49 research outputs found
Goldstini
Supersymmetric phenomenology has been largely bound to the hypothesis that
supersymmetry breaking originates from a single source. In this paper, we relax
this underlying assumption and consider a multiplicity of sectors which
independently break supersymmetry, thus yielding a corresponding multiplicity
of goldstini. While one linear combination of goldstini is eaten via the
super-Higgs mechanism, the orthogonal combinations remain in the spectrum as
physical degrees of freedom. Interestingly, supergravity effects induce a
universal tree-level mass for the goldstini which is exactly twice the
gravitino mass. Since visible sector fields can couple dominantly to the
goldstini rather than the gravitino, this framework allows for substantial
departures from conventional supersymmetric phenomenology. In fact, this even
occurs when a conventional mediation scheme is augmented by additional
supersymmetry breaking sectors which are fully sequestered. We discuss a number
of striking collider signatures, including various novel decay modes for the
lightest observable-sector supersymmetric particle, gravitinoless
gauge-mediated spectra, and events with multiple displaced vertices. We also
describe goldstini cosmology and the possibility of goldstini dark matter.Comment: 14 pages, 7 figures; references adde
Thermal quenches in N=2* plasmas
We exploit gauge/gravity duality to study `thermal quenches' in a plasma of
the strongly coupled N=2* gauge theory. Specifically, we consider the response
of an initial thermal equilibrium state of the theory under variations of the
bosonic or fermionic mass, to leading order in m/T<<1. When the masses are made
to vary in time, novel new counterterms must be introduced to renormalize the
boundary theory. We consider transitions the conformal super-Yang-Mills theory
to the mass deformed gauge theory and also the reverse transitions. By
construction, these transitions are controlled by a characteristic time scale
\calt and we show how the response of the system depends on the ratio of this
time scale to the thermal time scale 1/T. The response shows interesting
scaling behaviour both in the limit of fast quenches with T\calt<<1 and slow
quenches with T\calt>>1. In the limit that T\calt\to\infty, we observe the
expected adiabatic response. For fast quenches, the relaxation to the final
equilibrium is controlled by the lowest quasinormal mode of the bulk scalar
dual to the quenched operator. For slow quenches, the system relaxes with a
(nearly) adiabatic response that is governed entirely by the late time profile
of the mass. We describe new renormalization scheme ambiguities in defining
gauge invariant observables for the theory with time dependant couplings.Comment: 78 pages, 17 figure
Brane-World Gravity
The observable universe could be a 1+3-surface (the "brane") embedded in a
1+3+\textit{d}-dimensional spacetime (the "bulk"), with Standard Model
particles and fields trapped on the brane while gravity is free to access the
bulk. At least one of the \textit{d} extra spatial dimensions could be very
large relative to the Planck scale, which lowers the fundamental gravity scale,
possibly even down to the electroweak ( TeV) level. This revolutionary
picture arises in the framework of recent developments in M theory. The
1+10-dimensional M theory encompasses the known 1+9-dimensional superstring
theories, and is widely considered to be a promising potential route to quantum
gravity. At low energies, gravity is localized at the brane and general
relativity is recovered, but at high energies gravity "leaks" into the bulk,
behaving in a truly higher-dimensional way. This introduces significant changes
to gravitational dynamics and perturbations, with interesting and potentially
testable implications for high-energy astrophysics, black holes, and cosmology.
Brane-world models offer a phenomenological way to test some of the novel
predictions and corrections to general relativity that are implied by M theory.
This review analyzes the geometry, dynamics and perturbations of simple
brane-world models for cosmology and astrophysics, mainly focusing on warped
5-dimensional brane-worlds based on the Randall--Sundrum models. We also cover
the simplest brane-world models in which 4-dimensional gravity on the brane is
modified at \emph{low} energies -- the 5-dimensional Dvali--Gabadadze--Porrati
models. Then we discuss co-dimension two branes in 6-dimensional models.Comment: A major update of Living Reviews in Relativity 7:7 (2004)
"Brane-World Gravity", 119 pages, 28 figures, the update contains new
material on RS perturbations, including full numerical solutions of
gravitational waves and scalar perturbations, on DGP models, and also on 6D
models. A published version in Living Reviews in Relativit
The origin of multicellularity in cyanobacteria
Background: Cyanobacteria are one of the oldest and morphologically most diverse prokaryotic phyla on our planet. The early development of an oxygen-containing atmosphere approximately 2.45 - 2.22 billion years ago is attributed to the photosynthetic activity of cyanobacteria. Furthermore, they are one of the few prokaryotic phyla where multicellularity has evolved. Understanding when and how multicellularity evolved in these ancient organisms would provide fundamental information on the early history of life and further our knowledge of complex life forms.
Results: We conducted and compared phylogenetic analyses of 16S rDNA sequences from a large sample of taxa representing the morphological and genetic diversity of cyanobacteria. We reconstructed ancestral character states on 10,000 phylogenetic trees. The results suggest that the majority of extant cyanobacteria descend from multicellular ancestors. Reversals to unicellularity occurred at least 5 times. Multicellularity was established again at least once within a single-celled clade. Comparison to the fossil record supports an early origin of multicellularity, possibly as early as the “Great Oxygenation Event” that occurred 2.45 - 2.22 billion years ago.
Conclusions: The results indicate that a multicellular morphotype evolved early in the cyanobacterial lineage and was regained at least once after a previous loss. Most of the morphological diversity exhibited in cyanobacteria today —including the majority of single-celled species— arose from ancient multicellular lineages. Multicellularity could have conferred a considerable advantage for exploring new niches and hence facilitated the diversification of new lineages