Yu-Dai Tsai (Cornell): "Respect the ELDER: New Thermal Target for Dark Matter Direct Detections and Going Beyond with Astrophysical Signatures" | 9/13/17
A less explored procedure for a thermal relic to reach its current abundance is that it first elastically (thermally) decouples from the relativistic species before it freezes out from the number-changing processes. Here we present a novel dark matter (DM) candidate, an Elastically Decoupling Relic (ELDER), which is a thermal relic whose present-day abundance is determined by the cross-section of its elastic scattering on Standard Model particles, based on the aforementioned procedure.
Assuming that this scattering is mediated by a kinetically mixed dark photon, the ELDER scenario makes robust predictions for electron-recoil direct-detection experiments, as well as for dark photon searches. These predictions are independent of the details of interactions within the dark sector. The ELDER predictions provide a target region that will be almost entirely accessible to the next generation of searches for sub-GeV dark matter and dark photons.
If time permits, I will talk briefly about optical, gravitational, and radio signatures of DM-induced neutron star (NS) Implosions. The Astrophysical signatures (NS-NS mergers included!) are ways to go orders beyond the DM direct-detection limits.
This talk is based on Phys. Rev. Lett. 116, 221302 (arXiv:1512.04545), JHEP, 08:078, 2017 (arXiv:1706.05381), and arXiv:1706.00001
Stefano Massai (UChicago): "Worldsheet CFTs for microstate geometries" | 9/20/17
In string theory, black hole microstates at finite coupling give rise to horizon-scale structures that might solve the information paradox. Explicit constructions of these solutions are based on configurations of branes puffed up by the supertube effect. In an appropriate duality frame, we can construct the simplest supertube by adding momentum to a symmetric distribution of NS5 branes on their Coulomb branch. This suggests an exact worldsheet description of the supertube as a null gauged Wess-Zumino-Witten model. Such exact treatment in worldsheet string theory also describes BPS and non-BPS three-charge microstate geometries. This construction reveals stringy structures that are invisibile in the supergravity approximation, and that play a crucial role in understanding the constituents that carry most of the entropy.
Andrew J. Long (UChicago): "Topics in Axion Cosmology" | 9/27/17
(Which topics? The interesting ones!) Light pseudo-Goldstone bosons arise in many compelling models of particle physics and string theory. These particles can modify cosmology in interesting, surprising, and testable ways. In this talk I will discuss three topics. First I’ll argue that the dynamics of an axion field during inflation can give rise to the matter / antimatter asymmetry of the universe via the production of helical magnetic fields (which persist today and might be detectable!). Second, I’ll discuss what goes wrong when you try to implement the same idea with chiral gravitational waves instead of magnetic fields. Finally, I’ll talk about new phenomena that arise in multi-axion models, such as the recent proposed `clockwork’ axion, that have a large hierarchy between the scale of PQ-breaking and the axion decay constant.
Alex Streicher (UCSB): "Operator Dynamics in Quantum Chaos: Part I - Internal Degrees of Freedom" | 10/4/17
We study operator growth in quantum chaos by considering the SYK model, a toy model of holography containing only internal degrees of freedom which evolve via q-local interactions. First, we note that the product length of an operator is directly related to its sensitivity to small perturbations. This reveals that in the SYK model, the commutator-squared/out-of-time-ordered correlator - a new diagnostic of quantum chaos - is literally measuring the effective length of the operator. It is known that this quantity grows exponentially in time with a "Lyapunov exponent", and thus we conclude that lengths of operators grows exponentially in time (amongst the internal degrees of freedom). Motivated by this, we group the operators into families defined by their lengths, thereby explicitly solving for the coarse-grained dynamics of an operator in the large N, large q limit. We also note that one can understand the time evolution of operators by relating it to the quantum mechanics of a particle on a graph with a nontrivial topology. Lastly, we may make some comments on the bulk interpretation of operator growth in SYK.
Gordon Semenoff (UBC): "SOFT PHOTONS, SOFT GRAVITONS AND DECOHERENCE" | 10/11/17
Central to the solution of the infrared catastrophe of quantum electrodynamics and perturbative quantum gravity is the idea that detection apparatus inevitably have limited resolution and, in any scattering process, an infinite number of arbitrarily soft photons and gravitons are produced and escape detection. Photons and gravitons have polarizations and momenta and one might suspect that those which escape can carry away a significant amount of information. In this talk, I will examine the question as to the quantity of this information loss, its consequences.
