9,904 research outputs found
Photon locking
A novel observation of photon lockingâthe optical analog of spin lockingâis reported, demonstrating the applicability of phase-coherent pulse sequences. The experiments are reported for the optical transition of iodine gas at 589.7 nm using the pulse sequence XYX-XYXÌ. Locking decay rates are presented as a function of pressure and compared with optical dephasing (echo-decay) rates
Sinorhizobium Meliloti, A Bacterium Lacking The Autoinducer-2 (AI-2) Synthase, Responds To AI-2 Supplied By Other Bacteria
Many bacterial species respond to the quorum-sensing signal autoinducer-2 (AI-2) by regulating different niche-specific genes. Here, we show that Sinorhizobium meliloti, a plant symbiont lacking the gene for the AI-2 synthase, while not capable of producing AI-2 can nonetheless respond to AI-2 produced by other species. We demonstrate that S. meliloti has a periplasmic binding protein that binds AI-2. The crystal structure of this protein (here named SmlsrB) with its ligand reveals that it binds (2R,4S)-2-methyl-2,3,3,4-tetrahydroxytetrahydrofuran (R-THMF), the identical AI-2 isomer recognized by LsrB of Salmonella typhimurium. The gene encoding SmlsrB is in an operon with orthologues of the lsr genes required for AI-2 internalization in enteric bacteria. Accordingly, S. meliloti internalizes exogenous AI-2, and mutants in this operon are defective in AI-2 internalization. S. meliloti does not gain a metabolic benefit from internalizing AI-2, suggesting that AI-2 functions as a signal in S. meliloti. Furthermore, S. meliloti can completely eliminate the AI-2 secreted by Erwinia carotovora, a plant pathogen shown to use AI-2 to regulate virulence. Our findings suggest that S. meliloti is capable of \u27eavesdropping\u27 on the AI-2 signalling of other species and interfering with AI-2-regulated behaviours such as virulence
Grand unification through gravitational effects
We systematically study the unification of gauge couplings in the presence of
(one or more) effective dimension-5 operators cHGG/4MPl, induced into the grand
unified theory by gravitational interactions at the Planck scale MPl. These
operators alter the usual condition for gauge coupling unification, which can,
depending on the Higgs content H and vacuum expectation value, result in
unification at scales MX significantly different than naively expected. We find
non-supersymmetric models of SU(5) and SO(10) unification, with natural Wilson
coefficients c, that easily satisfy the constraints from proton decay.
Furthermore, gauge coupling unification at scales as high as the Planck scale
seems feasible, possibly hinting at simultaneous unification of gauge and
gravitational interactions. In the Appendix we work out the group theoretical
aspects of this scenario for SU(5) and SO(10) unified groups in detail; this
material is also relevant in the analysis of non-universal gaugino masses
obtained from supergravity.Comment: 27 pages, 5 figures, 8 tables, 1 appendix, revtex; v2: introduction
and conclusion expanded, references added, minor changes, version published
in PR
Relativistic cosmological perturbation scheme on a general background: scalar perturbations for irrotational dust
In standard perturbation approaches and N-body simulations, inhomogeneities
are described to evolve on a predefined background cosmology, commonly taken as
the homogeneous-isotropic solutions of Einstein's field equations
(Friedmann-Lema\^itre-Robertson-Walker (FLRW) cosmologies). In order to make
physical sense, this background cosmology must provide a reasonable description
of the effective, i.e. spatially averaged, evolution of structure
inhomogeneities also in the nonlinear regime. Guided by the insights that (i)
the average over an inhomogeneous distribution of matter and geometry is in
general not given by a homogeneous solution of general relativity, and that
(ii) the class of FLRW cosmologies is not only locally but also globally
gravitationally unstable in relevant cases, we here develop a perturbation
approach that describes the evolution of inhomogeneities on a general
background being defined by the spatially averaged evolution equations. This
physical background interacts with the formation of structures. We derive and
discuss the resulting perturbation scheme for the matter model `irrotational
dust' in the Lagrangian picture, restricting our attention to scalar
perturbations.Comment: 18 pages. Matches published version in CQ
Global gravitational instability of FLRW backgrounds - interpreting the dark sectors
The standard model of cosmology is based on homogeneous-isotropic solutions
of Einstein's equations. These solutions are known to be gravitationally
unstable to local inhomogeneous perturbations, commonly described as evolving
on a background given by the same solutions. In this picture, the FLRW
backgrounds are taken to describe the average over inhomogeneous perturbations
for all times. We study in the present article the (in)stability of FLRW dust
backgrounds within a class of averaged inhomogeneous cosmologies. We examine
the phase portraits of the latter, discuss their fixed points and orbital
structure and provide detailed illustrations. We show that FLRW cosmologies are
unstable in some relevant cases: averaged models are driven away from them
through structure formation and accelerated expansion. We find support for the
proposal that the dark components of the FLRW framework may be associated to
these instability sectors. Our conclusion is that FLRW cosmologies have to be
considered critically as for their role to serve as reliable models for the
physical background.Comment: 15 pages, 13 figures, 1 table. Matches published version in CQ
Do metals exist in two dimensions? A study of many-body localisation in low density electron gas
Using a combination of ground state quantum Monte-Carlo and finite size
scaling techniques, we perform a systematic study of the effect of Coulomb
interaction on the localisation length of a disordered two-dimensional electron
gas. We find that correlations delocalise the 2D system. In the absence of
valley degeneracy (as in GaAs heterostructures), this delocalization effect
corresponds to a finite increase of the localization length. The delocalisation
is much more dramatic in the presence of valley degeneracy (as in Si MOSFETSs)
where the localization length increases drastically. Our results suggest that a
rather simple mechanism can account for the main features of the metallic
behaviour observed in high mobility Si MOSFETs. Our findings support the claim
that this behaviour is indeed a genuine effect of the presence of
electron-electron interactions, yet that the system is not a ``true'' metal in
the thermodynamic sense.Comment: 5 pages 4 figure
A review of Quantum Gravity at the Large Hadron Collider
The aim of this article is to review the recent developments in the
phenomenology of quantum gravity at the Large Hadron Collider. We shall pay
special attention to four-dimensional models which are able to lower the
reduced Planck mass to the TeV region and compare them to models with a large
extra-dimensional volume. We then turn our attention to reviewing the emission
of gravitons (massless or massive) at the LHC and to the production of small
quantum black holes.Comment: 32 pages, invited revie
Genetic Rat Models of Parkinson's Disease
Parkinson's disease (PD) is a neurodegenerative disease characterized by a specific loss of dopaminergic neurons. Although the vast majority of PD cases are idiopathic in nature, there is a subset that contains genetic links. Of the genes that have been linked to PD, α-synuclein and leucine-rich repeat kinase 2 have been used to develop transgenic rat models of the disease. In this paper we focused on the various transgenic rat models of PD in terms of their ability to mimic key symptoms of PD in a progressive manner. In general, we found that most of these models provided useful tools for the early stages of PD, but the development of new transgenic rats that present significant neuropathologic and motoric deficits in a progressive manner that more accurately mimics PD is needed
One-particle inclusive CP asymmetries
One-particle inclusive CP asymmetries in the decays of the type B -> D(*) X
are considered in the framework of a QCD based method to calculate the rates
for one-particle inclusive decays.Comment: Latex, 13 pages, 6 figures (eps). Analytical and numerical results
unchanged, extended discussion of model assumptions and systematic
uncertainties. Version to be published in Phys. Rev. D 62, 0960xx. Additional
transparencies are available via the WWW at
http://www-ttp.physik.uni-karlsruhe.de/Slides
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