Minimal area surfaces in AdS_{n+1} and Wilson loops

The AdS/CFT correspondence relates the expectation value of Wilson loops in N=4 SYM to the area of minimal surfaces in AdS_5 In this paper we consider minimal area surfaces in generic Euclidean AdS_{n+1} using the Pohlmeyer reduction in a similar way as we did previously in Euclidean AdS_3. As in that case, the main obstacle is to find the correct parameterization of the curve in terms of a conformal parameter. Once that is done, the boundary conditions for the Pohlmeyer fields are obtained in terms of conformal invariants of the curve. After solving the Pohlmeyer equations, the area can be expressed as a boundary integral involving a generalization of the conformal arc-length, curvature and torsion of the curve. Furthermore, one can introduce the \lambda-deformation symmetry of the contours by a simple change in the conformal invariants. This determines the \lambda-deformed contours in terms of the solution of a boundary linear problem. In fact the condition that all \lambda deformed contours are periodic can be used as an alternative to solving the Pohlmeyer equations and is equivalent to imposing the vanishing of an infinite set of conserved charges derived from integrability.Comment: 29 pages, LaTeX, 1 figur

Minimal Area Surfaces Corresponding to Wilson Loops in Gauge Theories

In this dissertation, we study the properties of Wilson loops in strongly coupled gauge theories by using one of the most important results of the AdS/CFT correspondence, namely, that the expectation value of the Wilson loop operator is related to the area of the minimal area surface ending on the Wilson loop. In particular, we concentrate on the main example of AdS/CFT correspondence which is the duality between the N = 4 Super Yang-Mills (SYM) theory in 4 dimensions and type IIB string theory in AdS5 × S5 space. In this case, the minimal area surfaces live in Anti de Sitter (AdS) space and therefore have integrability properties we explore in this dissertation. Furthermore, N = 4 SYM is a conformal theory and therefore the expectation value of Wilson loop operators is conformally invariant, a property that we make manifest in all the results. The work consists of two main projects where we find new solutions for minimal area surfaces dual to Wilson loops and study the conformal and integrability properties of minimal area surfaces in general Euclidean AdS space. In the first project, we use the Mathieu equation, a standard example of a periodic potential, to obtain a new class of Wilson loops such that the area of the dual minimal area surface can be computed analytically in terms of eigenvalues of such equation. The area of the minimal surface is invariant under λ-deformations, a hidden symmetry that arises due to integrability. This result provides an important example to check the perturbative method developed by Dekel [18] to compute near circular Wilson loops. It can also be reduced in a special case to an early result proposed by Toledo [19]. There are several interesting limits of the Wilson loop solutions we found, including circular and multi-wound cases, as well as null-polygons studied by Alday and Maldacena [20], which are related to scattering amplitudes of even number of gluons. Finally, in the near null-case, we found that the potential becomes a series of separated wells each associated with two light-like segments leading to a simple perturbative method for these Wilson loops. In the second project we consider minimal area surfaces in generic Euclidean AdSd in a similar way as we did previously in Euclidean AdS3. We give a formula for the area in terms of a generalized form of the conformal arc-length, conformal curvature and conformal torsion of the Wilson loop. This formula is explicitly reparameterization invariant, conformally invariant as well as invariant under λ-deformations and therefore should be a main starting point to study Wilson loops in conformal field theories. To obtain this result, we use the Pohlmeyer reduction method. We show how an arbitrary Wilson loop living at the boundary determines the boundary conditions for the fields appearing in the Pohlmeyer reduction. Solving the Pohlmeyer equations determines an extra set of boundary conditions and allows to set up a linear problem along the Wilson loop that determines the shape of the λ-deformed curves from the original loop. This set of boundary conditions can also be determined by the condition that all λ-deformed contours are periodic, or equivalently the vanishing of an infinite set of conserved charges derived from integrability

Studies on Utilizing the Three Famous International Index Systems to Evaluate Scientific Research Level of Higher Learning Institutions

Science Citation Index (SCI), The Engineering Index (EI) and Index to Scientific & Technical Proceeding (ISTP) are widely accepted and used to evaluate the scientific research level of higher learning institutions by many country's science and technology field currently. After research, we point out the blemishes in this method and put forward the problems that need to be noticed, and then, under current conditions, bring forward brand-new standard and method to estimate research level, efficiency, fund exploitation and so on. One shouldn't over-emphasize the total amount of papers collected in SCI, EI & ISTP when evaluating the scientific research level of higher learning institutions, whereas using ‘comprehensive factor’ analysis method can make it more scientific and efficient

