1,563 research outputs found

### Metastable Gravitons and Infinite Volume Extra Dimensions

We address the issue of whether extra dimensions could have an infinite
volume and yet reproduce the effects of observable four-dimensional gravity on
a brane. There is no normalizable zero-mode graviton in this case, nevertheless
correct Newton's law can be obtained by exchanging bulk gravitons. This can be
interpreted as an exchange of a single {\it metastable} 4D graviton. Such
theories have remarkable phenomenological signatures since the evolution of the
Universe becomes high-dimensional at very large scales. Furthermore, the bulk
supersymmetry in the infinite volume limit might be preserved while being
completely broken on a brane. This gives rise to a possibility of controlling
the value of the bulk cosmological constant. Unfortunately, these theories have
difficulties in reproducing certain predictions of Einstein's theory related to
relativistic sources. This is due to the van Dam-Veltman-Zakharov discontinuity
in the propagator of a massive graviton. This suggests that all theories in
which contributions to effective 4D gravity come predominantly from the bulk
graviton exchange should encounter serious phenomenological difficulties.Comment: 9 LaTex pages; One reference and a comment adde

### Creating semiclassical black holes in collider experiments and keeping them on a string

We argue that a simple modification of the TeV scale quantum gravity scenario
allows production of semiclassical black holes in particle collisions at the
LHC. The key idea is that in models with large extra dimensions the strength of
gravity in the bulk can be higher than on the brane where we live. A well-known
example of this situation is the case of warped extra dimensions. Even if the
energy of the collision is not sufficient to create a black hole on the brane,
it may be enough to produce a particle which accelerates into the bulk up to
trans-Planckian energy and creates a large black hole there. In a concrete
model we consider, the black hole is formed in a collision of the particle with
its own image at an orbifold plane. When the particle in question carries some
Standard Model gauge charges the created black hole gets attached to our brane
by a string of the gauge flux. For a 4-dimensional observer such system looks
as a long-lived charged state with the mass continuously decreasing due to
Hawking evaporation of the black hole. This provides a distinctive signature of
black hole formation in our scenario.Comment: Journal version, a misprint correcte

### A Vacuum Accumulation Solution to the Strong CP Problem

We suggest a solution to the strong CP problem in which there are no axions
involved. The superselection rule of the \theta-vacua is dynamically lifted in
such a way that an infinite number of vacua are accumulated within the
phenomenologically acceptable range of \theta < 10^{-9}, whereas only a
measure-zero set of vacua remains outside of this interval. The general
prediction is the existence of membranes to which the standard model gauge
fields are coupled. These branes may be light enough for being produced at the
particle accelerators in form of the resonances with a characteristic membrane
spectrum.Comment: 17 page

### Families as Neighbors in Extra Dimension

We propose a new mechanism for explanation of the fermion hierarchy without
introducing any family symmetries. Instead, we postulate that different
generations live on different branes embedded in a relatively large extra
dimension, where the gauge fields can propagate. The electroweak symmetry is
broken on a separate brane, which is a source of exponentially decaying Higgs
profile in the bulk. The resulting fermion masses and mixings are determined by
an exponentially suppressed overlap of the fermion and Higgs wave functions and
are automatically hierarchical even if all copies are identical and there is no
hierarchy of distances. In this framework the well known pattern of the
"nearest neighbor mixing" is predicted due to the fact that the families are
literally neighbors in the extra space. This picture may also provide a new way
of a hierarchically weak supersymmetry breaking, provided that the combination
of three family branes is a non-BPS configuration, although each of them,
individually taken, is. This results in exponentially weak supersymmetry
breaking. We also discuss the issue of embedding identical branes in the
compact spaces and localization of the fermionic zero modes.Comment: Latex, 11 pages, no figure

### Dynamical classicalization

We integrate numerically the nonlinear equation of motion for a collapsing
spherical wavepacket in the context of theories that are expected to display
behavior characteristic of classicalization. The classicalization radius sets
the scale for the onset of significant deformations of the collapsing
configuration, which result in the formation of shock fronts. A characteristic
observable feature of the classicalization process is the creation of an
outgoing field configuration that extends far beyond the classicalization
radius. This feature develops before the deformed wavepacket reaches distances
of the order of the fundamental scale. We find that in some models the
scattering problem may not have real solutions over the whole space at late
times. We determine the origin of this behavior and discuss the consistency of
the underlying models.Comment: 16 pages, 6 figures, published versio

