Mukund Rangamani (UC Davis) : A paradigm for open quantum systems with memory from holography | 9/10/2021
The late-time, long-distance behaviour of gravitational perturbations of planar AdS black holes is controlled by hydrodynamic quasinormal modes. In the dual CFT these are modes of the conserved currents which control the infra-red dynamics. While the on-shell structure of these modes has been understood for a long-time, we have recently begun to understand how to construct an off-shell low energy effective action, an effective hydrodynamic field theory, from holography. I will describe the general lessons learnt from the gravitational analysis, sketching out a paradigm that provides a template for the analysis of general quantum systems with dissipation and long-time memory.
Yanou Cui (University of California, Riverside) : The Impact of a Midband Gravitational Wave Experiment On Detectability of Cosmological Stochastic Gravitational Wave Backgrounds |9/17/2021
I will discuss the impact of a future midband gravitational wave experiment on improving detectability of a cosmologically sourced stochastic GW background. Specific proposed midband experiments considered are TianGo, B-DECIGO and AEDGE. We propose a combined power-law integrated sensitivity (CPLS) curve combining GW experiments over different frequency bands, which shows significant improvement in sensitivity to SGWBs with the aid of a midband experiment. We consider GW emission from cosmic strings and phase transitions as benchmark examples of cosmological SGWBs. We explicitly model various astrophysical SGWB sources, most importantly from unresolved black hole mergers. Based on analysis using Markov Chain Monte Carlo, we demonstrated that midband experiments can, when combined with LIGO A+ and LISA, can significantly improve sensitivities to cosmological SGWBs and better separate them from astrophysical SGWBs
Patrick Draper (University of Illinois Urbana-Champaign) : de Sitter decays to infinity | 9/24/2021
The bubble of nothing is a gravitational instanton that can be thought of as describing tunneling through a vanishing scalar potential barrier to a vacuum at infinity. I will discuss generalizations of this process to nonzero scalar potentials with metastable de Sitter vacua. In simple cases, approximate bubble-of-nothing solutions can be constructed analytically. More generally, the problem can be formulated as a set of CdL equations with singular boundary
Oscar Varela (Utah State University) : Super-Chern-Simons spectra from Exceptional Field Theory | 10/1/2021
The spectrum of single-trace operators of holographic CFTs at strong coupling and large N can be mapped to the spectrum of Kaluza-Klein (KK) excitations over the dual AdS supergravity solutions. Unfortunately, computing these KK spectra is usually a prohibitively difficult task even for the simplest AdS solutions. In this talk, I will introduce new spectral methods based on Exceptional Field Theory that, for certain AdS/CFT dual pairs, bypass all difficulties and reduce the KK spectral problem to a simple diagonalisation of suitable mass matrices. I will illustrate these methods for a class of super-Chern-Simons CFT3s with AdS4 duals in M-theory and type IIA string theory.
Matthew McCullough (Cern) : Self-Organised Localisation | 10/6/2021
I will describe a new phenomenon in quantum cosmology: self-organised localisation. When the fundamental parameters of a theory are functions of a scalar field subject to large fluctuations during inflation, quantum phase transitions can act as dynamical attractors. As a result, the theory parameters are probabilistically localised around the critical value and the Universe finds itself at the edge of a phase transition. We illustrate how self-organised localisation could account for the observed near-criticality of the Higgs self-coupling, the naturalness of the Higgs mass, or the smallness of the cosmological constant.
Pedro Machado (Fermilab) : Quantum effects, neutrino masses and new oscillation phenomena | 10/22/2021
In this seminar I will discuss how quantum effects sourced by the neutrino mass mechanism can leave imprints on neutrino oscillations. More precisely, in the presence of relatively light new physics, the scale dependence of the mixing parameters can lead to observable effects in neutrino experiments. We discuss some of the experimental signatures of this scenario and present simple, UV complete models of neutrino masses which lead to observable running of the neutrino mixing matrix below the weak scale.
Chris Quigg (Fermilab) : Perspectives and Questions | 10/27/2021
Meditations on the future of particle physics.
