688 research outputs found
Brane gravity, massless bulk scalar and self-tuning of the cosmological constant
We show that a self-tuning mechanism of the cosmological constant could work
in 5D non-compact space-time with a symmetry in the presence of a
massless scalar field. The standard model matter fields live only on the 4D
brane. The change of vacuum energy on the brane (brane cosmological constant)
by, for instance, electroweak and QCD phase transitions, just gives rise to
dynamical shifts of the profiles of the background metric and the scalar field
in the extra dimension, keeping 4D space-time flat without any fine-tuning. To
avoid naked singularities in the bulk, the brane cosmological constant should
be negative. We introduce an additional brane-localized 4D Einstein-Hilbert
term so as to provide the observed 4D gravity with the non-compact extra
dimension. With a general form of brane-localized gravity term allowed by the
symmetries, the low energy Einstein gravity is successfully reproduced on the
brane at long distances. We show this phenomenon explicitly for the case of
vanishing bulk cosmological constant.Comment: 1+15 pages, no figure, Version to appear in PR
Anomaly Mediated Supersymmetry Breaking in Four Dimensions, Naturally
We present a simple four-dimensional model in which anomaly mediated
supersymmetry breaking naturally dominates. The central ingredient is that the
hidden sector is near a strongly-coupled infrared fixed-point for several
decades of energy below the Planck scale. Strong renormalization effects then
sequester the hidden sector from the visible sector. Supersymmetry is broken
dynamically and requires no small input parameters. The model provides a
natural and economical explanation of the hierarchy between the
supersymmetry-breaking scale and the Planck scale, while allowing anomaly
mediation to address the phenomenological challenges posed by weak scale
supersymmetry. In particular, flavor-changing neutral currents are naturally
near their experimental limits.Comment: 14 pages, Late
"Gaugomaly" Mediated SUSY Breaking and Conformal Sequestering
Anomaly-mediated supersymmetry breaking in the context of 4D conformally
sequestered models is combined with Poppitz-Trivedi D-type gauge-mediation. The
implementation of the two mediation mechanisms naturally leads to visible soft
masses at the same scale so that they can cooperatively solve the mu and flavor
problems of weak scale supersymmetry, as well as the tachyonic slepton problem
of pure anomaly-mediation. The tools are developed in a modular fashion for
more readily fitting into the general program of optimizing supersymmetric
dynamics in hunting for the most attractive weak scale phenomenologies combined
with Planck-scale plausibility.Comment: 14 pages, Late
Visible Effects of the Hidden Sector
The renormalization of operators responsible for soft supersymmetry breaking
is usually calculated by starting at some high scale and including only visible
sector interactions in the evolution equations, while ignoring hidden sector
interactions. Here we explain why this is correct only for the most trivial
structures in the hidden sector, and discuss possible implications. This
investigation was prompted by the idea of conformal sequestering. In that
framework hidden sector renormalizations by nearly conformal dynamics are
critical. In the original models of conformal sequestering it was necessary to
impose hidden sector flavor symmetries to achieve the sequestered form. We
present models which can evade this requirement and lead to no-scale or anomaly
mediated boundary conditions; but the necessary structures do not seem generic.
More generally, the ratios of scalar masses to gaugino masses, the -term,
the -term, -terms, and the gravitino mass can be significantly
affected.Comment: 23 pages, no figure
Improved Single Sector Supersymmetry Breaking
Building on recent work by N. Arkani-Hamed and the present authors, we
construct realistic models that break supersymmetry dynamically and give rise
to composite quarks and leptons, all in a single strongly-coupled sector. The
most important improvement compared to earlier models is that the
second-generation composite states correspond to dimension-2 "meson" operators
in the ultraviolet. This leads to a higher scale for flavor physics, and gives
a completely natural suppression of flavor-changing neutral currents. We also
construct models in which the hierarchy of Yukawa couplings is explained by the
dimensionality of composite states. These models provide an interesting and
viable alternative to gravity- and gauge-mediated models. The generic
signatures are unification of scalar masses with different quantum numbers at
the compositeness scale, and lighter gaugino, Higgsino, and third-generation
squark and slepton masses. We also analyze large classes of models that give
rise to both compositeness and supersymmetry breaking, based on gauge theories
with confining, fixed-point, or free-magnetic dynamics.Comment: 34 pages, LaTeX2
Phase Transition in a One-Dimensional Extended Peierls-Hubbard Model with a Pulse of Oscillating Electric Field: II. Linear Behavior in Neutral-to-Ionic Transition
Dynamics of charge density and lattice displacements after the neutral phase
