340 research outputs found
On Lorentz violation in Horava-Lifshitz type theories
We show that coupling the Standard Model to a Lorentz symmetry violating
sector may co-exist with viable phenomenology, provided that the interaction
between the two is mediated by higher-dimensional operators. In particular, if
the new sector acquires anisotropic scaling behavior above a "Horava-Lifshitz"
energy scale L_HL and couples to the Standard Model through interactions
suppressed by M_P, the transmission of the Lorentz violation into the Standard
Model is protected by the ratio L_HL^2/M_P^2. A wide scale separation,
L_HL<<M_P, can then make Lorentz-violating terms in the Standard Model sector
within experimental bounds without fine-tuning. We first illustrate our point
with a toy example of Lifshitz-type neutral fermion coupled to photon via the
magnetic moment operator, and then implement similar proposal for the
Ho\v{r}ava-Lifshitz gravity coupled to conventional Lorentz-symmetric matter
fields. We find that most radiatively induced Lorentz violation can be
controlled by a large scale separation, but the existence of instantaneously
propagating non-Lifshitz modes in gravity can cause a certain class of diagrams
to remain quadratically divergent above L_HL. Such problematic quadratic
divergence, however, can be removed by extending the action with terms of
higher Lifshitz dimension, resulting in a completely consistent setup that can
cope with the stringent tests of Lorentz invariance.Comment: 24 pages, Main results are very much strengthened in this new
version, where the gauge-choice independence of the main conclusion has been
proven. Furthermore a new extension that could improve the original model,
which completely eliminates the need of fine-tuning, is propose
"Light from chaos" in two dimensions
We perform a Monte-Carlo study of the lattice two-dimensional gauged
XY-model. Our results confirm the strong-coupling expansion arguments that for
sufficiently small values of the spin-spin coupling the ``gauge symmetry
breaking" terms decouple and the long-distance physics is that of the unbroken
pure gauge theory. We find no evidence for the existence, conjectured earlier,
of massless states near a critical value of the spin-spin coupling. We comment
on recent remarks in the literature on the use of gauged XY-models in proposed
constructions of chiral lattice gauge theories.Comment: 6 pages, 7 figure
On 1-loop diagrams in AdS space and the random disorder problem
We study the complex scalar loop corrections to the boundary-boundary gauge
two point function in pure AdS space in Poincare coordinates, in the presence
of a boundary quadratic perturbation to the scalar. These perturbations
correspond to double trace perturbations in the dual CFT and modify the
boundary conditions of the bulk scalars in AdS. We find that, in addition to
the usual UV divergences, the 1-loop calculation suffers from a divergence
originating in the limit as the loop vertices approach the AdS horizon. We show
that this type of divergence is independent of the boundary coupling, and
making use of which we extract the finite relative variation of the imaginary
part of the loop via Cutkosky rules as the boundary perturbation varies.
Applying our methods to compute the effects of a time-dependent impurity to the
conductivities using the replica trick in AdS/CFT, we find that generally an
IR-relevant disorder reduces the conductivity and that in the extreme low
frequency limit the correction due to the impurities overwhelms the planar CFT
result even though it is supposedly suppressed. Comments on the effect
of time-independent impurity in such a system are presented.Comment: 22 pages, 3 figures, Boundary conditions clarified, some typos fixed,
presentations improved and references adde
Lattice chirality and the decoupling of mirror fermions
We show, using exact lattice chirality, that partition functions of lattice
gauge theories with vectorlike fermion representations can be split into
"light" and "mirror" parts, such that the "light" and "mirror" representations
are chiral. The splitting of the full partition function into "light" and
"mirror" is well defined only if the two sectors are separately anomaly free.
We show that only then is the generating functional, and hence the spectrum, of
the mirror theory a smooth function of the gauge field background. This
explains how ideas to use additional non-gauge, high-scale mirror-sector
dynamics to decouple the mirror fermions without breaking the gauge
symmetry--for example, in symmetric phases at strong mirror Yukawa
coupling--are forced to respect the anomaly-free condition when combined with
the exact lattice chiral symmetry. Our results also explain a paradox posed by
a recent numerical study of the mirror-fermion spectrum in a toy
would-be-anomalous two-dimensional theory. In passing, we prove some general
properties of the partition functions of arbitrary chiral theories on the
lattice that should be of interest for further studies in this field.Comment: 29 pages, 2 figures; published version, new addendu
Resonance in Asymmetric Warped Geometry
We study the spectrum of an asymmetric warped braneworld model with different
AdS curvatures on either side of the brane. In addition to the RS-like modes we
find a resonance state. Its mass is proportional to the geometric mean of the
two AdS curvature scales, while the difference between them determines the
strength of the resonance peak. There is a complementarity between the RS
zero-mode and the resonance: making the asymmetry stronger weakens the
zero-mode but strengthens the resonance, and vice versa. We calculate
numerically the braneworld gravitational potential and discuss the holographic
correspondence for the asymmetric model.Comment: 17 pages, 9 figures; v2 references with comments added; v3 two
references added, JHEP versio
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