Repulsively bound atom pairs in an optical lattice

Throughout physics, stable composite objects are usually formed via attractive forces, which allow the constituents to lower their energy by binding together. Repulsive forces separate particles in free space. However, in a structured environment such as a periodic potential and in the absence of dissipation, stable composite objects can exist even for repulsive interactions. Here we report on the first observation of such an exotic bound state, comprised of a pair of ultracold atoms in an optical lattice. Consistent with our theoretical analysis, these repulsively bound pairs exhibit long lifetimes, even under collisions with one another. Signatures of the pairs are also recognised in the characteristic momentum distribution and through spectroscopic measurements. There is no analogue in traditional condensed matter systems of such repulsively bound pairs, due to the presence of strong decay channels. These results exemplify on a new level the strong correspondence between the optical lattice physics of ultracold bosonic atoms and the Bose-Hubbard model, a correspondence which is vital for future applications of these systems to the study of strongly correlated condensed matter systems and to quantum information.Comment: 5 pages, 4 figure

Very Cold Gas and Dark Matter

We have recently proposed a new candidate for baryonic dark matter: very cold molecular gas, in near-isothermal equilibrium with the cosmic background radiation at 2.73 K. The cold gas, of quasi-primordial abundances, is condensed in a fractal structure, resembling the hierarchical structure of the detected interstellar medium. We present some perspectives of detecting this very cold gas, either directly or indirectly. The H$_2$ molecule has an "ultrafine" structure, due to the interaction between the rotation-induced magnetic moment and the nuclear spins. But the lines fall in the km domain, and are very weak. The best opportunity might be the UV absorption of H$_2$ in front of quasars. The unexpected cold dust component, revealed by the COBE/FIRAS submillimetric results, could also be due to this very cold H$_2$ gas, through collision-induced radiation, or solid H$_2$ grains or snowflakes. The $\gamma$-ray distribution, much more radially extended than the supernovae at the origin of cosmic rays acceleration, also points towards and extended gas distribution.Comment: 16 pages, Latex pages, crckapb macro, 3 postscript figures, uuencoded compressed tar file. To be published in the proceeedings of the "Dust-Morphology" conference, Johannesburg, 22-26 January, 1996, D. Block (ed.), (Kluwer Dordrecht

The phase diagram of Yang-Mills theory with a compact extra dimension

We present a non-perturbative study of the phase diagram of SU(2) Yang-Mills theory in a five-dimensional spacetime with a compact extra dimension. The non-renormalizable theory is regularized on an anisotropic lattice and investigated through numerical simulations in a regime characterized by a hierarchy between the scale of low-energy physics, the inverse compactification radius, and the cutoff scale. We map out the structure of the phase diagram and the pattern of lines corresponding to fixed values of the ratio between the mass of the fifth component of the gauge field and the non-perturbative mass gap of the four-dimensional modes. We discuss different limits of the model, and comment on the implications of our findings.Comment: 17 pages, 9 figure

The kinases MSK1 and MSK2 act as negative regulators of Toll-like receptor signaling

The kinases MSK1 and MSK2 are activated 'downstream' of the p38 and Erk1/2 mitogen-activated protein kinases. Here we found that MSK1 and MSK2 were needed to limit the production of proinflammatory cytokines in response to stimulation of primary macrophages with lipopolysaccharide. By inducing transcription of the mitogen-activated protein kinase phosphatase DUSP1 and the anti-inflammatory cytokine interleukin 10, MSK1 and MSK2 exerted many negative feedback mechanisms. Deficiency in MSK1 and MSK2 prevented the binding of phosphorylated transcription factors CREB and ATF1 to the promoters of the genes encoding interleukin 10 and DUSP1. Mice doubly deficient in MSK1 and MSK2 were hypersensitive to lipopolysaccharide-induced endotoxic shock and showed prolonged inflammation in a model of toxic contact eczema induced by phorbol 12-myristate 13-acetate. Our results establish MSK1 and MSK2 as key components of negative feedback mechanisms needed to limit Toll-like receptor-driven inflammation.</p

