2,193 research outputs found

### Gravitational Lorentz Violation and Superluminality via AdS/CFT Duality

A weak quantum mechanical coupling is constructed permitting superluminal
communication within a preferred region of a gravitating AdS_5 spacetime. This
is achieved by adding a spatially non-local perturbation of a special kind to
the Hamiltonian of a four-dimensional conformal field theory with a
weakly-coupled AdS dual, such as maximally supersymmetric Yang-Mills theory. In
particular, two issues are given careful treatment: (1) the UV-completeness of
our deformed CFT, guaranteeing the existence of a ``deformed string theory''
AdS dual, and (2) the demonstration that superluminal effects can take place in
AdS, both on its boundary as well as in the bulk. Exotic Lorentz-violating
properties such as these may have implications for tests of General Relativity,
addressing the cosmological constant problem, or probing "behind'' horizons.
Our construction may give insight into the interpretation of wormhole solutions
in Euclidean AdS gravity.Comment: 23 pages LaTex. Typo in Eq. (37) corrected. References adde

### Quasi-localized states on noncommutative solitons

We consider noncommutative gauge theories which have zero mass states
propagating along both commutative and noncommutative dimensions. Solitons in
these theories generically carry U(m) gauge group on their world-volume. From
the point of view of string theory, these solitons correspond to
``branes within branes''. We show that once the world-volume U(m) gauge
theory is in the Higgs phase, light states become quasi-localized, rather than
strictly localized on the soliton, i.e. they mix with light bulk modes and have
finite widths to escape into the noncommutative dimensions. At small values of
U(m) symmetry breaking parameters, these widths are small compared to the
corresponding masses. Explicit examples considered are adjoint scalar field in
the background of a noncommutative vortex in U(1)-Higgs theory, and gauge
fields in instanton backgrounds in pure gauge noncommutative theories.Comment: 27 pages, references and comments added, final version to appear in
JHE

### Quasilocalized gravity without asymptotic flatness

We present a toy model of a generic five-dimensional warped geometry in which
the 4D graviton is not fully localized on the brane. Studying the tensor sector
of metric perturbation around this background, we find that its contribution to
the effective gravitational potential is of 4D type (1/r) at the intermediate
scales and that at the large scales it becomes 1/r^{1+alpha}, 0<alpha=< 1 being
a function of the parameters of the model (alpha=1 corresponds to the
asymptotically flat geometry). Large-distance behavior of the potential is
therefore not necessarily five-dimensional. Our analysis applies also to the
case of quasilocalized massless particles other than graviton.Comment: 9 pages, 1 figure; to be published in Phys. Rev.

### Gauge theory solitons on noncommutative cylinder

We generalize to noncommutative cylinder the solution generation technique,
originally suggested for gauge theories on noncommutative plane. For this
purpose we construct partial isometry operators and complete set of orthogonal
projectors in the algebra of the cylinder, and an isomorphism between the free
module and its direct sum with the Fock module on the cylinder. We construct
explicitly the gauge theory soliton and evaluate the spectrum of perturbations
about this soliton.Comment: References added; to appear in Theor.Math.Phy

### Color Glass Condensate in Brane Models or Don't Ultra High Energy Cosmic Rays Probe $10^{15}eV$ Scale ?

In a previous work hep-ph/0203165 we have studied propagation of relativistic
particles in the bulk for some of most popular brane models. Constraints have
been put on the parameter space of these models by calculating the time delay
due to propagation in the bulk of particles created during the interaction of
Ultra High Energy Cosmic Rays with protons in the terrestrial atmosphere. The
question was however raised that probability of hard processes in which bulk
modes can be produced is small and consequently, the tiny flux of UHECRs can
not constrain brane models. Here we use Color Glass Condensate (CGC) model to
show that effects of extra dimensions are visible not only in hard processes
when the incoming particle hits a massive Kaluza-Klein mode but also through
the modification of soft/semi-hard parton distribution. At classical level, for
an observer in the CM frame of UHECR and atmospheric hadrons, color charge
sources are contracted to a thin sheet with a width inversely proportional to
the energy of the ultra energetic cosmic ray hadron and consequently they can
see an extra dimension with comparable size. Due to QCD interaction a short
life swarm of partons is produced in front of the sheet and its partons can
penetrate to the extra-dimension bulk. This reduces the effective density of
partons on the brane or in a classical view creates a delay in the arrival of
the most energetic particles if they are reflected back due to the warping of
the bulk. In CGC approximation the density of swarm at different distance from
the classical sheet can be related and therefore it is possible (at least
formally) to determine the relative fraction of partons in the bulk and on the
brane at different scales. Results of this work are also relevant to the test
of brane models in hadron colliders like LHC.Comment: 17 pages, 3 figures. Text is modified to highlight the relation
between the distribution gluons at high and low rapidity scales. v3:
published versio

