95 research outputs found
Perturbing the Ground Ring of 2-D String Theory
We use free field techniques in D=2 string theory to calculate the
perturbation of the special state algebras when the cosmologi- cal constant is
turned on. In particular, we find that the "ground cone" preserved by the ring
structure is promoted to a three dimen- sional hyperboloid as conjectured by
Witten. On the other hand, the perturbed (1,1) a three dimensional hyperboloid
as conjectured by Witten. On the other hand, the perturbed (1,1) current
algebra of moduli deformations is computed completely, and no simple
geometrical inter- pretation is found. We also quote some facts concerning the
Liouville/matrix model dictio- nary in this class of theories.Comment: 23 page
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
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
Very Long Time Scales and Black Hole Thermal Equilibrium
We estimate the very long time behaviour of correlation functions in the
presence of eternal black holes. It was pointed out by Maldacena (hep-th
0106112) that their vanishing would lead to a violation of a unitarity-based
bound. The value of the bound is obtained from the holographic dual field
theory. The correlators indeed vanish in a semiclassical bulk approximation. We
trace the origin of their vanishing to the continuum energy spectrum in the
presence of event horizons. We elaborate on the two very long time scales
involved: one associated with the black hole and the other with a thermal gas
in the vacuum background. We find that assigning a role to the thermal gas
background, as suggested in the above work, does restore the compliance with a
time-averaged unitarity bound. We also find that additional configurations are
needed to explain the expected time dependence of the Poincar\'e recurrences
and their magnitude. It is suggested that, while a semiclassical black hole
does reproduce faithfully ``coarse grained'' properties of the system,
additional dynamical features of the horizon may be necessary to resolve a
finer grained information-loss problem. In particular, an effectively formed
stretched horizon could yield the desired results.Comment: 30 pages, harvmac, 1 eps figur
Non-linear Vacuum Phenomena in Non-commutative QED
We show that the classic results of Schwinger on the exact propagation of
particles in the background of constant field-strengths and plane waves can be
readily extended to the case of non-commutative QED. It is shown that
non-perturbative effects on constant backgrounds are the same as their
commutative counterparts, provided the on-shell gauge invariant dynamics is
referred to a non-perturbatively related space-time frame. For the case of the
plane wave background, we find evidence of the effective extended nature of
non-commutative particles, producing retarded and advanced effects in
scattering. Besides the known `dipolar' character of non-commutative neutral
particles, we find that charged particles are also effectively extended, but
they behave instead as `half-dipoles'.Comment: LaTeX, 23 p
A Comment on the Geometric Entropy and Conical Space
It has been recently pointed out that a definition of the geometric entropy
using the partition function in a conical space does not in general lead to a
positive definite quantity. For a scalar field model with a non-minimal
coupling we clarify the origin of the anomalous behavior from the viewpoint of
the canonical formulation.Comment: No Figures. To appear in Classical and Quantum Gravit
On the stringy nature of winding modes in noncommutative thermal field theories
We show that thermal noncommutative field theories admit a version of `channel duality' reminiscent of open/closed string duality, where non-planar thermal loops can be replaced by an infinite tower of tree-level exchanges of effective fields. These effective fields resemble closed strings in three aspects: their mass spectrum is that of closed-string winding modes, their interaction vertices contain extra moduli, and they can be regarded as propagating in a higher-dimensional `bulk' space-time. In noncommutative models that can be embedded in a D-brane, we show the precise relation between the effective `winding fields' and closed strings propagating off the D-brane. The winding fields represent the coherent coupling of the infinite tower of closed-string oscillator states. We derive a sum rule that expresses this effective coupling in terms of the elementary couplings of closed strings to the D-brane. We furthermore clarify the relation between the effective propagating dimension of the winding fields and the true codimension of the D-brane
Quantum black hole entropy and Newton constant renormalization
We discuss the status of the black hole entropy formula in low energy effective field theory. The low energy expansion of the
black hole entropy is studied in a non-equilibrium situation: the semiclassical
decay of hot flat space by black hole nucleation. In this context the entropy
can be defined as an enhancement factor in the semiclassical decay rate, which
is dominated by a sphaleron-like saddle point. We find that all perturbative
divergences appearing in Euclidean calculations of the entropy can be
renormalized in low energy couplings. We also discuss some formal aspects of
the relation between the Euclidean and Hamiltonian approaches to the one loop
corrections to black hole entropy and geometric entropy, and we emphasize the
virtues of the use of covariant regularization prescriptions. In fact, the
definition of black hole entropy in terms of decay rates {\it requires} the use
of covariant measures and accordingly, covariant regularizations in path
integrals. Finally, we speculate on the possibility that low energy effective
field theory could be sufficient to understand the microscopic degrees of
freedom underlying black hole entropy. We propose a qualitative physical
picture in which black hole entropy refers to a space of quasi-coherent states
of infalling matter, together with its gravitational field. We stress that this
scenario might provide a low energy explanation of both the black hole entropy
and the information puzzle.Comment: 24 pages, LaTeX. Several points have been clarified, though results
remain the same. Minor typos corrected, and references updated. Version to
appear in Phys. Rev.
A note on supersymmetric Yang-Mills thermodynamics
The thermodynamics of supersymmetric Yang-Mills theories is studied by
computing the two-loop correction to the canonical free energy and to the
equation of state for theories with 16, 8 and 4 supercharges in any dimension
, and in two dimensions at finite volume. In the
four-dimensional case we also evaluate the first non-analytic contribution in
the 't Hooft coupling to the free energy, arising from the resummation of ring
diagrams. To conclude, we discuss some applications to the study of the
Hagedorn transition in string theory in the context of Matrix strings and
speculate on the possible physical meaning of the transition.Comment: 19 pages, harvmac, epsf. 1 figure included. Minor changes: typos
corrected; references, a footonote and a note adde
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