Temperature-dependence of the QCD topological susceptibility

We recently obtained an estimate of the axion mass based on the hypothesis that axions make up most of the dark matter in the universe. A key ingredient for this calculation was the temperature-dependence of the topological susceptibility of full QCD. Here we summarize the calculation of the susceptibility in a range of temperatures from well below the finite temperature cross-over to around 2 GeV. The two main difficulties of the calculation are the unexpectedly slow convergence of the susceptibility to its continuum limit and the poor sampling of nonzero topological sectors at high temperature. We discuss how these problems can be solved by two new techniques, the first one with reweighting using the quark zero modes and the second one with the integration method.Comment: 9 pages, 6 figures, to be published in Proceedings of the 35th International Symposium on Lattice Field Theory (Lattice2017)}: Granada, Spain}, to appear in EPJ Web Con

Structure of the QCD Vacuum As Seen By Lattice Simulations

This talk is a review of our studies of instantons and their properties as seen in our lattice simulations of SU(2) gauge theory. We have measured the topological susceptibility and the size distribution of instantons in the QCD vacuum. We have also investigated the properties of quarks moving in instanton background field configurations, where the sizes and locations of the instantons are taken from simulations of the full gauge theory. By themselves, these multi-instanton configurations do not confine quarks, but they induce chiral symmetry breaking.Comment: 18 pages, LaTeX, 8 figures, uses epsf, Talk given at YKIS9

The localization transition in SU(3) gauge theory

We study the Anderson-like localization transition in the spectrum of the Dirac operator of quenched QCD. Above the deconfining transition we determine the temperature dependence of the mobility edge separating localized and delocalized eigenmodes in the spectrum. We show that the temperature where the mobility edge vanishes and localized modes disappear from the spectrum, coincides with the critical temperature of the deconfining transition. We also identify topological charge related close to zero modes in the Dirac spectrum and show that they account for only a small fraction of localized modes, a fraction that is rapidly falling as the temperature increases.Comment: 7 pages, 5 figures, v3: additional data on finer lattice; final, published versio

Wind stress forcing of the freshwater distribution in the Arctic and North Atlantic Oceans

The hypothesis of this study was that there is a potential for an oscillating covariability between the freshwater content of the Arctic Ocean and the Subarctic North Atlantic, and the redistribution between their basins is governed by wind stress forcing associated with large-scale patterns of atmospheric variability. In order to test this hypothesis, numerical model simulations were performed using the Max Planck Institute Earth System Model (MPI-ESM) with the objectives to 1) analyze the link between Arctic and Subarctic North Atlantic freshwater anomalies, to 2) identify key patterns of atmospheric variability that govern these anomalies through wind forcing, and to 3) explain the physical mechanisms of coupling between freshwater and near-surface winds associated with these key patterns. The results showed that even though there is a stable sign of freshwater redistribution between the Arctic and the Subarctic North Atlantic on a multidecadal time scale, this sign is mostly obscured by large anomalies in the North Atlantic that are transported from the south. A comprehensive statistical analysis revealed that the main statistical modes of large-scale atmospheric variability do not represent those modes that are best connected to freshwater anomalies. Such modes were identified in this work by performing a redundancy analysis of atmospheric variability and freshwater content, separately for its liquid and solid components. The impact of wind stress forcing was demonstrated by further simulations. These used prescribed wind data based on observations, but unlike the otherwise identically set- up unconstrained fully coupled runs, they could reproduce the observed freshwater anomalies of the 1990s. This confirmed the key role of wind stress forcing. Additional experiments with prescribed idealized wind perturbations enabled the isolation of the effect of certain wind forcing patterns on freshwater variability