Design principles for riboswitch function

Scientific and technological advances that enable the tuning of integrated regulatory components to match network and system requirements are critical to reliably control the function of biological systems. RNA provides a promising building block for the construction of tunable regulatory components based on its rich regulatory capacity and our current understanding of the sequence–function relationship. One prominent example of RNA-based regulatory components is riboswitches, genetic elements that mediate ligand control of gene expression through diverse regulatory mechanisms. While characterization of natural and synthetic riboswitches has revealed that riboswitch function can be modulated through sequence alteration, no quantitative frameworks exist to investigate or guide riboswitch tuning. Here, we combined mathematical modeling and experimental approaches to investigate the relationship between riboswitch function and performance. Model results demonstrated that the competition between reversible and irreversible rate constants dictates performance for different regulatory mechanisms. We also found that practical system restrictions, such as an upper limit on ligand concentration, can significantly alter the requirements for riboswitch performance, necessitating alternative tuning strategies. Previous experimental data for natural and synthetic riboswitches as well as experiments conducted in this work support model predictions. From our results, we developed a set of general design principles for synthetic riboswitches. Our results also provide a foundation from which to investigate how natural riboswitches are tuned to meet systems-level regulatory demands

The Mechanisms of Codon Reassignments in Mitochondrial Genetic Codes

Many cases of non-standard genetic codes are known in mitochondrial genomes. We carry out analysis of phylogeny and codon usage of organisms for which the complete mitochondrial genome is available, and we determine the most likely mechanism for codon reassignment in each case. Reassignment events can be classified according to the gain-loss framework. The gain represents the appearance of a new tRNA for the reassigned codon or the change of an existing tRNA such that it gains the ability to pair with the codon. The loss represents the deletion of a tRNA or the change in a tRNA so that it no longer translates the codon. One possible mechanism is Codon Disappearance, where the codon disappears from the genome prior to the gain and loss events. In the alternative mechanisms the codon does not disappear. In the Unassigned Codon mechanism, the loss occurs first, whereas in the Ambiguous Intermediate mechanism, the gain occurs first. Codon usage analysis gives clear evidence of cases where the codon disappeared at the point of the reassignment and also cases where it did not disappear. Codon disappearance is the probable explanation for stop to sense reassignments and a small number of reassignments of sense codons. However, the majority of sense to sense reassignments cannot be explained by codon disappearance. In the latter cases, by analysis of the presence or absence of tRNAs in the genome and of the changes in tRNA sequences, it is sometimes possible to distinguish between the Unassigned Codon and Ambiguous Intermediate mechanisms. We emphasize that not all reassignments follow the same scenario and that it is necessary to consider the details of each case carefully.Comment: 53 pages (45 pages, including 4 figures + 8 pages of supplementary information). To appear in J.Mol.Evo

Uso de fármacos psicoestimulantes en drogodependencias

El uso de medicamentos estimulantes es una cuestión de plena actualidad en psiquiatría, aunque su utilización y prescripción es controvertida . Fármacos como el metilfenidato, las anfetaminas, o el modafinilo están siendo utilizados y estudiados en distintas enfermedades psiquiátricas como el trastorno por déficit de atención e hiperactividad (TDAH), la dependencia de cocaína, en trastornos del sueño y en la depresión resistente. Todos estos fármacos tienen en común, igual que las drogas de abuso, que son medicamentos que actúan sobre el sistema dopaminérgico, que constituye la base neurobiológica del refuerzo fisiológico. Los estimulantes como el metilfenidato o el modafinilo son fármacos eficaces en el TDAH y han sido estudiados en el tratamiento de la dependencia de cocaína. En niños con TDAH el metilfenidato es un factor protector para el desarrollo de fármaco en la dependencia de cocaína, aunque son estudios preliminares, por lo que no se debe considerar que este totalmente demostrado que los fármacos psicoestimulantes sean eficaces en el tratamiento de esta dependencia. Aunque no son conocidos todos los mecanismos fisiopatológicos, parece crítico que el refuerzo, y por lo tanto el riesgo de dependencia, aparece cuando se producen incrementos rápidos dopaminérgicos y que los efectos terapéuticos aparecen cuando son lentos y mantenidos. Las características de uso a dosis bajas administradas por vía oral disminuyen el riesgo de abuso. Para realizar una adecuada prescripción es necesario aclarar, definitivamente, los mecanismos neuroquímicos en los que intervienen, y sus indicaciones en drogodependenciasStimulant drugs prescription is a controversial and current topic in psychiatry. Drugs such as methylphenidate, amphetamine compounds and modafinil have been trialed and used in attention deficit hyperactivity disorder (ADHD), sleep conditions, cocaine dependence and as an adjunct to antidepressants for depression. All these drugs, like stimulant drugs abuse, increase extracellular dopamine in the brain.This effect is associated with reinforcing as well as therapeutic effects. Methylphenidate and modafinil treatment of ADHD are associated with a reduced risk for later substance abuse among ADHD patients. There is evidence of the beneficial effects of the use of modafinil in cocaine dependence, altough there isn't conclusive evidence for the stimulants' efficacy in treatment of the stimulants' dependence. At this time, the physiopathology of drug abuse and dependence is unknown, but it's known that the very critical point is that the reinforcing effects are associated with rapid changes in dopamine increases, whereas the therapeutic effects are associated with slowly and smoothly rising dopamine levels, such as are achieved with low doses and oral administration. Due to this, it's necessary to study the neurobiological bases on which stimulants drugs are related, and their clinical use in dependence treatment

