98,588 research outputs found
New Concepts in Particle Physics from Solution of an Old Problem
Recent ideas on modular localization in local quantum physics are used to
clarify the relation between on- and off-shell quantities in particle physics;
in particular the relation between on-shell crossing symmetry and off-shell
Einstein causality. Among the collateral results of this new nonperturbative
approach are profound relations between crossing symmetry of particle physics
and Hawking-Unruh like thermal aspects (KMS property, entropy attached to
horizons) of quantum matter behind causal horizons, aspects which hitherto were
exclusively related with Killing horizons in curved spacetime rather than with
localization aspects in Minkowski space particle physics. The scope of this
modular framework is amazingly wide and ranges from providing a conceptual
basis for the d=1+1 bootstrap-formfactor program for factorizable d=1+1 models
to a decomposition theory of QFT's in terms of a finite collection of unitarily
equivalent chiral conformal theories placed a specified relative position
within a common Hilbert space (in d=1+1 a holographic relation and in higher
dimensions more like a scanning). The new framework gives a spacetime
interpretation to the Zamolodchikov-Faddeev algebra and explains its thermal
aspects.Comment: In this form it will appear in JPA Math Gen, 47 pages tcilate
Dimension and Dimensional Reduction in Quantum Gravity
If gravity is asymptotically safe, operators will exhibit anomalous scaling
at the ultraviolet fixed point in a way that makes the theory effectively
two-dimensional. A number of independent lines of evidence, based on different
approaches to quantization, indicate a similar short-distance dimensional
reduction. I will review the evidence for this behavior, emphasizing the
physical question of what one means by `dimension' in a quantum spacetime, and
will discuss possible mechanisms that could explain the universality of this
phenomenon.Comment: For proceedings of the conference in honor of Martin Reuter: "Quantum
Fields---From Fundamental Concepts to Phenomenological Questions"; 14 pages;
based in part on my review article arXiv:1705.0541
The String Tension in Gauge Theories
A review article on string tension concept and their relevance as
non-perturbative quantity on the study of quark confinement in lattice gauge
theories. A detailed description of a variety of methods to measure the string
tension on the lattice and an indication of the most promising developments is
proposed.Comment: Postscript file, 46 pages and 14 figure
The AdS/QCD Correspondence: Still Undelivered
We consider the particle spectrum and event shapes in large N gauge theories
in different regimes of the short-distance 't Hooft coupling, lambda. The
mesons in the small lambda limit should have a Regge spectrum in order to agree
with perturbation theory, while generically the large lambda theories with
gravity duals produce spectra reminiscent of KK modes. We argue that these
KK-like states are qualitatively different from QCD modes: they are deeply
bound states which are sensitive to short distance interactions rather than the
flux tube-like states expected in asymptotically free, confining gauge
theories. In addition, we also find that the characteristic event shapes for
the large lambda theories with gravity duals are close to spherical, very
different from QCD-like (small lambda, small N) and Nambu-Goto-like (small
lambda, large N) theories which have jets. This observation is in agreement
with the conjecture of Strassler on event shapes in large 't Hooft coupling
theories, which was recently proved by Hofman and Maldacena for the conformal
case. This conclusion does not change even when considering soft-wall
backgrounds in the gravity dual. The picture that emerges is the following:
theories with small and large lambda are qualitatively different, while
theories with small and large N are qualitatively similar. Thus it seems that
it is the relative smallness of the 't Hooft coupling in QCD that prevents a
reliable AdS/QCD correspondence from emerging, and that reproducing
characteristic QCD-like behavior will require genuine stringy dynamics to be
incorporated into any putative dual theory.Comment: 32 pages, 15 figures; references added, minor changes, history
clarifie
Screening in Strongly Coupled Plasmas: Universal Properties from Strings in Curved Space
We use the gauge/gravity correspondence to study the screening of a heavy
quark-antiquark pair in various strongly coupled plasmas. Besides N=4 super
Yang-Mills theory and the corresponding AdS_5 space we also study theories
obtained as deformations of AdS_5, among them in particular a class of
deformations solving supergravity equations of motion. We consider the
dependence of the screening distance on the velocity and the orientation of the
pair in the plasma. The value of the screening distance in N=4 SYM is found to
be a minimum in the class of theories under consideration for all kinematic
parameters.Comment: 10 pages, Talk presented by C.E. at Gribov-80 Memorial Workshop, ICTP
Trieste, Italy, May 201
Infinite N phase transitions in continuum Wilson loop operators
We define smoothed Wilson loop operators on a four dimensional lattice and
check numerically that they have a finite and nontrivial continuum limit. The
continuum operators maintain their character as unitary matrices and undergo a
phase transition at infinite N reflected by the eigenvalue distribution closing
a gap in its spectrum when the defining smooth loop is dilated from a small
size to a large one. If this large N phase transition belongs to a solvable
universality class one might be able to calculate analytically the string
tension in terms of the perturbative Lambda-parameter. This would be achieved
by matching instanton results for small loops to the relevant large-N-universal
function which, in turn, would be matched for large loops to an effective
string theory. Similarities between our findings and known analytical results
in two dimensional space-time indicate that the phase transitions we found only
affect the eigenvalue distribution, but the traces of finite powers of the
Wilson loop operators stay smooth under scaling.Comment: 31 pages, 9 figures, typos and references corrected, minor
clarifications adde
Dimension and Dimensional Reduction in Quantum Gravity
A number of very different approaches to quantum gravity contain a common
thread, a hint that spacetime at very short distances becomes effectively two
dimensional. I review this evidence, starting with a discussion of the physical
meaning of "dimension" and concluding with some speculative ideas of what
dimensional reduction might mean for physics.Comment: 33 page draft of solicited review article -- comments and added
references welcome; v2: many added references, minor clean-u
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