### On Gauge Dynamics and SUSY Breaking in Orientiworld

In the Orientiworld framework the Standard Model fields are localized on
D3-branes sitting on top of an orientifold 3-plane. The transverse
6-dimensional space is a non-compact orbifold (or a more general conifold). The
4-dimensional gravity on D3-branes is reproduced due to the 4-dimensional
Einstein-Hilbert term induced at the quantum level. The orientifold 3-plane
plays a crucial role, in particular, without it the D3-brane world-volume
theories would be conformal due to the tadpole cancellation. We study
non-perturbative gauge dynamics in various N=1 supersymmetric orientiworld
models based on the Z_3 as well as Z_5 and Z_7 orbifold groups. Our discussions
illustrate that there is a rich variety of supersymmetry preserving dynamics in
some of these models. On the other hand, we also find some models with
dynamical supersymmetry breaking.Comment: 21 pages, revtex, minor misprints corrected (to appear in Int. J.
Mod. Phys. A

### Large Hierarchies from Attractor Vacua

We discuss a mechanism through which the multi-vacua theories, such as String
Theory, could solve the Hierarchy Problem, without any UV-regulating physics at
low energies. Because of symmetry the number density of vacua with a certain
hierarchically-small Higgs mass diverges, and is an attractor on the vacuum
landscape.The hierarchy problem is solved in two steps. It is first promoted
into a problem of the super-selection rule among the infinite number of vacua
(analogous to theta-vacua in QCD), that are finely scanned by the Higgs mass.
This rule is lifted by heavy branes, which effectively convert the Higgs mass
into a dynamical variable. The key point is that a discrete
"brane-charge-conjugation" symmetry guarantees that the fineness of the
vacuum-scanning is set by the Higgs mass itself. On a resulting landscape in
all, but a measure-zero set of vacua the Higgs mass has a common
hierarchically-small value. In minimal models this value is controlled by the
QCD scale and is of the right magnitude. Although in each particular vacuum
there is no visible UV-regulating low energy physics, the realistic models are
predictive. For example, we show that in the minimal case the "charge
conjugation" symmetry is automatically a family symmetry, and imposes severe
restrictions on quark Yukawa matrices.Comment: 33 pages, Late

### Consistency of Relevant Cosmological Deformations on all Scales

Using cosmological perturbation theory we show that the most relevant defor-
mation of gravity is consistent at the linear level. In particular, we prove
the absence of uni- tarity violating negative norm states in the weak coupling
regime from sub- to super-Hubble scales. This demonstrates that the recently
proposed classical self-protection mechanism of deformed gravity extends to the
entire kinematical domain.Comment: 22 pages, 4 figure

### Ultra-High Energy Probes of Classicalization

Classicalizing theories are characterized by a rapid growth of the scattering
cross section. This growth converts these sort of theories in interesting
probes for ultra-high energy experiments even at relatively low luminosity,
such as cosmic rays or Plasma Wakefield accelerators. The microscopic reason
behind this growth is the production of N-particle states, classicalons, that
represent self-sustained lumps of soft Bosons. For spin-2 theories this is the
quantum portrait of what in the classical limit are known as black holes. We
emphasize the importance of this quantum picture which liberates us from the
artifacts of the classical geometric limit and allows to scan a much wider
landscape of experimentally-interesting quantum theories. We identify a
phenomenologically-viable class of spin-2 theories for which the growth of
classicalon production cross section can be as efficient as to compete with QCD
cross section already at 100 TeV energy, signaling production of quantum black
holes with graviton occupation number of order 10^4.Comment: 23 pages, late

### Symmetries within domain walls

The comparison of symmetries in the interior and the exterior of a domain
wall is relevant when discussing the correspondence between domain walls and
branes, and also when studying the interaction of walls and magnetic monopoles.
I discuss the symmetries in the context of an SU(N) times Z_2 model (for odd N)
with a single adjoint scalar field. Situations in which the wall interior has
less symmetry than the vacuum are easy to construct while the reverse situation
requires significant engineering of the scalar potential.Comment: 5 pages. Added reference

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