Real-time intermembrane force measurements and imaging of lipid domain morphology during hemifusion

Membrane fusion is the core process in membrane trafficking and is essential for cellular transport of proteins and other biomacromolecules. During protein-mediated membrane fusion, membrane proteins are often excluded from the membrane-membrane contact, indicating that local structural transformations in lipid domains play a major role. However, the rearrangements of lipid domains during fusion have not been thoroughly examined. Here using a newly developed Fluorescence Surface Forces Apparatus (FL-SFA), migration of liquid-disordered clusters and depletion of liquid-ordered domains at the membrane-membrane contact are imaged in real time during hemifusion of model lipid membranes, together with simultaneous force-distance and lipid membrane thickness measurements. The load and contact time-dependent hemifusion results show that the domain rearrangements decrease the energy barrier to fusion, illustrating the significance of dynamic domain transformations in membrane fusion processes. Importantly, the FL-SFA can unambiguously correlate interaction forces and in situ imaging in many dynamic interfacial systems.open0

Study of the reaction e^{+}e^{-} -->J/psi\pi^{+}\pi^{-} via initial-state radiation at BaBar

We study the process $e^+e^-\to J/\psi\pi^{+}\pi^{-}$ with initial-state-radiation events produced at the PEP-II asymmetric-energy collider. The data were recorded with the BaBar detector at center-of-mass energies 10.58 and 10.54 GeV, and correspond to an integrated luminosity of 454 $\mathrm{fb^{-1}}$. We investigate the $J/\psi \pi^{+}\pi^{-}$ mass distribution in the region from 3.5 to 5.5 $\mathrm{GeV/c^{2}}$. Below 3.7 $\mathrm{GeV/c^{2}}$ the $\psi(2S)$ signal dominates, and above 4 $\mathrm{GeV/c^{2}}$ there is a significant peak due to the Y(4260). A fit to the data in the range 3.74 -- 5.50 $\mathrm{GeV/c^{2}}$ yields a mass value $4244 \pm 5$ (stat) $\pm 4$ (syst)$\mathrm{MeV/c^{2}}$ and a width value $114 ^{+16}_{-15}$ (stat)$\pm 7$(syst)$\mathrm{MeV}$ for this state. We do not confirm the report from the Belle collaboration of a broad structure at 4.01 $\mathrm{GeV/c^{2}}$. In addition, we investigate the $\pi^{+}\pi^{-}$ system which results from Y(4260) decay

Observation of associated near-side and away-side long-range correlations in √sNN=5.02 TeV proton-lead collisions with the ATLAS detector

Two-particle correlations in relative azimuthal angle (Δϕ) and pseudorapidity (Δη) are measured in √sNN=5.02  TeV p+Pb collisions using the ATLAS detector at the LHC. The measurements are performed using approximately 1  μb-1 of data as a function of transverse momentum (pT) and the transverse energy (ΣETPb) summed over 3.1<η<4.9 in the direction of the Pb beam. The correlation function, constructed from charged particles, exhibits a long-range (2<|Δη|<5) “near-side” (Δϕ∼0) correlation that grows rapidly with increasing ΣETPb. A long-range “away-side” (Δϕ∼π) correlation, obtained by subtracting the expected contributions from recoiling dijets and other sources estimated using events with small ΣETPb, is found to match the near-side correlation in magnitude, shape (in Δη and Δϕ) and ΣETPb dependence. The resultant Δϕ correlation is approximately symmetric about π/2, and is consistent with a dominant cos⁡2Δϕ modulation for all ΣETPb ranges and particle pT