logarithmically with the horizon thickness,
and a local Hagedorn transition occurs in higher dimensional models. We also
observe that supersymmetry improves the regularity of strings at the horizon.Comment: 13 pages, PUPT-94-147
Fermion Exchange between D-instantons
We define fermionic collective coordinates for type II-B Dirichlet instantons and discuss some effects of the associated zero modes within the dilute gas framework. We show that the standard rules for clustering of zero modes in the dilute limit, and the fermion exchange interactions follow from world-sheet Ward identities. Fermion exchange is strongly attractive at string scale distances, which makes the short distance Hagedorn singularity between instantons and anti-instantons even stronger
Rotated Branes and N=1 Duality
We consider configurations of rotated NS-branes leading to a family of four-dimensional N=1 super-QCD theories, interpolating between four-dimensional analogues of the Hanany-Witten vacua, and the Elitzur-Giveon-Kutasov configuration for N=1 duality. The rotation angle is the N=2 breaking parameter, the mass of the adjoint scalar in the N=2 vector multiplet. We add some comments on the relevance of these configurations as possible stringy proofs of N=1 duality
Closed String Tachyons and Their Implications for Non-Supersymmetric Strings
Closed string tachyons have long been somewhat mysterious. We note that there
is often a regime in the classical moduli space in which one can systematically
compute the effective action for such fields. In this regime, the tachyon is
light, and cannot be integrated out. Instead, one must consider the combined
dynamics of gravitons, moduli, tachyons and other light fields. We compute the
action and find that the quartic term for the tachyon is positive in the field
definition where the tachyon has no derivative coupling to the radion. We study
the evolution of isotropic, homogeneous configurations and find that typically
the system is driven to regions where the calculation is no longer under
control.Comment: 10 pages, no figures, references adde
On the Naturalness of Higgs Inflation
We critically examine the recent claim that the Standard Model Higgs boson
could drive inflation in agreement with observations if has a strong coupling to the Ricci curvature scalar. We
first show that the effective theory approach upon which that claim is based
ceases to be valid beyond a cutoff scale , where is the
reduced Planck mass. We then argue that knowing the Higgs potential profile for
the field values relevant for inflation () requires knowledge of the ultraviolet completion of the SM beyond
. In absence of such microscopic theory, the extrapolation of the pure
SM potential beyond is unwarranted and the scenario is akin to other
ad-hoc inflaton potentials afflicted with significant fine-tuning. The
appealing naturalness of this minimal proposal is therefore lost.Comment: 9 pages. Replaced with published version, plus a footnote clarifying
the use of power counting estimate
Multitrace AdS/CFT and Master Field Dynamics
We consider gauge theories with multitrace deformations in the context of certain AdS/CFT models with explicit breaking of conformal symmetry and supersymmetry. In particular, we study the standard four-dimensional confining model based on the D4-brane metric at finite temperature. We work in the self-consistent Hartree approximation, which becomes exact in the large-N limit and is equivalent to the AdS/CFT multitrace prescription that has been proposed in the literature. We show that generic multitrace perturbations have important effects on the phase structure of these models. Most notably they can induce new types of large-N first-order phase transitions
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