Multi-Brane Worlds and modification of gravity at large scales

We discuss the implications of multi-brane constructions involving combinations of positive and negative tension brane and show how anomalously light KK states emerge when negative tension ''-'' branes are sandwiched between ''+'' branes. We present a detailed study of a ''+--+'' brane assignment which interpolates between two models that have been previously proposed in which gravity is modified at large scales due to the anomalously light states. We show that it has the peculiar characteristic that gravity changes from four dimensional (4D) to 5D at large distances and returns to 4D at even larger scales. We also consider a crystalline universe which leads to a similar structure for gravity. The problems associated with intermediate negative tension branes are discussed and a possible resolution suggested.Comment: 28 pages, 6 figures,references adde

Brane Universe and Multigravity: Modification of gravity at large and small distances

We consider a modification of gravity at large distances in a Brane Universe which was discussed recently. In these models the modification of gravity at large distances is ultimately connected to existence of negative tension brane(s) and exponentially small tunneling factor. We discuss a general model which interpolates between Bi-gravity model and GRS model. We also discuss the possible mechanism of stabilization for negative tension branes in AdS background. Finally we show that extra degrees of freedom of massive gravitons do not lead to disastrous contradiction with General Relativity if the stabilization condition $\int dy \sqrt{-G^{(5)}} (T^\mu_\mu-2T^5_5)=0$ is implemented.Comment: 12 pages, 4 eps figures, LaTe

AIDS-defining illnesses at initial diagnosis of HIV in a large Guatemalan cohort

AbstractBackgroundAnecdotal evidence suggests that a high proportion of patients diagnosed with HIV in Guatemala present with AIDS. There remain limited data on the epidemiology of AIDS-defining illnesses (ADIs) in Central America.MethodsWe conducted a retrospective cohort study of all patients living with HIV at the largest HIV clinic in Guatemala. Charts were analyzed for clinical and demographic data. Presence of an ADI was assessed by US Centers for Disease Control definitions; patients who presented with an ADI were compared with those without ADI using descriptive statistics.ResultsOf 3686 patients living with HIV, 931 (25.3%) had an ADI at HIV diagnosis, 748 (80.3%) of whom had CD4 counts lower than 200 cells/mm3. Those with ADIs were more likely to be male (67.5% vs 54.6%; P &amp;lt; .0001) and heterosexual (89.4% vs 85.0%; P = .005). The most common ADIs were Mycobacterium tuberculosis (55.0%), Pneumocystis jirovecii pneumonia (13.7%), esophageal candidiasis (13.4%), and histoplasmosis (11.4%). Histoplasmosis and HIV wasting syndrome were both more common among rural patients.ConclusionsIn this large Guatemalan cohort of patients currently living with HIV, a significant portion presented with an ADI. These data inform the most common ADIs diagnosed among survivors, show that histoplasmosis is more commonly diagnosed in rural patients, and suggest that HIV wasting syndrome may reflect missed histoplasmosis diagnoses.</jats:sec

Bulk Fermi surface coexistence with Dirac surface state in Bi2_2Se3_3: a comparison of photoemission and Shubnikov-de Haas measurements

Shubnikov de Haas (SdH) oscillations and Angle Resolved PhotoEmission Spectroscopy (ARPES) are used to probe the Fermi surface of single crystals of Bi2Se3. We find that SdH and ARPES probes quantitatively agree on measurements of the effective mass and bulk band dispersion. In high carrier density samples, the two probes also agree in the exact position of the Fermi level EF, but for lower carrier density samples discrepancies emerge in the position of EF. In particular, SdH reveals a bulk three-dimensional Fermi surface for samples with carrier densities as low as 10^17cm-3. We suggest a simple mechanism to explain these differences and discuss consequences for existing and future transport studies of topological insulators.Comment: 5 mages, 5 figure

The surface-state of the topological insulator Bi2_2Se3_3 revealed by cyclotron resonance

To date transport measurements of topological insulators have been dominated by the conductivity of the bulk, leading to substantial difficulties in resolving the properties of the surface. To this end, we use high magnetic field, rf- and microwave-spectroscopy to selectively couple to the surface conductivity of Bi$_2$Se$_3$ at high frequency. In the frequency range of a few GHz we observe a crossover from quantum oscillations indicative of a small 3D Fermi surface, to cyclotron resonance indicative of a 2D surface state

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Evidence from in vivo 31-phosphorus magnetic resonance spectroscopy phosphodiesters that exhaled ethane is a biomarker of cerebral n-3 polyunsaturated fatty acid peroxidation in humans

<p>Abstract</p> <p>Background</p> <p>This study tested the hypothesis that exhaled ethane is a biomarker of cerebral <it>n</it>-3 polyunsaturated fatty acid peroxidation in humans. Ethane is released specifically following peroxidation of <it>n</it>-3 polyunsaturated fatty acids. We reasoned that the cerebral source of ethane would be the docosahexaenoic acid component of membrane phospholipids. Breakdown of the latter also releases phosphorylated polar head groups, giving rise to glycerophosphorylcholine and glycerophosphorylethanolamine, which can be measured from the 31-phosphorus neurospectroscopy phosphodiester peak. Schizophrenia patients were chosen because of evidence of increased free radical-mediated damage and cerebral lipid peroxidation in this disorder.</p> <p>Methods</p> <p>Samples of alveolar air were obtained from eight patients and ethane was analyzed and quantified by gas chromatography and mass spectrometry (<it>m</it>/<it>z </it>= 30). Cerebral 31-phosphorus spectra were obtained from the same patients at a magnetic field strength of 1.5 T using an image-selected <it>in vivo </it>spectroscopy sequence (TR = 10 s; 64 signal averages localized on a 70 × 70 × 70 mm³voxel). The quantification of the 31-phosphorus signals using prior knowledge was carried out in the temporal domain after truncating the first 1.92 ms of the signal to remove the broad component present in the 31-phosphorus spectra.</p> <p>Results</p> <p>The ethane and phosphodiester levels, expressed as a percentage of the total 31-phosphorus signal, were positively and significantly correlated (<it>r</it>_{<it>s </it>}= 0.714, <it>p </it>< 0.05).</p> <p>Conclusion</p> <p>Our results support the hypothesis that the measurement of exhaled ethane levels indexes cerebral <it>n</it>-3 lipid peroxidation. From a practical viewpoint, if human cerebral <it>n</it>-3 polyunsaturated fatty acid catabolism can be measured by ethane in expired breath, this would be more convenient than determining the area of the 31-phosphorus neurospectroscopy phosphodiester peak.</p

Value-related neuronal responses in the human amygdala during observational learning

The amygdala plays an important role in many aspects of social cognition and reward learning. Here, we aimed to determine whether human amygdala neurons are involved in the computations necessary to implement learning through observation. We performed single-neuron recordings from the amygdalae of human neurosurgical patients (male and female) while they learned about the value of stimuli through observing the outcomes experienced by another agent interacting with those stimuli. We used a detailed computational modeling approach to describe patients' behavior in the task. We found a significant proportion of amygdala neurons whose activity correlated with both expected rewards for oneself and others, and in tracking outcome values received by oneself or other agents. Additionally, a population decoding analysis suggests the presence of information for both observed and experiential outcomes in the amygdala. Encoding and decoding analyses suggested observational value coding in amygdala neurons occurred in a different subset of neurons than experiential value coding. Collectively, these findings support a key role for the human amygdala in the computations underlying the capacity for learning through observation