Maxim Perelstein (Cornell) : Continuum Dark Matter | 11/5/2021
The microscopic nature of dark matter remains a major outstanding question in particle physics and cosmology, and many simple models are now in tension with experimental constraints such as direct detection bounds. In this talk, I will introduce the notion of “gapped continuum” in quantum field theories, and explain how it can be realized explicitly by a field propagating on a 5D warped space background with a soft wall. I will then describe how to calculate cross sections and decay rates involving gapped continuum states, as well as study their equilibrium and non-equilibrium thermodynamics. With this theoretical framework in hand, I will present a fully realistic model of dark matter consisting of gapped continuum states, interacting with the Standard Model via the Z portal. Direct detection bounds, which normally rule out Z portal interactions for thermal-relic dark matter, are avoided due to a strong suppression of the relevant cross sections by a mechanism peculiar to gapped continuum. Some interesting and potentially testable predictions of this model, such as continuous decays of the dark matter states throughout the history of the universe, as well as striking collider signatures, will also be discussed.
Igor Klebanov (Princeton) : Confining or Not? | 11/12/2021
The problem of Color Confinement in Yang-Mills theory is one of the deepest problems in theoretical physics. There is convincing numerical evidence from Lattice Gauge Theory, yet the proof of Confinement in Asymptotically Free theories has not been found. I will briefly introduce the Confinement problem and review some results on large N theories using the gauge/gravity duality. I will then discuss two-dimensional SU(N) theory coupled to an adjoint Majorana fermion. I will show that, when the adjoint mass is sent to zero, the spectrum retains a mass gap but the confinement disappears. Using the Discretized Light-Cone Quantization, I will discuss the spectrum of color singlet states and exhibit certain threshold states. Similar threshold states are also present in a model with a massless adjoint and a massive fundamental fermion. They provide new evidence for the lack of confinement. When the adjoint mass is turned on, the theory becomes confining, and the spectrum of bound states becomes discrete.
Gabriel Larios (IFT UAM-CSIC and LCTP) : Lessons and surprises from Kaluza-Klein spectra | 11/19/2021
Infinite towers of massive modes arise for every compactification of higher dimensional theories. Understanding the properties of these Kaluza-Klein towers on non-trivial solutions with an AdS factor has been a longstanding issue with clear holographic interest, as they describe the spectrum of single-trace operators of the dual CFTs at strong coupling and large N. In this talk, I will focus on two classes of solutions of such kind. The first class consists of AdS4 S-fold solutions of Type IIB supergravity that can be obtained from maximal gauged supergravity in D=4. For the later part, I will describe new families of solutions in N=(1,1) supergravity in D=6 which uplift from half-maximal supergravity in D=3. In both cases, the spectra can be computed using recent techniques from Exceptional Field Theory, and the information thus obtained leads to several unexpected conclusions.
Julio Parra Martinez (Caltech) : Bounds on effective theories of gravity | 11/30/2021
Modifications of Einstein’s theory of gravity can be systematically analyzed using the framework of effective field theory (EFT). In this setup, new physics is captured in a set of higher-dimension operators whose coefficients must be measured experimentally, or matched from a UV completion. It has been known for some time that basic principles, such as unitarity and causality, impose constraints on the allowed values of such coefficients, but a framework to exhaustively explore said constraints has only emerged recently. In this talk I will explain how developments in scattering amplitudes and the conformal bootstrap allowed us to compute sharp numerical bounds on these EFT coefficients. Our results have direct implications for the possibility of testing modifications of General Relativity using gravitational waves and in other astrophysical settings. In addition they also connect with the swampland program. Finally, I will briefly comment on the possibility of obtaining similar results for the Standard Model Effective Theory (SMEFT).
Geoffrey Compere ( Universite Libre De Bruxelles) : Kerr Geodesics and Self-consistent match between Inspiral and Transition-to-merger | 12/10/2021
The two-body motion in General Relativity can be solved perturbatively in the small mass ratio expansion. Kerr geodesics describe the leading order motion.
After a short summary of the classification of polar and radial Kerr geodesic motion, I will consider the inspiral motion of a point particle around the Kerr black hole
subjected to the self-force. I will describe its quasi-circular inspiral motion in the radiation timescale expansion. I will describe in parallel the transition-to-merger motion around the last stable
circular orbit and prove that it is controlled by the Painlevé transcendental equation of the first kind. I will then prove that one can consistently match the two motions using
the method of asymptotically matched expansions.
