Search Talks

When the wavefunction of a macroscopic system unitarily evolves from a low-entropy initial state, there is good circumstantial evidence it develops "branches", i.e., a decomposition into orthogonal components that can't be distinguished from the corresponding incoherent mixture by feasible observations, with each component a simultaneous eigenstate of preferred macroscopic observables.  Is this decomposition unique?  Can the number of branches decrease in time?

Answering these questions is hard because branches are defined only intuitively, much like early investigations of algorithms prior to the Church–Turing thesis.  A rigorous definition could give a speed up to certain non-stationary matrix-product state numerical simulations, as well as solve Kent's Set Selection problem in the consistent histories formalism, a 20-year-old open challenge in the foundations of quantum mechanics. I introduce some tentative definitions based on the idea of redundant information, and establishing a uniqueness theorems. A key counterexample is provided by the Shor error-correction code, which demonstrates that branch structure with robust, redundant records on macroscopic scales can hide incompatible (noncommuting) structure on microscopic scales.

We study the effective twisted superpotential of 3d N=2 gauge theories compactified on a circle.  This is a rich object which encodes much of the protected information in these theories.  We review its properties, and survey some applications, including the algebra of Wilson loops, computation of supersymmetric partition functions on S^1 bundles, and the reduction of 3d dualities to two dimensions.

We introduce a simple and modern discussion of rotational superradiance based on quantum field theory. We work with an effective theory valid at scales much larger than the size of the spinning object responsible for superradiance. Within this framework, the probability of absorption by an object at rest completely determines the superradiant amplification rate when that same object is spinning. We first discuss in detail superradiant scattering of spin 0 particles with orbital angular momentum l = 1, and then extend our analysis to higher values of orbital angular momentum and spin. Along the way, we provide a simple derivation of vacuum friction—a “quantum torque” acting on spinning objects in empty space. Our results apply not only to black holes but to arbitrary spinning objects. We also discuss superradiant instability due to formation of bound states and, as an illustration, we calculate the instability rate Γ for bound states with massive spin 1 particles.

Abstract: We present the foundation for a holographic dictionary with depth perception. The dictionary consists of natural operators associated with CFT bilocals whose duals are simple, diffeomorphism-invariant bulk operators.  These objects admit a description as fields in kinematic space, a phase space for such probes.  The framework of kinematic space allows for conceptually simple derivations of many results known in the literature, including linearized Einstein’s equations, the relationship between conformal blocks and geodesic Witten diagrams, and the CFT representation of bulk local operators.   Reference: https://arxiv.org/abs/1604.03110

Soft theorems for the scattering of low energy photons and gravitons and cosmological consistency conditions on the squeezed-limit correlation functions are both understood to be consequences of invariance under large gauge transformations. I apply the same method used in cosmology -- based on the identification of an infinite set of "adiabatic modes" and the corresponding conserved currents -- to derive flat space soft theorems for electrodynamics and gravity. I discuss how the recent derivations based on the asymptotic symmetry groups (BMS) can be continued to a finite size sphere surrounding the scattering event. Finally, I comment on Hawking-Perry-Strominger proposal of "soft hair on black holes".

The space of causal diamonds recently brought to attention by de Boer et al. and Czech et al. provides an organizing principle for the dependence of entanglement entropy in conformal field theories on the spatial subregion considered. I will show that the inclusion relation of causal diamonds does not give rise to a consistent notion of a causal structure and thus does not provide an alternate metric on this space. I will also show that the entanglement entropy of ball shaped regions is not enough to reconstruct the areas of higher dimensional bulk surfaces in a static geometry. Instead, I will provide a new direction for the reconstruction of the areas of bulk surfaces from boundary data in holography, using the shape derivatives of the entanglement entropy of nearly ball shaped regions.

I will discuss the properties, and constraints on, new light

particles, which appear in many extensions of the Standard Model. An

especially well motivated example is the QCD axion, and I will show how

its mass and couplings can be extracted at high precision. I will also

discuss its properties at finite temperature, and possible distinguishing

features if it makes up dark matter. More generally, strong constraints on

the couplings of new light particles to the Standard Model come from their

production in the hot cores of stars, and the effect on stellar cooling.

The large electron density in stellar cores gives large thermal effects,

and I will show how these can parametrically change the production rates

compared to a 'tree level' calculation.

We consider conformal blocks of two heavy operators and an arbitrary  number of light operators in a 1+1 dimensional CFT with large central charge. Using the monodromy method, these higher-point  conformal blocks are shown to factorize into products of 4-point conformal blocks in the heavy-light limit for a class of OPE channels. This result is reproduced by considering suitable worldline configurations in the bulk conical defect geometry. We apply the CFT results to calculate the entanglement entropy of an arbitrary number of  disjoint intervals for heavy states. The corresponding holographic entanglement entropy calculated via the minimal area prescription precisely matches these results from CFT.

We describe a new solution to the strong CP problem inspired by the massless up quark solution. At high energies, QCD is embedded in a SU(3)xSU(3)xSU(3) model, with each matter generation charged under a different site. Instanton effects are unsuppressed at the scale of Higgsing to the SM diagonal QCD, and a set of anomalous U(1)_PQ symmetries removes the low-energy strong CP phase. A non-zero theta parameter is generated at loop level near current bounds. Similar models can also lead to a heavy axion solution to the strong CP problem.

What natural CFT quantities can “see” in the interior of the bulk AdS in a diffeomorphism invariant way? And how can we use them to learn about the emergence of local bulk physics? Inspired by the Ryu-Takayanagi relation, we construct a class of simple non-local operators on both sides of the duality and demonstrate their equivalence. Integrals of free bulk fields along geodesics/minimal surfaces are dual to what we will call “OPE blocks”: Individual conformal family contributions to the OPE of local operators. Our findings can be utilized to reconstruct local bulk operators and unify a number of previously disconnected AdS/CFT results. We extend this kinematic correspondence to incorporate gravitational interactions in AdS3 by relating geodesic operators to the Virasoro OPE blocks, hence providing a natural CFT prescription for “gravitational dressing”. We conclude with discussion of preliminary results on an interesting CFT structure that generalizes our dictionary to include arbitrary local bulk interactions. 

Painlevé equations can be obtained both from time-dependent classical Hamiltonian systems and from isomonodromic deformation problems. These realizations lead to a precise matching between Painlevé equations and Hitchin systems associated to four-dimensional N=2 SQCD as well as Argyres-Douglas theories. Long-time analysis of the Painlevé Hamiltonians dynamics allows to extract the unrefined "instanton" partition function for these theories at all strong-coupling points

The T2K experiment studies neutrino properties by producing a beam of muon neutrinos and sending them 295 km across Japan to the Super-Kamiokande detector. En route, neutrinos undergo a transmutation known as “neutrino oscillations” wherein they can transition to two other species or flavours, electron and tau neutrinos. Starting in 2014, T2K has run with an antineutrino beam to study the corresponding antineutrino oscillations and the possibility that a  complex phase in the neutrino mass and flavour mixing may lead to differences in neutrino and antineutrino oscillations. Such a difference may provide a critical clue into how our universe came to be dominated by matter.  I will present the latest results from an analysis of T2K neutrino and antineutrino data taken through 2016 along with other recent results from other experiments, and discuss their implications for our understanding of neutrinos.