Andrew Mcleod (Stanford): "Extended Steinmann Relations and Cosmic Galois Theory in Planar N = 4" | 10/25/17
While traditional methods for calculating scattering amplitudes prove too computationally intensive to be useful at higher loop orders, a great deal is now known about the analytic and kinematic properties of amplitudes to all orders in planar N=4. This information can be leveraged to construct these amplitudes directly, by putting together an ansatz of the relevant class of functions and requiring that it share the distinctive properties of a given amplitude. In this talk, I will describe how this bootstrap-type approach can be used to uniquely determine all six-particle amplitudes in this theory through (at least) five loops, focusing on how these methods make transparent the Steinmann relations and bear out the predictions of cosmic Galois theory. I will also discuss how these methods can be generalized to quantities of direct relevance to particle physics experiments.
Nick Llewellyn Rodd (MIT): " Searching for Dark Matter in Distant Galaxies" | 11/1/17
Galaxies beyond our own represent some of the brightest potential sources of dark matter flux on the sky. As such they represent excellent candidates for indirect detection and in this talk I will demonstrate how to exploit this information to search for dark matter using the Fermi telescope. In particular I will outline how to map from an observed baryonic galaxy to its underlying dark matter distribution and a demonstration that our methods work in a simulated N-body environment.
Robert Lasenby (Perimeter): "Searching for weakly-coupled particles: from stars to colliders" | 11/8/17
Many theories of beyond Standard Model physics include new light, weakly-coupled particles, which can be challenging to search for experimentally. I will talk about two observational probes of such particles. The first is based on “stellar cooling”: if new particles are produced in the hot cores of stars, they can escape from the star and carry away energy, affecting its structure and evolution. I’ll describe how the plasma environment in stellar cores can parametrically alter the rates for these process, and how this can significantly change the constraints and discovery potential for some new particle candidates.
I will also discuss searches for light vectors at colliders. Unless these couple to a fully conserved SM current, the production rate for longitudinal modes is enhanced by (energy / vector mass)^2. This is true even if the current is only broken at loop level, as for anomalous vectors, and can result in significantly improved constraints on many models of phenomenological interest.
Nirmal Raj (Notre Dame): "Dark Fires in the Sky: Model-Independent Dark Matter Detection via Kinetic Heating of Neutron Stars" | 11/29/17
I present a largely model-independent probe of dark matter-nucleon interactions. Accelerated by gravity to relativistic speeds, local dark matter scattering against old neutron stars deposits kinetic energy at a rate that heats them to infrared blackbody temperatures. The resulting radiation is detectable by next generation telescopes such as James Webb and the Thirty Meter Telescope. While underground direct detection searches are not (or poorly) sensitive to dark matter with sub-GeV masses, higher-than-weak-scale masses, scattering with strong cross-sections, scattering below neutrino floors, spin-dependent per-nucleon scattering below per-nuclear cross-sections, velocity-dependent scattering, and inelastic scattering for inter-state transitions exceeding O(100 keV), the (non-)observation of dark kinetic heating of neutron stars should advance these frontiers by orders of magnitude. Popular dark matter candidates previously suspected elusive, such as the thermal Higgsino, may be discovered.
Cheng Peng (Brown): "From higher spins to generalized SYK models" | 12/6/17
The spectrum of the Sachdev-Ye-Kitaev (SYK) model consists of an infinite tower of operators, which resembles the spectra of various vector models that are holographically dual to higher spin gravity theories. In this talk, I will discuss a direct connection between SYK-like tensor models and the Gross-Neveu vector model. This is achieved by studying a toy model where a tensor field is coupled with some vector fields. By integrating out the tensor field, the toy model reduces to the Gross-Neveu model in 1 dimension. At a different corner of the moduli space of this toy model, a perturbation can be turned on and the toy model flows to an SYK-like model at low energy. In addition, a chaotic-nonchaotic phase transition is observed as the sign of the perturbation is altered. If time permitted, I will briefly discuss some aspects of supersymmetric SYK-like models.
