1,027 research outputs found
The Gluonic Field of a Heavy Quark in Conformal Field Theories at Strong Coupling
We determine the gluonic field configuration sourced by a heavy quark
undergoing arbitrary motion in N=4 super-Yang-Mills at strong coupling and
large number of colors. More specifically, we compute the expectation value of
the operator tr[F^2+...] in the presence of such a quark, by means of the
AdS/CFT correspondence. Our results for this observable show that signals
propagate without temporal broadening, just as was found for the expectation
value of the energy density in recent work by Hatta et al. We attempt to shed
some additional light on the origin of this feature, and propose a different
interpretation for its physical significance. As an application of our general
results, we examine when the quark undergoes oscillatory motion,
uniform circular motion, and uniform acceleration. Via the AdS/CFT
correspondence, all of our results are pertinent to any conformal field theory
in 3+1 dimensions with a dual gravity formulation.Comment: 1+38 pages, 16 eps figures; v2: completed affiliation; v3: corrected
typo, version to appear in JHE
Human hippocampal neurogenesis drops sharply in children to undetectable levels in adults.
New neurons continue to be generated in the subgranular zone of the dentate gyrus of the adult mammalian hippocampus. This process has been linked to learning and memory, stress and exercise, and is thought to be altered in neurological disease. In humans, some studies have suggested that hundreds of new neurons are added to the adult dentate gyrus every day, whereas other studies find many fewer putative new neurons. Despite these discrepancies, it is generally believed that the adult human hippocampus continues to generate new neurons. Here we show that a defined population of progenitor cells does not coalesce in the subgranular zone during human fetal or postnatal development. We also find that the number of proliferating progenitors and young neurons in the dentate gyrus declines sharply during the first year of life and only a few isolated young neurons are observed by 7 and 13 years of age. In adult patients with epilepsy and healthy adults (18-77 years; n = 17 post-mortem samples from controls; n = 12 surgical resection samples from patients with epilepsy), young neurons were not detected in the dentate gyrus. In the monkey (Macaca mulatta) hippocampus, proliferation of neurons in the subgranular zone was found in early postnatal life, but this diminished during juvenile development as neurogenesis decreased. We conclude that recruitment of young neurons to the primate hippocampus decreases rapidly during the first years of life, and that neurogenesis in the dentate gyrus does not continue, or is extremely rare, in adult humans. The early decline in hippocampal neurogenesis raises questions about how the function of the dentate gyrus differs between humans and other species in which adult hippocampal neurogenesis is preserved
Early-Time Energy Loss in a Strongly-Coupled SYM Plasma
We carry out an analytic study of the early-time motion of a quark in a
strongly-coupled maximally-supersymmetric Yang-Mills plasma, using the AdS/CFT
correspondence. Our approach extracts the first thermal effects as a small
perturbation of the known quark dynamics in vacuum, using a double expansion
that is valid for early times and for (moderately) ultrarelativistic quark
velocities. The quark is found to lose energy at a rate that differs
significantly from the previously derived stationary/late-time result: it
scales like T^4 instead of T^2, and is associated with a friction coefficient
that is not independent of the quark momentum. Under conditions representative
of the quark-gluon plasma as obtained at RHIC, the early energy loss rate is a
few times smaller than its late-time counterpart. Our analysis additionally
leads to thermally-corrected expressions for the intrinsic energy and momentum
of the quark, in which the previously discovered limiting velocity of the quark
is found to appear naturally.Comment: 39 pages, no figures. v2: Minor corrections and clarifications.
References added. Version to be published in JHE
Multifrequency Strategies for the Identification of Gamma-Ray Sources
More than half the sources in the Third EGRET (3EG) catalog have no firmly
established counterparts at other wavelengths and are unidentified. Some of
these unidentified sources have remained a mystery since the first surveys of
the gamma-ray sky with the COS-B satellite. The unidentified sources generally
have large error circles, and finding counterparts has often been a challenging
job. A multiwavelength approach, using X-ray, optical, and radio data, is often
needed to understand the nature of these sources. This chapter reviews the
technique of identification of EGRET sources using multiwavelength studies of
the gamma-ray fields.Comment: 35 pages, 22 figures. Chapter prepared for the book "Cosmic Gamma-ray
Sources", edited by K.S. Cheng and G.E. Romero, to be published by Kluwer
Academic Press, 2004. For complete article and higher resolution figures, go
to: http://www.astro.columbia.edu/~muk/mukherjee_multiwave.pd
Quantum Fluctuations and the Unruh Effect in Strongly-Coupled Conformal Field Theories
Through the AdS/CFT correspondence, we study a uniformly accelerated quark in
the vacuum of strongly-coupled conformal field theories in various dimensions,
and determine the resulting stochastic fluctuations of the quark trajectory.