is photoexcited is studied by solving the time-dependent Schr\"odinger equation
for a one-dimensional extended Peierls-Hubbard model with alternating
potentials. In contrast to the ionic-to-neutral transition studied previously,
the neutral-to-ionic transition proceeds in an uncooperative manner as far as
the one-dimensional system is concerned. The final ionicity is a linear
function of the increment of the total energy. After the electric field is
turned off, the electronic state does not significantly change, roughly keeping
the ionicity, even if the transition is not completed, because the ionic
domains never proliferate. As a consequence, an electric field with frequency
just at the linear absorption peak causes the neutral-to-ionic transition the
most efficiently. These findings are consistent with the recent experiments on
the mixed-stack organic charge-transfer complex, TTF-CA. We artificially modify
or remove the electron-lattice coupling to discuss the origin of such
differences between the two transitions.Comment: 17 pages, 9 figure
A Minimal Superstring Standard Model I: Flat Directions
Three family SU(3)_C x SU(2)_L x U(1)_Y string models in several
constructions generically possess two features: (i) an extra local anomalous
U(1)_A and (ii) numerous (often fractionally charged) exotic particles beyond
those in the minimal supersymmetric model (MSSM). Recently, we demonstrated
that the observable sector effective field theory of such a free fermionic
string model can reduce to that of the MSSM, with the standard observable gauge
group being just SU(3)_C x SU(2)_L x U(1)_Y and the SU(3)_C x SU(2)_L x
U(1)_Y-charged spectrum of the observable sector consisting solely of the MSSM
spectrum. An example of a model with this property was shown. We continue our
investigation of this model by presenting a large set of different flat
directions of the same model that all produce the MSSM spectrum. Our results
suggest that even after imposing the conditions for the decoupling of exotic
states, there may remain sufficient freedom to satisfy the remaining
phenomenological constraints imposed by the observed data.Comment: 64 pages. Latex. Revisions to match version in Int. J. Mod. Physics
Composite Quarks and Leptons from Dynamical Supersymmetry Breaking without Messengers
We present new theories of dynamical SUSY breaking in which the strong
interactions that break SUSY also give rise to composite quarks and leptons
with naturally small Yukawa couplings. In these models, SUSY breaking is
communicated directly to the composite fields without ``messenger''
interactions. The compositeness scale can be anywhere between 10 TeV and the
Planck scale. These models can naturally solve the supersymmetric flavor
problem, and generically predict sfermion mass unification independent from
gauge unification.Comment: 27 pages, LaTeX; Clarified flavor symmetry of strong interactions;
corrected overestimate of FCNC's; conclusions strengthene
Baryons with Many Colors and Flavors
Using recently-developed diagrammatic techniques, I derive some general
results concerning baryons in the expansion, where is the number of
QCD colors. I show that the spin-flavor relations which hold for baryons in the
large- limit, as well as the form of the corrections to these relations at
higher orders in , hold even if , where is the
number of light quark flavors. I also show that the amplitude for a baryon to
emit mesons is , and that meson loops attached to
baryon lines are unsupressed in the large- limit, independent of . For
, there are ambiguities in the extrapolation away from because
the baryon flavor multiplets for a given spin grow with . I argue that the
expansion is valid for baryons with spin and {\it arbitrary}
flavor quantum numbers, including e.g. baryons with isospin and/or strangeness
. This allows the formulation of a large- expansion in which it is not
necessary to identify the physical baryons with particular large- states.
symmetry can be made manifest to all orders in , yet group
theory factors must be evaluated explicitly only for . To
illustrate this expansion, I consider the non-singlet axial currents, baryon
mass splittings, and matrix elements of \mybar ss and \mybar s \gam_\mu
\gam_5 s in the nucleon.Comment: 19 pages, plain TeX, 4 uuencoded postscrip figures, LBL-35539,
NSF-ITP-94-4
New Mechanisms of Dynamical Supersymmetry Breaking and Direct Gauge Mediation
We construct supersymmetric gauge theories with new mechanisms of dynamical
supersymmetry breaking. The models have flat directions at the classical level,
and different mechanisms lift these flat directions in different regions of the
classical moduli space. In one branch of the moduli space, supersymmetry is
broken by confinement in a novel manner. The models contain only dimensionless
couplings and have large groups of unbroken global symmetries, making them
potentially interesting for model-building. As an illustrative application, we
couple the standard model gauge group to a model with an SU(5) global symmetry,
resulting in a model with composite messengers and a non-minimal spectrum of
superpartner masses.Comment: 18 pages, LaTeX2e, no figures. Minor corrections; version to be
published in Phys. Rev.
- …