### Limit on the electric charge-nonconserving $\mu^+ \to invisible$ decay

The first limit on the branching ratio of the electric charge-nonconserving
invisible muon decay $Br(\mu^+ \to invisible) < 5.2 \times 10^{-3}$ is obtained
from the recently reported results on new determination of the Fermi constant
from muon decays. The results of a feasibility study of a new proposed
experiment for a sensitive search for this decay mode at the level of a few
parts in 10^{11} are presented. Constrains on the $\tau \to invisible$ decay
rate are discussed. These leptonic charge-nonconserving processes may hold in
four-dimensional world in models with infinite extra dimensions, thus making
their searches complementary to collider experiments probing new physics.Comment: 5 pages, 2 Figure, to appear in PR

### Localized U(1) Gauge Fields, Millicharged Particles, and Holography

We consider U(1) gauge fields in a slice of AdS_5 with bulk and boundary mass
parameters. The zero mode of a bulk U(1) gauge field can be localized either on
the UV or IR brane. This leads to a simple model of millicharged particles in
which fermions can have arbitrarily small electric charge. In the electroweak
sector we also discuss phenomenological implications of a localized U(1)_Y
gauge boson. Using the AdS/CFT correspondence we present the 4D holographic
interpretation of the 5D model. In particular the photon is shown to be a
composite particle when localized near the IR brane, whereas it is elementary
when localized near the UV brane. In the dual interpretation the
``millicharge'' results from an elementary fermion coupling to a composite
photon via a vector current with large anomalous dimension.Comment: 26 pages, 1 figur

### QED from six-dimensional vortex and gauge anomalies

Starting from an anomaly-free Abelian Higgs model coupled to gravity in a
6-dimensional space-time we construct an effective four-dimensional theory of
charged fermions interacting with U(1) Abelian gauge field and gravity, both
localised near the core of a Nielsen-Olesen vortex configuration. We show that
an anomaly free theory in 6-dimensions can give rise to an anomalous theory in
D=4, which suggests a possibility of consistent regularisation of abelian
anomalous chiral gauge theories in four dimensions. We also show that the
spectrum of charged bulk fermions has a mass gap.Comment: Latex, 19 page

### Causality, Analyticity and an IR Obstruction to UV Completion

We argue that certain apparently consistent low-energy effective field
theories described by local, Lorentz-invariant Lagrangians, secretly exhibit
macroscopic non-locality and cannot be embedded in any UV theory whose S-matrix
satisfies canonical analyticity constraints. The obstruction involves the signs
of a set of leading irrelevant operators, which must be strictly positive to
ensure UV analyticity. An IR manifestation of this restriction is that the
"wrong" signs lead to superluminal fluctuations around non-trivial backgrounds,
making it impossible to define local, causal evolution, and implying a
surprising IR breakdown of the effective theory. Such effective theories can
not arise in quantum field theories or weakly coupled string theories, whose
S-matrices satisfy the usual analyticity properties. This conclusion applies to
the DGP brane-world model modifying gravity in the IR, giving a simple
explanation for the difficulty of embedding this model into controlled stringy
backgrounds, and to models of electroweak symmetry breaking that predict
negative anomalous quartic couplings for the W and Z. Conversely, any
experimental support for the DGP model, or measured negative signs for
anomalous quartic gauge boson couplings at future accelerators, would
constitute direct evidence for the existence of superluminality and macroscopic
non-locality unlike anything previously seen in physics, and almost
incidentally falsify both local quantum field theory and perturbative string
theory.Comment: 34 pages, 10 figures; v2: analyticity arguments improved, discussion
on non-commutative theories and minor clarifications adde

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