492 research outputs found
Regge behavior saves string theory from causality violations
Higher-derivative corrections to the Einstein-Hilbert action are present in bosonic string theory leading to the potential causality violations recently pointed out by Camanho et al. [1]. We analyze in detail this question by considering high-energy string-brane collisions at impact parameters b ≤ l s (the string-length parameter) with l s ≫ R p (the characteristic scale of the D p -brane geometry). If we keep only the contribution of the massless states causality is violated for a set of initial states whose polarization is suitably chosen with respect to the impact parameter vector. Such violations are instead neatly avoided when the full structure of string theory — and in particular its Regge behavior — is taken into account
Phases of planar 5-dimensional supersymmetric Chern-Simons theory
In this paper we investigate the large- behavior of 5-dimensional
super Yang-Mills with a level Chern-Simons term and an
adjoint hypermultiplet. As in three-dimensional Chern-Simons theories, one must
choose an integration contour to completely define the theory. Using
localization, we reduce the path integral to a matrix model with a cubic action
and compute its free energy in various scenarios. In the limit of infinite
Yang-Mills coupling and for particular choices of the contours, we find that
the free-energy scales as for gauge groups with large values
of the Chern-Simons 't\,Hooft coupling, . If we also
set the hypermultiplet mass to zero, then this limit is a superconformal fixed
point and the behavior parallels other fixed points which have known
supergravity duals. We also demonstrate that gauge groups cannot have
this scaling for their free-energy. At finite Yang-Mills coupling we
establish the existence of a third order phase transition where the theory
crosses over from the Yang-Mills phase to the Chern-Simons phase. The phase
transition exists for any value of , although the details differ
between small and large values of . For pure Chern-Simons
theories we present evidence for a chain of phase transitions as
is increased.
We also find the expectation values for supersymmetric circular Wilson loops
in these various scenarios and show that the Chern-Simons term leads to
different physical properties for fundamental and anti-fundamental Wilson
loops. Different choices of the integration contours also lead to different
properties for the loops.Comment: 40 pages, 17 figures, Minor corrections, Published versio
D-brane Charges in Gravitational Duals of 2+1 Dimensional Gauge Theories and Duality Cascades
We perform a systematic analysis of the D-brane charges associated with
string theory realizations of d=3 gauge theories, focusing on the examples of
the N=4 supersymmetric U(N)xU(N+M) Yang-Mills theory and the N=3 supersymmetric
U(N)xU(N+M) Yang-Mills-Chern-Simons theory. We use both the brane construction
of these theories and their dual string theory backgrounds in the supergravity
approximation. In the N=4 case we generalize the previously known gravitational
duals to arbitrary values of the gauge couplings, and present a precise mapping
between the gravity and field theory parameters. In the N=3 case, which (for
some values of N and M) flows to an N=6 supersymmetric Chern-Simons-matter
theory in the IR, we argue that the careful analysis of the charges leads to a
shift in the value of the B-field in the IR solution by 1/2, in units where its
periodicity is one, compared to previous claims. We also suggest that the N=3
theories may exhibit, for some values of N and M, duality cascades similar to
those of the Klebanov-Strassler theory.Comment: 47 pages, 9 figures; minor changes, references adde
Holographic three-point functions for short operators
We consider holographic three-point functions for operators dual to short
string states at strong coupling in N=4 super Yang-Mills. We treat the states
as point-like as they come in from the boundary but as strings in the
interaction region in the bulk. The interaction position is determined by
saddle point, which is equivalent to conservation of the canonical momentum for
the interacting particles, and leads to conservation of their conformal
charges. We further show that for large dimensions the rms size of the
interaction region is small compared to the radius of curvature of the AdS
space, but still large compared to the string Compton wave-length. Hence, one
