657 research outputs found

### On the thin-string limit of the 6d stringlike defect model

We show that in 6d models localizing gravity on stringlike defects and
satisfying the dominant energy condition, the metric exterior to the string
inevitably depends on the string's thickness. As a consequence, in the limit of
thin string either the gravity delocalizes, or the six-dimensional Planck scale
must be much larger that the four-dimensional one.Comment: 3 pages; v2: an alternative interpretation of our results, overlooked
in the first version, is adde

### Universal contributions to scalar masses from five dimensional supergravity

We compute the effective Kahler potential for matter fields in warped
compactifications, starting from five dimensional gauged supergravity, as a
function of the matter fields localization. We show that truncation to zero
modes is inconsistent and the tree-level exchange of the massive gravitational
multiplet is needed for consistency of the four-dimensional theory. In addition
to the standard Kahler coming from dimensional reduction, we find the quartic
correction coming from integrating out the gravity multiplet. We apply our
result to the computation of scalar masses, by assuming that the SUSY breaking
field is a bulk hypermultiplet. In the limit of extreme opposite localization
of the matter and the spurion fields, we find zero scalar masses, consistent
with sequestering arguments. Surprisingly enough, for all the other cases the
scalar masses are tachyonic. This suggests the holographic interpretation that
a CFT sector always generates operators contributing in a tachyonic way to
scalar masses. Viability of warped su- persymmetric compactifications
necessarily asks then for additional contributions. We discuss the case of
additional bulk vector multiplets with mixed boundary conditions, which is a
partic- ularly simple and attractive way to generate large positive scalar
masses. We show that in this case successful fermion mass matrices implies
highly degenerate scalar masses for the first two generations of squarks and
sleptons.Comment: 23 pages. v2: References added, new section on effect of additional
bulk vector multiplets and phenomenolog

### Dirac Neutrino Masses with Planck Scale Lepton Number Violation

It is shown how pure Dirac neutrino masses can naturally occur at low
energies even in the presence of Planck scale lepton number violation. The
geometrical picture in five dimensions assumes that the lepton number symmetry
is explicitly broken on the Planck brane while the right-handed neutrino is
localised on the TeV brane. This physical separation in the bulk causes the
global lepton number to be preserved at low energies. A small wavefunction
overlap between the left-handed and right-handed neutrinos then naturally leads
to a small Dirac Yukawa coupling. By the AdS/CFT correspondence there exists a
purely four-dimensional dual description in which the right-handed neutrino is
a composite CFT bound state. The global lepton number is violated at the Planck
scale in a fundamental sector whose mixing into the composite sector is highly
suppressed by CFT operators with large anomalous dimensions. A similar small
mixing is then also responsible for generating a naturally small Dirac Yukawa
coupling between the fundamental left-handed neutrino and the composite
right-handed neutrino.Comment: 12 pages, LaTeX; v2: references added; v3: another reference adde

### The \tau -> \mu \bar{\nu_i} \nu_i decay in the Randall Sundrum background with localized U(1)_Y gauge boson

We study the effects of localization of the U(1)_Y gauge boson around the
visible brane and the contributions of the KK modes of Z bosons on the BR of
the LFV \tau -> \mu \bar{\nu_i} \nu_i decay. We observe that the BR is
sensitive to the amount of localization of Z boson in the bulk of the Randall
Sundrum background.Comment: 13 pages, 4 figures,1 tabl

### Higgs as a Holographic Pseudo-Goldstone Boson

The AdS/CFT correspondence allows to relate 4D strongly coupled theories to
weakly coupled theories in 5D AdS. We use this correspondence to study a
scenario in which the Higgs appears as a composite pseudo-Goldstone boson (PGB)
of a strongly coupled theory. We show how a non-linearly realized global
symmetry protects the Higgs mass and guarantees the absence of quadratic
divergences at any loop order. The gauge and Yukawa interactions for the PGB
Higgs are introduced in a simply way in the 5D AdS theory, and their one-loop
contributions to the Higgs potential are calculated using perturbation theory.
These contributions are finite, giving a squared-mass to the Higgs which is
one-loop smaller than the mass of the first Kaluza-Klein state. We also show
that if the symmetry breaking is caused by boundary conditions in the extra
dimension, the PGB Higgs corresponds to the fifth component of the bulk gauge
boson. To make the model fully realistic, a tree-level Higgs quartic coupling
must be induced. We present a possible mechanism to generate it and discuss the
conditions under which an unwanted large Higgs mass term is avoided.Comment: 31 pages, 6 figure

