115,352 research outputs found
High Energy Bounds on Soft N=4 SYM Amplitudes from AdS/CFT
Using the AdS/CFT correspondence, we study the high-energy behavior of
colorless dipole elastic scattering amplitudes in N=4 SYM gauge theory through
the Wilson loop correlator formalism and Euclidean to Minkowskian analytic
continuation. The purely elastic behavior obtained at large impact-parameter L,
through duality from disconnected AdS_5 minimal surfaces beyond the
Gross-Ooguri transition point, is combined with unitarity and analyticity
constraints in the central region. In this way we obtain an absolute bound on
the high-energy behavior of the forward scattering amplitude due to the
graviton interaction between minimal surfaces in the bulk. The dominant
"Pomeron" intercept is bounded by alpha less than or equal to 11/7 using the
AdS/CFT constraint of a weak gravitational field in the bulk. Assuming the
elastic eikonal approximation in a larger impact-parameter range gives alpha
between 4/3 and 11/7. The actual intercept becomes 4/3 if one assumes the
elastic eikonal approximation within its maximally allowed range L larger than
exp{Y/3}, where Y is the total rapidity. Subleading AdS/CFT contributions at
large impact-parameter due to the other d=10 supergravity fields are obtained.
A divergence in the real part of the tachyonic KK scalar is cured by
analyticity but signals the need for a theoretical completion of the AdS/CFT
scheme.Comment: 25 pages, 3 eps figure
On the running coupling constant in QCD
We try to review the main current ideas and points of view on the running
coupling constant in QCD. We begin by recalling briefly the classic analysis
based on the Renormalization Group with some emphasis on the exact solutions of
the RG equation for a given number of loops, in comparison with the usual
approximate expressions. We give particular attention to the problem of
eliminating the unphysical Landau singularities, and of defining a coupling
that remains significant at the infrared scales. We consider various proposals
of couplings directly related to the quark-antiquark potential or to other
physical quantities (effective charges) and discuss optimization in the choice
of the scale parameter and of the RS. Our main focus is, however, on dispersive
methods, their application, their relation with non-perturbative effects. We
try also to summarize the main results obtained by Lattice simulations in
various MOM schemes. We conclude briefly recalling the traditional comparison
with the experimental data.Comment: 75 pages, 8 figures. Corrected typos, added references, replaced 1
figure. Accepted for publication in Progress in Particle and Nuclear Physic
Knot theory and quantum gravity in loop space: a primer
These notes summarize the lectures delivered in the V Mexican School of
Particle Physics, at the University of Guanajuato. We give a survey of the
application of Ashtekar's variables to the quantization of General Relativity
in four dimensions with special emphasis on the application of techniques of
analytic knot theory to the loop representation. We discuss the role that the
Jones Polynomial plays as a generator of nondegenerate quantum states of the
gravitational field.Comment: 44 pages. v2: figures added, also available as PDF at
http://www.phys.psu.edu/~pullin/primerfigs.pd
Analytic resummation for the quark form factor in QCD
The quark form factor is known to exponentiate within the framework of
dimensionally regularized perturbative QCD. The logarithm of the form factor is
expressed in terms of integrals over the scale of the running coupling. I show
that these integrals can be evaluated explicitly and expressed in terms of
renormalization group invariant analytic functions of the coupling and of the
space--time dimension, to any order in renormalized perturbation theory.
Explicit expressions are given up two loops. To this order, all the infrared
and collinear singularities in the logarithm of the form factor resum to a
single pole in epsilon, whose residue is determined at one loop, plus powers of
logarithms of epsilon. This behavior is conjectured to extend to all loops.Comment: 24 pages, LaTeX, no figure
High-energy hadron-hadron (dipole-dipole) scattering from lattice QCD
In this paper the problem of high-energy hadron-hadron (dipole-dipole)
scattering is approached (for the first time) from the point of view of lattice
QCD, by means of Monte Carlo numerical simulations. In the first part, we give
a brief review of how high-energy scattering amplitudes can be reconstructed,
using a functional-integral approach, in terms of certain correlation functions
of two Wilson loops and we also briefly recall some relevant analyticity and
crossing-symmetry properties of these loop-loop correlation functions, when
going from Euclidean to Minkowskian theory. In the second part, we shall see
how these (Euclidean) loop-loop correlation functions can be evaluated in
lattice QCD and we shall compare our numerical results with some
nonperturbative analytical estimates that appeared in the literature,
discussing in particular the question of the analytic continuation from
Euclidean to Minkowskian theory and its relation to the still unsolved problem
of the asymptotic s-dependence of the hadron-hadron total cross sections.Comment: Revised version (to be published in Phys. Rev. D) with new comments
in section 4, a new figure [Fig. 6], two new references in Refs. [3] and
[34], and some other minor changes; 27 pages, 17 figure
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