can approximate the string vertex operators as flat-space vertex operators with
a definite momentum, which depends on the conformal and R-charges of the
operator. We then argue that the string vertex operator dual to a primary
operator is chosen by satisfying a twisted version of Q^L=Q^R, up to spurious
terms. This leads to a unique choice for a scalar vertex operator with the
appropriate charges at the first massive level. We then comment on some
features of the corresponding three-point functions, including the application
of these results to Konishi operators.Comment: 24 pages; v2: References added, typos fixed, minor change
Phenotypic Variation and Bistable Switching in Bacteria
Microbial research generally focuses on clonal populations. However, bacterial cells with identical genotypes frequently display different phenotypes under identical conditions. This microbial cell individuality is receiving increasing attention in the literature because of its impact on cellular differentiation, survival under selective conditions, and the interaction of pathogens with their hosts. It is becoming clear that stochasticity in gene expression in conjunction with the architecture of the gene network that underlies the cellular processes can generate phenotypic variation. An important regulatory mechanism is the so-called positive feedback, in which a system reinforces its own response, for instance by stimulating the production of an activator. Bistability is an interesting and relevant phenomenon, in which two distinct subpopulations of cells showing discrete levels of gene expression coexist in a single culture. In this chapter, we address techniques and approaches used to establish phenotypic variation, and relate three well-characterized examples of bistability to the molecular mechanisms that govern these processes, with a focus on positive feedback.
Impact of safety-related dose reductions or discontinuations on sustained virologic response in HCV-infected patients: Results from the GUARD-C Cohort
BACKGROUND:
Despite the introduction of direct-acting antiviral agents for chronic hepatitis C virus (HCV) infection, peginterferon alfa/ribavirin remains relevant in many resource-constrained settings. The non-randomized GUARD-C cohort investigated baseline predictors of safety-related dose reductions or discontinuations (sr-RD) and their impact on sustained virologic response (SVR) in patients receiving peginterferon alfa/ribavirin in routine practice.
METHODS:
A total of 3181 HCV-mono-infected treatment-naive patients were assigned to 24 or 48 weeks of peginterferon alfa/ribavirin by their physician. Patients were categorized by time-to-first sr-RD (Week 4/12). Detailed analyses of the impact of sr-RD on SVR24 (HCV RNA <50 IU/mL) were conducted in 951 Caucasian, noncirrhotic genotype (G)1 patients assigned to peginterferon alfa-2a/ribavirin for 48 weeks. The probability of SVR24 was identified by a baseline scoring system (range: 0-9 points) on which scores of 5 to 9 and <5 represent high and low probability of SVR24, respectively.
RESULTS:
SVR24 rates were 46.1% (754/1634), 77.1% (279/362), 68.0% (514/756), and 51.3% (203/396), respectively, in G1, 2, 3, and 4 patients. Overall, 16.9% and 21.8% patients experienced 651 sr-RD for peginterferon alfa and ribavirin, respectively. Among Caucasian noncirrhotic G1 patients: female sex, lower body mass index, pre-existing cardiovascular/pulmonary disease, and low hematological indices were prognostic factors of sr-RD; SVR24 was lower in patients with 651 vs. no sr-RD by Week 4 (37.9% vs. 54.4%; P = 0.0046) and Week 12 (41.7% vs. 55.3%; P = 0.0016); sr-RD by Week 4/12 significantly reduced SVR24 in patients with scores <5 but not 655.
CONCLUSIONS:
In conclusion, sr-RD to peginterferon alfa-2a/ribavirin significantly impacts on SVR24 rates in treatment-naive G1 noncirrhotic Caucasian patients. Baseline characteristics can help select patients with a high probability of SVR24 and a low probability of sr-RD with peginterferon alfa-2a/ribavirin
Comparative effectiveness of Cladribine tablets vs other drugs in relapsing-remitting multiple sclerosis: an approach merging randomized controlled trial with real life data
OBAYA (obesity and adverse health outcomes in young adults): feasibility of a population-based multiethnic cohort study using electronic medical records
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