Table of contents

The focus of this paper is on recent progress in muon spin rotation (µSR) studies of the vortex cores in type-II superconductors. By comparison of µSR measurements of the vortex core size in a variety of materials with results from techniques that directly probe electronic states, the effect of delocalized quasiparticles on the spatial variation of field in a lattice of interacting vortices has been determined for both single-band and multi-band superconductors. These studies demonstrate the remarkable accuracy of what some still consider an exotic technique. In recent years µSR has also been used to search for magnetism in and around the vortex cores of high-temperature superconductors. As a local probe µSR is specially suited for detecting static or quasistatic magnetism having short-range or random spatial correlations. As discussed in this review, µSR experiments support a generic phase diagram of competing superconducting and magnetic order parameters, characterized by a quantum phase transition to a state where the competing order is spatially nonuniform.

This paper is a review of the present status of neutrino mass physics, which grew out of an APS sponsored study of neutrinos in 2004. After a discussion of the present knowledge of neutrino masses and mixing and some popular ways to probe the new physics implied by recent data, it summarizes what can be learned about neutrino interactions as well as the nature of new physics beyond the Standard Model from the various proposed neutrino experiments. The intriguing possibility that neutrino mass physics may be at the heart of our understanding of a long standing puzzle of cosmology, i.e. the origin of matter–antimatter asymmetry is also discussed.

Meson–antimeson oscillations are a pure quantum phenomenon involving many of quantum mechanics' 'spooky' features like state mixing and EPR correlations; the subtle implementation of (approximate) symmetries also plays a crucial role. The analysis of oscillations represents a high sensitivity probe of nature's fundamental forces and at the same time provides experimental validation of subtle features of quantum mechanics. The theoretical framework and experimental signatures are described in detail. Meson–antimeson oscillations have also formed essential ingredients in the discovery of CP violation, a delicate, yet profound feature of our universe. These phenomena have been crucial for the evolution of the Standard Model of high energy physics and have more recently provided impressive validation for its CKM dynamics. Nevertheless these successes do not invalidate the arguments for the Standard Model being incomplete already at 'nearby' energy scales. Oscillation phenomena and CP violation open up new portals for the emergence of the anticipated new physics. Other incarnations of matter–antimatter oscillations, as for neutrons, are briefly commented on.

The limitations and possibilities that the concept of quantum interference offers as a tool for testing fundamental physics are explored here. The use of neutron interference as an instrument to compare measurement readouts with some of the principles behind metric theories of gravity will be analyzed, as will some discrepancies between theory and experiment. The main restrictions that this model embodies for the study of some of the features of the structure of space–time will be explicitly pointed out. For instance, the conditions imposed by the necessary use of the semiclassical approximation. Additionally, the role that photon interference could play as an element in this context is also considered. In this realm we explore the differences between first-order and second-order coherence experiments, and underline the fact that the Hanbury–Brown–Twiss effect could open up some interesting experimental possibilities in the analysis of the structure of space–time. The void, in connection with the description of wave phenomena, implicit in the principles of metric theories is analyzed. The conceptual difficulties that this void entails are commented upon.