Winter 2017

I will discuss work with Cordova and Dumitrescu.  We systematically analyze the possible operator content of unitary superconformal multiplets in  three, and higher, spacetime dimensions.  There is a surprisingly rich and exotic zoo of multiplets, and sporadic phenomena.  As applications, we classify supersymmetry preserving deformations, the absolutely protected multiplets, and the conserved currents or free fields that can appear in superconformal theories.  I will also discuss earlier work on the 6d a-theorem for supersymmetric theories, and some recent extensions. 

Precision calculations in de Sitter space (such as of inflationary predictions for primordial fluctuations) are often plagued by infrared problems and issues of secular time dependence. Similar issues about the breakdown of perturbation theory seem also to arise for information loss in black holes. This talk briefly summarizes how similar problems can arise in other areas of physics, and how they are dealt with when they do. It is argued that Master-Equation techniques used in areas like optics also apply to cosmology (and possibly black holes) and can tell us how to extract reliably late-time predictions. Applied to inflation they lead to Starobinsky's stochastic methods (plus small but important corrections). This is argued to provide an explanation for why stochastic inflation seems to resum IR effects in simple examples, and allows these tools to be generalized to apply more broadly. I mention in passing the relevance of these tools to the problem of Schrodinger's Cosmologist: how primordial fluctuations decohere sometime between their production during inflation and their observation early in the present Big Bang Epoch.

The search for physics beyond the Standard Model at the LHC is largely oriented towards new particles associated with solutions to the electroweak hierarchy problem. While the precise character of these partner states may vary from model to model, they typically possess large QCD production rates favorable for detection at hadron colliders. Null results in searches for partner particles during Run 1 of the LHC have placed the idea of electroweak naturalness under increasing strain. In this talk I'll discuss a broad class of natural theories where the new degrees of freedom relevant for naturalness lie in hidden sectors and are largely unconstrained by LHC data. Rather than rendering electroweak naturalness untestable, they give rise to entirely new signs of naturalness at the LHC, including displaced decays and other exotic signatures. They also furnish a variety of viable dark matter candidates testable at current and future experiments.

The black hole information paradox is really a combination of two problems: the causality paradox and the entanglement problem. The causality paradox arises because in the semiclassical approximation infalling matter gets causally trapped inside its own horizon; it is therefore unable to send its information back to infinity if we disallow propagation outside the light cone. We show that  the causality paradox can be resolved by local effects in the fuzzball paradigm, and contrast this resolution with other proposed paradigms where nonlocal effects like wormholes are required to exist. 

A very close supernova explosion could have caused a mass extinction of life in Earth. In 1996, Brian Fields, the late Dave Schramm and the speaker proposed looking for unstable isotopes such as Iron 60 that could have been deposited by a recent nearby supernova explosion. A group from the Technical University of Munich has discovered Iron 60 in deep-ocean sediments and ferromanganese crusts due to one or more supernovae that exploded O(100) parsecs away about 2.5 million years ago. These results have recently been confirmed by a group from the Australian National University, and the Munich group has also discovered supernova Iron 60 in lunar rock samples and in microfossils of magnetotactic bacteria. 

This colloquium will discuss the data and their interpretation in terms of supernova models, and the possible implications for life on Earth.

After recalling a definition of a black hole "hair" we explain why the only interesting hairs are those that are also remnants. Next, we will examine a recent proposal by Hawking, Perry and Strominger, who suggest that soft photons and soft gravitons can be regarded as black hole hairs that may be relevant to the black hole information paradox. We will make use of factorization theorems for infrared divergences of the S-matrix to argue that, by appropriately dressing in and out hard states, the soft-quanta-dependent part of the S-matrix becomes essentially trivial, in the sense that the information paradox can be fully formulated in terms of dressed hard states, which do not depend on soft quanta.

A sonic analogy for Hawking radiation was introduced nearly 40 years ago, motivated in part by the “trans-Planckian puzzle”. This has shed light on the puzzle (which, however, remains enigmatic), and it suggested that analog Hawking radiation could one day be observed in a laboratory. That day has come. In two recent papers, observations of Hawking radiation in rubidium condensates have been reported. The first attributed the observed features to the "black hole laser" effect, while the second reported measurements of the quantum entanglement of Hawking phonons with their partners. I'll explain this circle of ideas, describe the experiments, and report on theoretical analyses showing that, in fact, the laser effect was most likely not behind the observations, and that more work is needed to determine whether the measurements actually demonstrated entanglement.

I discuss CP violating oscillations of neutral baryons into anti-baryons, and propose an experimentally allowed  and conceivably  testable scenario where some heavy flavored baryons oscillate at rates which are within a few orders of magnitude of their lifetimes, while the flavor structure of the baryon violation suppresses neutron oscillations and baryon violating nuclear decays. I describe a scenario for producing such baryons in the early universe via the out of equilibrium decays of a neutral particle, after the hadronization temperature but before nucleosynthesis, and the prospects for baryogenesis.

An ultra light axion dark matter (DM) may avoid certain problems of the cold DM paradigm with observations at galactic scales.  Such an axion, often referred to as "Fuzzy DM (FDM)," may get its tiny mass from large masses suppressed by non-perturbative stringy effects.  We examine an alternative possibility that the mass of FDM is generated by infrared confining dynamics, in analogy with the QCD axion. We find that cosmological constraints are suggestive of a period of mild of inflation that reheats the Standard Model (SM) sector only. A typical prediction of the scenario, broadly speaking, is a larger effective number of neutrinos compared to the SM value Neff≈3, as inferred from precision measurements of the cosmic microwave background. Some of the new degrees of freedom may be identified as "sterile neutrinos," which may be required to explain certain neutrino oscillation anomalies.

Heavy dark matter coupled to much lighter force carriers exhibits a range of interesting behaviors that are not well-characterized by the usual perturbative Feynman diagram expansion, including long-range interactions, annihilation rates modified by both large velocity-dependent enhancements and large logarithms, and the presence of bound states. There has been great interest in recent years in dark sectors containing both dark matter and much lighter particles; as the LHC continues to constrain low-scale supersymmetry, even classic weakly interacting dark matter candidates may need to be heavy relative to the weak gauge bosons. I will describe recent work to characterize the novel properties of heavy dark matter coupled to light force carriers, in two principal directions: (1) the precision calculation of heavy wino dark matter annihilation to line photons using effective field theory, now to NLL’ order, and (2) the properties of dark-sector bound states where multiple force carriers and several states in a dark matter multiplet may be involved, using the wino as an example. I will discuss the importance of these results for indirect detection experiments searching for the products of dark matter annihilation.