1,789 research outputs found
Cosmological Constraints on Theories with Large Extra Dimensions
In theories with large extra dimensions, constraints from cosmology lead to
non-trivial lower bounds on the fundamental scale M_F, corresponding to upper
bounds on the radii of the compact extra dimensions. These constraints are
especially relevant to the case of two extra dimensions, since only if M_F is
10 TeV or less do deviations from the standard gravitational force law become
evident at distances accessible to planned sub-mm gravity experiments. By
examining the graviton decay contribution to the cosmic diffuse gamma
radiation, we derive, for the case of two extra dimensions, a conservative
bound M_F > 110 TeV, corresponding to r_2 < 5.1 times 10^-5 mm, well beyond the
reach of these experiments. We also consider the constraint coming from
graviton overclosure of the universe and derive an independent bound M_F > 6.5
h^(-1/2) TeV, or r_2 < .015 h mm.Comment: 10 pages, references adde
Fermion Mass Hierarchies and Small Mixing Angles from Extra Dimensions
In this paper we study renormalization-group evolutions of Yukawa matrices
enhanced by Kaluza-Klein excited modes and analyze their infrared fixed-point
structure. We derive necessary conditions to obtain hierarchies between
generations on the fixed point. These conditions restrict how the fields in the
models can extend to higher dimension. Several specific mechanisms to realize
the conditions are presented. We also take account of generation mixing effects
and find a scenario where the mixing angles become small at low energy even
with large initial values at high-energy scale. A toy model is shown to lead
realistic quark mass matrices.Comment: 23 pages, 7 figures, LaTeX, a supplementary explanation and
references adde
Brane fluctuations and suppression of Kaluza-Klein mode couplings
In higher dimensional models where the gauge and gravity fields live in the
bulk and the matter fields only in a brane, we point out the importance of the
brane (transverse) coordinate modes, which are the Nambu-Goldstone bosons
appearing as a result of spontaneous breaking of the translation symmetry. The
brane recoil effect suppresses the couplings of higher Kaluza-Klein modes to
the matter, and gives a natural resolution to the divergence problem caused by
the exchange of infinitely many Kaluza-Klein modes.Comment: 11 pages, 1 eps figure, references adde
On High-Energy Behavior of Cross Sections in Theories with Large Extra Dimensions
We discuss the high-energy behavior of cross sections in theories with large
extra dimensions and low-scale quantum gravity, addressing two particular
issues: (i) the tension of the D-branes, and (ii) bounds on the cross section
and their relation to approximations in the mode sum over Kaluza-Klein-graviton
exchanges.Comment: 6 pages, late
4D Constructions of Supersymmetric Extra Dimensions and Gaugino Mediation
We present 4D gauge theories which at low energies coincide with higher
dimensional supersymmetric (SUSY) gauge theories on a transverse lattice. We
show that in the simplest case of pure 5D SUSY Yang-Mills there is an
enhancement of SUSY in the continuum limit without fine-tuning. This result no
longer holds in the presence of matter fields, in which case fine-tuning is
necessary to ensure higher dimensional Lorentz invariance and supersymmetry. We
use this construction to generate 4D models which mimic gaugino mediation of
SUSY breaking. The way supersymmetry breaking is mediated in these models to
the MSSM is by assuming that the physical gauginos are a mixture of a number of
gauge eigenstate gauginos: one of these couples to the SUSY breaking sector,
while another couples to the MSSM matter fields. The lattice can be as coarse
as just two gauge groups while still obtaining the characteristic
gaugino-mediated soft breaking terms.Comment: 32 pages LaTeX; missing factor in two-loop gauge-mediated scalar mass
estimate fixed, comments on unification revise
Top quark Kaluza-Klein mode mixing in the Randall-Sundrum bulk Standard Model and B -> X_s gamma
We study top quark Kaluza-Klein (KK) mode mixing in the Randall-Sundrum
scenario with all the SM fermions and gauge bosons in the bulk. Even though the
simple assumption of universal bulk fermion mass m_psi leads to the same KK
mass spectrum for all the SM fermions and thus suppresses new contributions to
the flavor changing neutral current and the rho parameter, large Yukawa