From the perspective of an inertial observer, these are quantum fluctuations
induced by the gluonic radiation emitted by the accelerated quark. From the
point of view of the quark itself, they originate from the thermal medium
predicted by the Unruh effect. We scrutinize the relation between these two
descriptions in the gravity side of the correspondence, and show in particular
that upon transforming the conformal field theory from Rindler space to the
open Einstein universe, the acceleration horizon disappears from the boundary
theory but is preserved in the bulk. This transformation allows us to directly
connect our calculation of radiation-induced fluctuations in vacuum with the
analysis by de Boer et al. of the Brownian motion of a quark that is on average
static within a thermal medium. Combining this same bulk transformation with
previous results of Emparan, we are also able to compute the stress-energy
tensor of the Unruh thermal medium.Comment: 1+31 pages; v2: reference adde
Turbulence and galactic structure
Interstellar turbulence is driven over a wide range of scales by processes
including spiral arm instabilities and supernovae, and it affects the rate and
morphology of star formation, energy dissipation, and angular momentum transfer
in galaxy disks. Star formation is initiated on large scales by gravitational
instabilities which control the overall rate through the long dynamical time
corresponding to the average ISM density. Stars form at much higher densities
than average, however, and at much faster rates locally, so the slow average
rate arises because the fraction of the gas mass that forms stars at any one
time is low, ~10^{-4}. This low fraction is determined by turbulence
compression, and is apparently independent of specific cloud formation
processes which all operate at lower densities. Turbulence compression also
accounts for the formation of most stars in clusters, along with the cluster
mass spectrum, and it gives a hierarchical distribution to the positions of
these clusters and to star-forming regions in general. Turbulent motions appear
to be very fast in irregular galaxies at high redshift, possibly having speeds
equal to several tenths of the rotation speed in view of the morphology of
chain galaxies and their face-on counterparts. The origin of this turbulence is
not evident, but some of it could come from accretion onto the disk. Such high
turbulence could help drive an early epoch of gas inflow through viscous
torques in galaxies where spiral arms and bars are weak. Such evolution may
lead to bulge or bar formation, or to bar re-formation if a previous bar
dissolved. We show evidence that the bar fraction is about constant with
redshift out to z~1, and model the formation and destruction rates of bars
required to achieve this constancy.Comment: in: Penetrating Bars through Masks of Cosmic Dust: The Hubble Tuning
Fork strikes a New Note, Eds., K. Freeman, D. Block, I. Puerari, R. Groess,
Dordrecht: Kluwer, in press (presented at a conference in South Africa, June
7-12, 2004). 19 pgs, 5 figure
FibroScan-AST (FAST) score for the non-invasive identification of patients with non-alcoholic steatohepatitis with significant activity and fibrosis: a prospective derivation and global validation study
BACKGROUND
The burden of non-alcoholic fatty liver disease (NAFLD) is increasing globally, and a major priority is to identify patients with non-alcoholic steatohepatitis (NASH) who are at greater risk of progression to cirrhosis, and who will be candidates for clinical trials and emerging new pharmacotherapies. We aimed to develop a score to identify patients with NASH, elevated NAFLD activity score (NAS≥4), and advanced fibrosis (stage 2 or higher [F≥2]).
METHODS
This prospective study included a derivation cohort before validation in multiple international cohorts. The derivation cohort was a cross-sectional, multicentre study of patients aged 18 years or older, scheduled to have a liver biopsy for suspicion of NAFLD at seven tertiary care liver centres in England. This was a prespecified secondary outcome of a study for which the primary endpoints have already been reported. Liver stiffness measurement (LSM) by vibration-controlled transient elastography and controlled attenuation parameter (CAP) measured by FibroScan device were combined with aspartate aminotransferase (AST), alanine aminotransferase (ALT), or AST:ALT ratio. To identify those patients with NASH, an elevated NAS, and significant fibrosis, the best fitting multivariable logistic regression model was identified and internally validated using boot-strapping. Score calibration and discrimination performance were determined in both the derivation dataset in England, and seven independent international (France, USA, China, Malaysia, Turkey) histologically confirmed cohorts of patients with NAFLD (external validation cohorts). This study is registered with ClinicalTrials.gov, number NCT01985009.
