1,122 research outputs found
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
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
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
A bound on the effective gravitational coupling from semiclassical black holes
We show that the existence of semiclassical black holes of size as small as a
minimal length scale implies a bound on a gravitational analogue of
't-Hooft's coupling at all scales . The proof is valid for any metric theory of gravity that consistently
extends Einstein's gravity and is based on two assumptions about semiclassical
black holes: i) that they emit as black bodies, and ii) that they are perfect
quantum emitters. The examples of higher dimensional gravity and of weakly
coupled string theory are used to explicitly check our assumptions and to
verify that the proposed bound holds. Finally, we discuss some consequences of
the bound for theories of quantum gravity in general and for string theory in
particular.Comment: 16 page
A Novel Approach to the Cosmological Constant Problem
We propose a novel infinite-volume brane world scenario where we live on a
non-inflating spherical 3-brane, whose radius is somewhat larger than the
present Hubble size, embedded in higher dimensional bulk. Once we include
higher curvature terms in the bulk, we find completely smooth solutions with
the property that the 3-brane world-volume is non-inflating for a continuous
range of positive values of the brane tension, that is, without fine-tuning. In
particular, our solution, which is a near-BPS background with supersymmetry
broken on the brane around TeV, is controlled by a single integration constant.Comment: 20 pages, revte
Blown-up p-Branes and the Cosmological Constant
We consider a blown-up 3-brane, with the resulting geometry R^(3,1) \times
S^(N-1), in an infinite-volume bulk with N > 2 extra dimensions. The action on
the brane includes both an Einstein term and a cosmological constant. Similar
setups have been proposed both to reproduce 4-d gravity on the brane, and to
solve the cosmological constant problem. Here we obtain a singularity-free
solution to Einstein's equations everywhere in the bulk and on the brane, which
allows us to address these question explicitely. One finds, however, that the
proper volume of S^(N-1) and the cosmological constant on the brane have to be
fine-tuned relatively to each other, thus the cosmological constant problem is
not solved. Moreover the scalar propagator on the brane behaves 4-dimensionally
over a phenomenologically acceptable range only if the warp factor on the brane
is huge, which aggravates the Weak Scale - Planck Scale hierarchy problem.Comment: 21 pages, no figure
DGP Brane as a Gravity Conductor
We study how the DGP (Dvali-Gabadadze-Porrati) brane affects particle
dynamics in linearized approximation. We find that once the particle is removed
from the brane it is repelled to the bulk. Assuming that the cutoff for
gravitational interaction is , we calculate the classical
self energy of a particle as the function of its position. Since the particle
wants to go to the region where its self energy is lower, it is repelled from
the brane to the bulk where it gains its 5D self energy. Cases when mass of the
particle are qualitatively different, and in
later case one has to take into account effects of strong gravity. In both
cases the particle is repelled from the brane. For we obtain the
same result from the 'electrostatic' analog of the theory. In that language
mass (charge) in the bulk induces charge distribution on the brane which
shields the other side of the brane and provides repulsive force. The DGP brane
acts as a conducting plane in electrostatics (keeping in mind that in gravity
different charges repel). The repulsive nature of the brane requires a certain
localization mechanism. When the particle overcomes the localizing potential it
rapidly moves to the bulk. Particles of mass form a black hole
within distance from the brane.Comment: 13 pages, 3 figure
Domain Walls in SU(5)
We consider the Grand Unified SU(5) model with a small or vanishing cubic
term in the adjoint scalar field in the potential. This gives the model an
approximate or exact Z symmetry whose breaking leads to domain walls. The
simplest domain wall has the structure of a kink across which the Higgs field
changes sign () and inside which the full SU(5) is restored.
The kink is shown to be perturbatively unstable for all parameters. We then
construct a domain wall solution that is lighter than the kink and show it to
be perturbatively stable for a range of parameters. The symmetry in the core of
this domain wall is smaller than that outside. The interactions of the domain
wall with magnetic monopole is discussed and it is shown that magnetic
monopoles with certain internal space orientations relative to the wall pass
through the domain wall. Magnetic monopoles in other relative internal space
orientations are likely to be swept away on collision with the domain walls,
suggesting a scenario where the domain walls might act like optical
polarization filters, allowing certain monopole ``polarizations'' to pass
through but not others. As SU(5) domain walls will also be formed at small
values of the cubic coupling, this leads to a very complicated picture of the
evolution of defects after the Grand Unified phase transition.Comment: 6 pages, 1 figure. Animations can be viewed at
http://theory4.phys.cwru.edu/~levon/figures.htm
Gravity induced over a smooth soliton
I consider gravity induced over a smooth (finite thickness) soliton. Graviton
kinetic term is coupled to bulk scalar that develops solitonic vacuum
expectation value. Couplings of Kaluza-Klein modes to soliton-localized matter
are suppressed, giving rise to crossover distance between
4D and 5D behavior. This system can be viewed as a finite thickness brane
regularization of the model of Dvali, Gabadadze and Porrati.Comment: 12 pages, 2 figure
Braneworld Flattening by a Cosmological Constant
We present a model with an infinite volume bulk in which a braneworld with a
cosmological constant evolves to a static, 4-dimensional Minkowski spacetime.
This evolution occurs for a generic class of initial conditions with positive
energy densities. The metric everywhere outside the brane is that of a
5-dimensional Minkowski spacetime, where the effect of the brane is the
creation of a frame with a varying speed of light. This fact is encoded in the
structure of the 4-dimensional graviton propagator on the braneworld, which may
lead to some interesting Lorentz symmetry violating effects. In our framework
the cosmological constant problem takes a different meaning since the flatness
of the Universe is guaranteed for an arbitrary negative cosmological constant.
Instead constraints on the model come from different concerns which we discuss
in detail.Comment: 18 pages, 3 figures RevTe
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