coupling of the top quark generates the mixing among its KK modes and breaks
the degeneracy: Unacceptably large contribution to rho occurs. In order to
satisfy the rho constraint, we relax the model by assigning a different bulk
fermion mass to SU(2) singlet bottom quark, and demonstrate that there exists
some limited parameter space where the constraint is satisfied. It
is also shown that the current measurement of Br(B -> X_s + gamma) can be
accommodated in this modified model, and one sigma level precision constrains
the effective weak scale k_{EW} such as k_{EW} ~ 3 TeV for m_psi/k=-0.4, where
k_{EW} is the warp-suppressed AdS_5 curvature.Comment: To satisfy the rho constraint, the model is minimally extended. The
final version to appear in PR
Photons, neutrinos and large compact space dimensions
We compute the contribution of Kaluza-Klein graviton exchange to the cross
section for photon-neutrino scattering. Unlike the usual situation where the
virtual graviton exchange represents a small correction to a leading order
electroweak or strong amplitude, in this case the graviton contribution is of
the same order as the electroweak amplitude, or somewhat larger. Inclusion of
the graviton contribution is not sufficient to allow high energy neutrinos to
scatter from relic neutrinos in processes such as
, but the photon-neutrino decoupling temperature
is substantially reduced.Comment: 8 pages, 3 figures LaTeX. Typos correcte
Gravity and Matter in Extra Dimensions
In this paper, we derive from the viewpoint of the effective 4D theory the
interaction terms between linearized gravity propagating in N>= 2 large extra
dimensions and matter propagating into one extra dimension. This generalizes
known results for the interactions between gravity and 4D matter in ADD-type
models. Although we assume that matter is described by an Universal Extra
Dimensions (UED) scenario (with all fields propagating into the fifth
dimension), we present our results in a general form that can be easily adapted
to various other scenarios of matter distribution. We then apply our results to
the UED model on a fat brane and consider some phenomenological applications.
Among these are the computation of the gravitational decay widths of the matter
KK excitations and the effect the width of the brane has on the interactions of
gravity with Standard Model particles. We also estimate the cross-section for
producing single KK excitations at colliders through KK number-violating
gravitational interaction.Comment: 21 pages, 6 figures, Late
Enhancement of the Higgs pair production at LHC; the MSSM and extra dimension effects
The neutral Higgs pair production at the LHC is studied in the MSSM, the
large extra dimensional (ADD) model and the Randall-Sundrum (RS) model, where
the total cross section can be significantly enhanced compared to that in the
SM. The , invariant mass and rapidity distributions of each model have
been shown to be distinctive: The ADD model raises the and invariant mass
distributions at high scales of and invariant mass; in the RS model
resonant peaks appear after the SM contribution dies away; the SM and the MSSM
distributions drop rapidly at those high scales; in the ADD and the RS models
the rapidity distributions congregate more around the center. It is concluded
that various distributions of the Higgs pair production at the LHC with
restrictive kinematic cuts would provide one of the most robust signals for the
extra dimensional effects.Comment: Revised version, 26 pages including 11 figures, ReVTe
Large Extra Dimension Effects on the Spin Configuration of the Top Quark Pair at e^+ e^- Colliders
Large extra dimension effects on the spin configuration of the top quark pair
at the process are studied. It is shown that the TeV
scale quantum gravity effects cause significant deviations from the Standard
Model predictions for the spin configuration in the off-diagonal basis: they
lead to substantial cross sections of the like-spin states of the top quark
pair, which vanish in the SM; they weaken the pure dominance of the processes,
the Up-Down (Down-Up) spin states for the left-handed (right-handed) beam. In
addition it is shown that the angular cut is very effective
to determine the sign of the quantum gravity corrections.Comment: A discussion on the angular distribution is added with a tabl
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