FINDINGS
Between March 20, 2014, and Jan 17, 2017, 350 patients with suspected NAFLD attending liver clinics in England were prospectively enrolled in the derivation cohort. The most predictive model combined LSM, CAP, and AST, and was designated FAST (FibroScan-AST). Performance was satisfactory in the derivation dataset (C-statistic 0·80, 95% CI 0·76–0·85) and was well calibrated. In external validation cohorts, calibration of the score was satisfactory and discrimination was good across the full range of validation cohorts (C-statistic range 0·74–0·95, 0·85; 95% CI 0·83–0·87 in the pooled external validation patients' cohort; n=1026). Cutoff was 0·35 for sensitivity of 0·90 or greater and 0·67 for specificity of 0·90 or greater in the derivation cohort, leading to a positive predictive value (PPV) of 0·83 (84/101) and a negative predictive value (NPV) of 0·85 (93/110). In the external validation cohorts, PPV ranged from 0·33 to 0·81 and NPV from 0·73 to 1·0.
INTERPRETATION
The FAST score provides an efficient way to non-invasively identify patients at risk of progressive NASH for clinical trials or treatments when they become available, and thereby reduce unnecessary liver biopsy in patients unlikely to have significant disease
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Projected WIMP sensitivity of the LUX-ZEPLIN dark matter experiment
LUX-ZEPLIN (LZ) is a next-generation dark matter direct detection experiment that will operate 4850 feet underground at the Sanford Underground Research Facility (SURF) in Lead, South Dakota, USA. Using a two-phase xenon detector with an active mass of 7 tonnes, LZ will search primarily for low-energy interactions with weakly interacting massive particles (WIMPs), which are hypothesized to make up the dark matter in our galactic halo. In this paper, the projected WIMP sensitivity of LZ is presented based on the latest background estimates and simulations of the detector. For a 1000 live day run using a 5.6-tonne fiducial mass, LZ is projected to exclude at 90% confidence level spin-independent WIMP-nucleon cross sections above 1.4×10-48 cm2 for a 40 GeV/c2 mass WIMP. Additionally, a 5σ discovery potential is projected, reaching cross sections below the exclusion limits of recent experiments. For spin-dependent WIMP-neutron(-proton) scattering, a sensitivity of 2.3×10-43 cm2 (7.1×10-42 cm2) for a 40 GeV/c2 mass WIMP is expected. With underground installation well underway, LZ is on track for commissioning at SURF in 2020
Utility of CD4 cell counts for early prediction of virological failure during antiretroviral therapy in a resource-limited setting
BACKGROUND: Viral load monitoring is not available for the vast majority of patients receiving antiretroviral therapy in resource-limited settings. However, the practical utility of CD4 cell count measurements as an alternative monitoring strategy has not been rigorously assessed. METHODS: In this study, we used a novel modelling approach that accounted for all CD4 cell count and VL values measured during follow-up from the first date that VL suppression was achieved. We determined the associations between CD4 counts (absolute values and changes during ART), VL measurements and risk of virological failure (VL > 1,000 copies/ml) following initial VL suppression in 330 patients in South Africa. CD4 count changes were modelled both as the difference from baseline (DeltaCD4 count) and the difference between consecutive values (CD4 count slope) using all 3-monthly CD4 count measurements during follow-up. RESULTS: During 7093.2 patient-months of observation 3756 paired CD4 count and VL measurements were made. In patients who developed virological failure (n = 179), VL correlated significantly with absolute CD4 counts (r = - 0.08, P = 0.003), DeltaCD4 counts (r = - 0.11, P < 0.01), and most strongly with CD4 count slopes (r = - 0.30, P < 0.001). However, the distributions of the absolute CD4 counts, DeltaCD4 counts and CD4 count slopes at the time of virological failure did not differ significantly from the corresponding distributions in those without virological failure (P = 0.99, P = 0.92 and P = 0.75, respectively). Moreover, in a receiver operating characteristic (ROC) curve, the association between a negative CD4 count slope and virological failure was poor (area under the curve = 0.59; sensitivity = 53.0%; specificity = 63.6%; positive predictive value = 10.9%). CONCLUSION: CD4 count changes correlated significantly with VL at group level but had very limited utility in identifying virological failure in individual patients. CD4 count is an inadequate alternative to VL measurement for early detection of virological failure
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