Nonperturbative renormalization group approach to frustrated magnets

This article is devoted to the study of the critical properties of classical XY and Heisenberg frustrated magnets in three dimensions. We first analyze the experimental and numerical situations. We show that the unusual behaviors encountered in these systems, typically nonuniversal scaling, are hardly compatible with the hypothesis of a second order phase transition. We then review the various perturbative and early nonperturbative approaches used to investigate these systems. We argue that none of them provides a completely satisfactory description of the three-dimensional critical behavior. We then recall the principles of the nonperturbative approach - the effective average action method - that we have used to investigate the physics of frustrated magnets. First, we recall the treatment of the unfrustrated - O(N) - case with this method. This allows to introduce its technical aspects. Then, we show how this method unables to clarify most of the problems encountered in the previous theoretical descriptions of frustrated magnets. Firstly, we get an explanation of the long-standing mismatch between different perturbative approaches which consists in a nonperturbative mechanism of annihilation of fixed points between two and three dimensions. Secondly, we get a coherent picture of the physics of frustrated magnets in qualitative and (semi-) quantitative agreement with the numerical and experimental results. The central feature that emerges from our approach is the existence of scaling behaviors without fixed or pseudo-fixed point and that relies on a slowing-down of the renormalization group flow in a whole region in the coupling constants space. This phenomenon allows to explain the occurence of generic weak first order behaviors and to understand the absence of universality in the critical behavior of frustrated magnets.Comment: 58 pages, 15 PS figure

Genome-wide asociation analysis identifies three psoriasis susceptibility loci

We carried out a meta-analysis of two recent psoriasis genome-wide asociation studies with a combined discovery sample of 1,831 affected individuals (cases) and 2,546 controls. One hundred and two loci selected based on P value rankings were foLowed up in a thrE-stage replication study including 4,064 cases and 4,685 controls from Michigan, Toronto, Newfoundland and Germany. In the combined meta-analysis, we identified thrE new susceptibility loci, including one at NOS2 (rs4795067, combined P = 4-10-11), one at FBXL19 (rs10782001, combined P = 9-10-10) and one near PSMA6-NFKBIA (rs12586317, combined P = 2-10-8). AL thrE loci were also aSociated with psoriatic arthritis (rs4795067, combined P = 1-10-5; rs10782001, combined P = 4-10-8; and rs12586317, combined P = 6-10-5) and purely cutaneous psoriasis (rs4795067, combined P = 1-10-8; rs10782001, combined P = 2-10-6; and rs12586317, combined P = 1-10-6). We also replicated a recently identified asociation signal near RNF114 (rs495337, combined P = 2-10-7)

Helium burning and neutron sources in the stars

Helium burning represents an important stage of stellar evolution as it contributes to the synthesis of key elements such as carbon, through the triple-alfa process, and oxygen, through the 12C(alfa, gamma)16O reaction. It is the ratio of carbon to oxygen at the end of the helium burning stage that governs the following phases of stellar evolution leading to different scenarios depending on the initial stellar mass. In addition, helium burning in Asymptotic Giant Branch stars, provides the two main sources of neutrons, namely the 13C(alfa, n)16O and the 22Ne(alfa, n)25Mg, for the synthesis of about half of all elements heavier than iron through the s-process. Given the importance of these reactions, much experimental work has been devoted to the study of their reaction rates over the last few decades. However, large uncertainties still remain at the energies of astrophysical interest which greatly limit the accuracy of stellar models predictions. Here, we review the current status on the latest experimental efforts and show how measurements of these important reaction cross sections can be significantly improved at next-generation deep underground laboratories