Non-Asymptotic Bounds for Adversarial Excess Risk under Misspecified Models

We propose a general approach to evaluating the performance of robust estimators based on adversarial losses under misspecified models. We first show that adversarial risk is equivalent to the risk induced by a distributional adversarial attack under certain smoothness conditions. This ensures that the adversarial training procedure is well-defined. To evaluate the generalization performance of the adversarial estimator, we study the adversarial excess risk. Our proposed analysis method includes investigations on both generalization error and approximation error. We then establish non-asymptotic upper bounds for the adversarial excess risk associated with Lipschitz loss functions. In addition, we apply our general results to adversarial training for classification and regression problems. For the quadratic loss in nonparametric regression, we show that the adversarial excess risk bound can be improved over those for a general loss.Comment: 27 pages, 3 table

Computer-generated reminders and quality of pediatric HIV care in a resource-limited setting

OBJECTIVES: To evaluate the impact of clinician-targeted computer-generated reminders on compliance with HIV care guidelines in a resource-limited setting. METHODS: We conducted this randomized, controlled trial in an HIV referral clinic in Kenya caring for HIV-infected and HIV-exposed children (<14 years of age). For children randomly assigned to the intervention group, printed patient summaries containing computer-generated patient-specific reminders for overdue care recommendations were provided to the clinician at the time of the child's clinic visit. For children in the control group, clinicians received the summaries, but no computer-generated reminders. We compared differences between the intervention and control groups in completion of overdue tasks, including HIV testing, laboratory monitoring, initiating antiretroviral therapy, and making referrals. RESULTS: During the 5-month study period, 1611 patients (49% female, 70% HIV-infected) were eligible to receive at least 1 computer-generated reminder (ie, had an overdue clinical task). We observed a fourfold increase in the completion of overdue clinical tasks when reminders were availed to providers over the course of the study (68% intervention vs 18% control, P < .001). Orders also occurred earlier for the intervention group (77 days, SD 2.4 days) compared with the control group (104 days, SD 1.2 days) (P < .001). Response rates to reminders varied significantly by type of reminder and between clinicians. CONCLUSIONS: Clinician-targeted, computer-generated clinical reminders are associated with a significant increase in completion of overdue clinical tasks for HIV-infected and exposed children in a resource-limited setting

Estrogen induces global reorganization of chromatin structure in human breast cancer cells

In the cell nucleus, each chromosome is confined to a chromosome territory. This spatial organization of chromosomes plays a crucial role in gene regulation and genome stability. An additional level of organization has been discovered at the chromosome scale: the spatial segregation into open and closed chromatins to form two genome-wide compartments. Although considerable progress has been made in our knowledge of chromatin organization, a fundamental issue remains the understanding of its dynamics, especially in cancer. To address this issue, we performed genome-wide mapping of chromatin interactions (Hi-C) over the time after estrogen stimulation of breast cancer cells. To biologically interpret these interactions, we integrated with estrogen receptor α (ERα) binding events, gene expression and epigenetic marks. We show that gene-rich chromosomes as well as areas of open and highly transcribed chromatins are rearranged to greater spatial proximity, thus enabling genes to share transcriptional machinery and regulatory elements. At a smaller scale, differentially interacting loci are enriched for cancer proliferation and estrogen-related genes. Moreover, these loci are correlated with higher ERα binding events and gene expression. Taken together these results reveal the role of a hormone--estrogen--on genome organization, and its effect on gene regulation in cancer

Lightweight conductive graphene/thermoplastic polyurethane foams with ultrahigh compressibility for piezoresistive sensing

Lightweight conductive porous graphene/thermoplastic polyurethane (TPU) foams with ultrahigh compressibility were successfully fabricated by using the thermal induced phase separation (TISP) technique. The density and porosity of the foams were calculated to be about 0.11 g cm−3 and 90% owing to the porous structure. Compared with pure TPU foams, the addition of graphene could effectively increase the thickness of the cell wall and hinder the formation of small holes, leading to a robust porous structure with excellent compression property. Meanwhile, the cell walls with small holes and a dendritic structure were observed due to the flexibility of graphene, endowing the foam with special positive piezoresistive behaviors and peculiar response patterns with a deflection point during the cyclic compression. This could effectively enhance the identifiability of external compression strain when used as piezoresistive sensors. In addition, larger compression sensitivity was achieved at a higher compression rate. Due to high porosity and good elasticity of TPU, the conductive foams demonstrated good compressibility and stable piezoresistive sensing signals at a strain of up to 90%. During the cyclic piezoresistive sensing test under different compression strains, the conductive foam exhibited good recoverability and reproducibility after the stabilization of cyclic loading. All these suggest that the fabricated conductive foam possesses great potential to be used as lightweight, flexible, highly sensitive, and stable piezoresistive sensors