2,431 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
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.
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
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
Color Glass Condensate in Brane Models or Don't Ultra High Energy Cosmic Rays Probe 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
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
Tunneling into Extra Dimension and High-Energy Violation of Lorentz Invariance
We consider a class of models with infinite extra dimension, where bulk space
does not possess SO(1,3) invariance, but Lorentz invariance emerges as an
approximate symmetry of the low-energy effective theory. In these models, the
maximum attainable speeds of the graviton, gauge bosons and scalar particles
are automatically equal to each other and smaller than the maximum speed in the
bulk. Additional fine-tuning is needed in order to assure that the maximum
attainable speed of fermions takes the same value. A peculiar feature of our
scenario is that there are no truly localized modes. All four-dimensional
particles are resonances with finite widths. The latter depends on the energy
of the particle and is naturally small at low energies.Comment: 21 pages, references and comments added, final version to appear in
JHE
Gauss-Bonnet gravity renders negative tension braneworlds unstable
We show that the Gauss-Bonnet correction to Einstein gravity induces a
gravitational tachyon mode, namely an unstable spin 2 fluctuation, in the
Randall-Sundrum I model. We demonstrate that this instability is generically
related to the presence of a negative tension brane in the set-up, with or
without -symmetry across it. Indeed it is shown that the tachyon mode is a
bound state localised on any negative tension brane of co-dimension one,
embedded in anti-de Sitter background. We discuss the possible resolution of
this instability by the inclusion of induced gravity terms on the branes or by
an effective four-dimensional cosmological constant.Comment: published versio
Massive Cosmologies
We explore the cosmological solutions of a recently proposed extension of
General Relativity with a Lorentz-invariant mass term. We show that the same
constraint that removes the Boulware-Deser ghost in this theory also prohibits
the existence of homogeneous and isotropic cosmological solutions.
Nevertheless, within domains of the size of inverse graviton mass we find
approximately homogeneous and isotropic solutions that can well describe the
past and present of the Universe. At energy densities above a certain crossover
value, these solutions approximate the standard FRW evolution with great
accuracy. As the Universe evolves and density drops below the crossover value
the inhomogeneities become more and more pronounced. In the low density regime
each domain of the size of the inverse graviton mass has essentially non-FRW
cosmology. This scenario imposes an upper bound on the graviton mass, which we
roughly estimate to be an order of magnitude below the present-day value of the
Hubble parameter. The bound becomes especially restrictive if one utilizes an
exact self-accelerated solution that this theory offers. Although the above are
robust predictions of massive gravity with an explicit mass term, we point out
that if the mass parameter emerges from some additional scalar field
condensation, the constraint no longer forbids the homogeneous and isotropic
cosmologies. In the latter case, there will exist an extra light scalar field
at cosmological scales, which is screened by the Vainshtein mechanism at
shorter distances.Comment: 21 page
Generation of 10^15 - 10^17 eV photons by UHE CR in the Galactic magnetic filed
We show that the deep expected in the diffuse photon spectrum above the
threshold of e+e- pair production, i.e., at energies 10^15 - 10^17 eV, may be
absent due to the synchrotron radiation by the electron component of the
extragalactic Ultra-High Energy Cosmic Rays (UHE CR) in the Galactic magnetic
filed. The mechanism we propose requires small (less than 2x10^-12 G)
extragalactic magnetic fields and large fraction of photons in the UHE CR. For
a typical photon flux expected in top-down scenarios of UHE CR, the predicted
flux in the region of the deep is close to the existing experimental limit. The
sensitivity of our mechanism to the extragalactic magnetic field may be used to
improve existing bounds on the latter by two orders of magnitude.Comment: 11 pages, LaTeX, 1 .ps figure. Numerical error corrected; references
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