Winter 2015

One of the most intriguing consequences of the Higgs boson discovery is that its mass places the Standard Model near the border between absolute stability and metastability, i.e. our universe is unstable, but should last for an astronomically long time. This conclusion is based on effective potential calculations which involve some suspicious elements: for example, the stability criteria apparently depends on gauge. Can we establish the fate of our universe using only self-consistent methods?

The discovery of the Higgs boson at the Large Hadron Collider (LHC) marks the culmination of a decades-long hunt for the last ingredient of the Standard Model. At the same time, this discovery has started a new era in the search for more fundamental physics. In this talk, I will discuss how tests of new physics beyond the Standard Model using the Higgs boson are complementary to direct searches for new particles. In particular, I will motivate new searches at the LHC that can discover otherwise undetected new phenomena.

The LHC has placed strong constraints on many signals of supersymmetry. This is beginning to lead to some tension in the idea of supersymmetry as a fully natural solution to the hierarchy problem. However, nonminimal models can evade direct searches, as can models that are somewhat fine-tuned. I will discuss some ways that SUSY can be hiding from us, and how a broader LHC search strategy as well as precision Standard Model measurements and dark matter experiments can explore these hiding places in the future.

A major goal of General Relativity (GR) is to precisely describe and finally observe gravitational radiation, one of the predictions of GR. In order to do so, one has to study the spacetimes for typical sources. Among the latter we find binary neutron stars and binary black hole mergers or core-collapse supernovae. In these processes typically mass and momenta are radiated away in form of gravitational waves. These waves leave a footprint in the spacetime, that is they displace test masses permanently. This effect is called the memory. In this talk, I will explore the gravitational memory. We will see that there are two types of memory, one going back to Ya. B. Zel'dovich and A. G. Polnarev and one to D. Christodoulou. Then I will discuss recent work in collaboration with D. Garfinkle, S.-T. Yau, P. Chen, focusing on how neutrinos or electromagnetic fields contribute to the memory effect. Moreover, recent results on memory in other fields will be mentioned such as my collaboration with D. Garfinkle concerning electromagnetic radiation, and A. Strominger with A. Zhiboedov in quantum field theory.

Dark Matter is the best evidence we have for physics beyond the Standard Model. Yet, we have little data concerning the nature of dark matter beyond its mass density and the fact that it is very weakly interacting, if at all. In this talk I will focus on the possibility that dark matter is a new fundamental particle and explore both the underlying theoretical structures as well as the variety of experimental frontiers associated with this possibility

After a brief summary of the current status of precision tests of the electroweak Standard Model, the new insights on electroweak physics expected from the LHC will be discussed. These include measurements of classic precision quantities, as well as genuine high-energy tests. It will be illustrated how existing e+e- data and future results from the LHC probe possible new physics effects in a complementary way. Finally, the improvements of electroweak precision tests from a future high-luminosity e+e- collider will be discussed and compared to existing theory predictions, in particular elucidating the available methods for computing high-order loop corrections and estimating their uncertainty.

For almost four decades, infrared and collinear (IRC) safety has been the guiding principle for determining which jet observables can be calculated using perturbative QCD.  Now in the LHC era, new jet substructure observables have emerged which are IRC unsafe, yet still calculable using perturbative techniques.  In this talk, I explain the origin of these "Sudakov safe" observables and show how they blur the boundary between perturbative and nonperturbative aspects of QCD.

Depending what future runs of the LHC may reveal, the most interesting possibility for a subsequent high-energy accelerator may be a larger circular tunnel housing an electron-positron collider and/or a proton-proton collider. CERN is coordinating studies of an electron-positron option that would yield much higher luminosities than a linear collider for Higgs, Z and W studies, and a proton-proton option capable of reaching 100 TeV in the centre of mass.Together they could explore directly and indirectly new physics at energies of 10 TeV or more. Examples of possible physics topics include precision Higgs and electroweak studies and searches for massive dark matter particles.

Effective Field Theory allows one to separate the known particles and interactions at low energy from the unknown physics at high energy. When applied to General Relativity it allows reliable calculation of quantum effects in the infrared. I will describe the methods and results of the EFT treatment, with an emphasis on non-local/non-analytic effects. When pushed to higher energies, the formalism points towards the quantum resolution of some gravitational singularities.

I will point out a connection between the emergence of bulk locality in AdS/CFT and the theory of quantum error correction.  Bulk notions such as Bogoliubov transformations, location in the radial direction, and the holographic entropy bound all have natural CFT interpretations in the language of quantum error correction.  Interestingly, the version of quantum error correction which is best suited to analyzing AdS/CFT is the somewhat nonstandard "operator algebra quantum error correction" of Beny, Kempf, and Kribs.  This connection gives a precise formulation of the idea of "subregion-subregion" duality in AdS/CFT, and clarifies the limits of its validity.

A common criticism of the amplitudes field is that the constant push to higher and higher loops seems to have no plausible application. In this talk, I will describe a counterexample: a time when the string theory community had a question that could only be answered by calculating the six-loop divergence of five-dimensional N=4 super Yang-Mills. I will discuss the methods that made this calculation possible, generalized unitarity and sector decomposition. I will then move on to briefly survey some of the research directions that sprung up in the wake of the calculation.