### Locally Localized Gravity Models in Higher Dimensions

We explore the possibility of generalizing the locally localized gravity
model in five space-time dimensions to arbitrary higher dimensions. In a
space-time with negative cosmological constant, there are essentially two kinds
of higher-dimensional cousins which not only take an analytic form but also are
free from the naked curvature singularity in a whole bulk space-time. One
cousin is a trivial extension of five-dimensional model, while the other one is
in essence in higher dimensions. One interesting observation is that in the
latter model, only anti-de Sitter ($AdS_p$) brane is physically meaningful
whereas de Sitter ($dS_p$) and Minkowski ($M_p$) branes are dismissed.
Moreover, for $AdS_p$ brane in the latter model, we study the property of
localization of various bulk fields on a single brane. In particular, it is
shown that the presence of the brane cosmological constant enables bulk gauge
field and massless fermions to confine to the brane only by a gravitational
interaction. We find a novel relation between mass of brane gauge field and the
brane cosmological constant.Comment: 20 pages, LaTex 2e, revised version (to appear in Phys. Rev. D

### Approximate gauge symmetry of composite vector bosons

It can be shown in a solvable field theory model that the couplings of the
composite vector bosons made of a fermion pair approach the gauge couplings in
the limit of strong binding. Although this phenomenon may appear accidental and
special to the vector boson made of a fermion pair, we extend it to the case of
bosons being constituents and find that the same phenomenon occurs in more an
intriguing way. The functional formalism not only facilitates computation but
also provides us with a better insight into the generating mechanism of
approximate gauge symmetry, in particular, how the strong binding and global
current conservation conspire to generate such an approximate symmetry. Remarks
are made on its possible relevance or irrelevance to electroweak and higher
symmetries.Comment: Correction of typos. The published versio

### A universe in a global monopole

We investigate brane physics in a universe with an extra dimensional global
monopole and negative bulk cosmological constant. The graviton zero mode is
naturally divergent; we thus invoke a physical cut-off to induce four
dimensional gravity on a brane at the monopole core. Independently, the massive
Kaluza-Klein modes have naturally compactified extra dimensions, inducing a
discrete spectrum. This spectrum remains consistent with four dimensional
gravity on the brane, even for small mass gap. Extra dimensional matter fields
also induce four dimensional matter fields on the brane, with the same
Kaluza-Klein spectrum of excited states. We choose parameters to solve the
hierarchy problem; that is, to induce the observed hierarchy between particle
and Planck scales in the effective four dimensional universe.Comment: 22 pages, 2 eps figures, revte

### Symmetry-preserving Loop Regularization and Renormalization of QFTs

A new symmetry-preserving loop regularization method proposed in \cite{ylw}
is further investigated. It is found that its prescription can be understood by
introducing a regulating distribution function to the proper-time formalism of
irreducible loop integrals. The method simulates in many interesting features
to the momentum cutoff, Pauli-Villars and dimensional regularization. The loop
regularization method is also simple and general for the practical calculations
to higher loop graphs and can be applied to both underlying and effective
quantum field theories including gauge, chiral, supersymmetric and
gravitational ones as the new method does not modify either the lagrangian
formalism or the space-time dimension of original theory. The appearance of
characteristic energy scale $M_c$ and sliding energy scale $\mu_s$ offers a
systematic way for studying the renormalization-group evolution of gauge
theories in the spirit of Wilson-Kadanoff and for exploring important effects
of higher dimensional interaction terms in the infrared regime.Comment: 13 pages, Revtex, extended modified version, more references adde

### Supergravity, Non-Conformal Field Theories and Brane-Worlds

We consider the supergravity dual descriptions of non-conformal super
Yang-Mills theories realized on the world-volume of Dp-branes. We use the dual
description to compute stress-energy tensor and current correlators. We apply
the results to the study of dilatonic brane-worlds described by non-conformal
field theories coupled to gravity. We find that brane-worlds based on D4 and D5
branes exhibit a localization of gauge and gravitational fields. We calculate
the corrections to the Newton and Coulomb laws in these theories.Comment: 24 pages, Latex, 2 figure

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