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Spring 2022

February 25, 2022 (Friday) 4:00-5:00p.m. 
Speaker: Julian Heeck, University of Virginia
Host: M. Sher
Title: Does matter decay?
Abstract: Life in our universe is only possible due to the stability of matter, notably that of electrons, protons, and bound neutrons. Particle-physics experiments provide lower bounds on the lifetimes of these typically-assumed-to-be-stable particles that greatly improve on such anthropic arguments and far exceed the age of our universe itself. We will discuss the experimental signatures of decaying matter and survey if such decays are allowed or even preferred from a theoretical perspective.

March 4, 2022 (Friday) 4:00-5:00p.m. 
Speaker: Prof. Yafis Barlas, University of Nevada Reno
Host: E. Rossi
Title: Topological Phases and Hofstadter butterflies in two-dimensional crystals  
Abstract: Large-scale interference Moiré patterns emerge when two-dimensional (2D) crystals are rotated at relative angles, generally referred to as Twistronics. Twistronics modifies the electron’s kinetic energy, and at some “magic angles’', the kinetic energy of the electrons becomes negligible compared to their mutual electron-electron interactions. Strong interactions in flat bands result in strongly correlated and topologically ordered states owing to the interplay of topology and electron correlations. In this talk, I will discuss our recent studies of interactions and topological phases in two-dimensional crystals. First, I will discuss a new class of interacting and non-interacting symmetry-protected topological phases stabilized by mirror symmetry in ABA-stacked trilayer graphene. This quantum parity Hall state exhibits two one-dimensional counter-propagating metallic edge states, distinguished by even or odd parity under the system’s mirror reflection symmetry. Then, I will discuss electron dispersion in twisted 2D crystals at high magnetic fields, which results in fractal Hofstadter butterfly patterns in the energy spectrum. Finally. I will discuss the topological properties of Hofstadter bands as a function of twist angles and magnetic fields. 

April 1, 2022 (Friday) 4:00-5:00p.m. 
Speaker: Andrei Afanasev, GWU
Host: I. Novikova
Title: Peculiar Quantum Features of Optical Vortices
Abstract: “Optical vortex” is a term describing photon beams with helical wavefronts, which quantized version is known as “twisted photons”. They are known for their ability to carry large values of angular momentum along the direction of propagation. In this presentation, history of optical vortices will be reviewed and their interactions with a variety of quantum systems will be discussed. We will demonstrate new quantum selection rules for absorption of these photons (that were recently confirmed experimentally with cold trapped ions); quantum ``superkick” effects; optical activity in chiral and non-chiral matter; and novel optical polarization phenomena with atoms and quantum dots. The talk is aimed at a broad audience of researchers and graduate students that do not specialize in this field.

About the speaker:
Andrei Afanasev is an Endowed Professor of Theoretical Physics with George Washington University in Washington, DC, USA. He received his PhD (1990) in Nuclear and Particle Physics in Kharkiv, Ukraine in Theory Division of Kharkiv Institute of Physics and Technology headed by A.Akhiezer (formerly by Lev Landau). His research in quantum electrodynamics of electron scattering brought him to Jefferson Lab (USA) in 1993, where he worked till 2011, until he accepted a faculty position with GWU. He has over 150 published papers in quantum electrodynamics, nuclear physics, quantum optics, accelerator physics, dark-matter searches, and condensed matter physics.

April 8, 2022 (Friday) 4:00-5:00p.m. 
Speaker: Ed Barnes, Virginia Tech
Host: E. Rossi
Title: Time crystals and quantum computation
Abstract: Quantum nonequilibrium phases of matter can exhibit a variety of phenomena that have no counterpart in equilibrium settings. One such phase, known as a discrete time crystal, arises when a periodically driven system spontaneously breaks discrete time-translation invariance. In this phase, the quantum many-body state becomes insensitive to disorder and driving errors, provided interactions are sufficiently strong. I will describe the history and main concepts behind time crystals and related phases, present new examples of such phases, and show how they can be used to improve the performance of quantum computational tasks in quantum dot spin arrays.

April 15, 2022 (Friday) 4:00-5:00p.m. 
Speaker: Craig Group, University of Virginia
Host: P. Vahle
Title: Expanding the search for dark matter with new accelerator-based experiments
Abstract: The evidence for dark matter is strong.  However, the constituents of dark matter are still unknown, and the viable possibilities span a very large mass range. Specific scenarios for the origin of dark matter sharpen the focus on a narrower range of masses:  the natural scenario where dark matter originates from thermal contact with familiar matter in the early Universe requires the DM mass to lie within about an MeV to 100 TeV. Considerable experimental attention has been given to exploring Weakly Interacting Massive Particles in the upper end of this range (few GeV – ~TeV), while the region ~MeV to ~GeV is largely unexplored. Most of the stable constituents of known matter have masses in this lower range, tantalizing hints for physics beyond the Standard Model have been found here, and a thermal origin for dark matter works in a simple and predictive manner in this mass range as well. It is therefore a priority to explore. If there is an interaction between light DM and ordinary matter, as there must be in the case of a thermal origin, then there necessarily is a production mechanism in accelerator-based experiments. The most sensitive way to search for this production (if there is a coupling to electrons) is to use a primary electron beam to produce DM in fixed-target collisions. The Light Dark Matter eXperiment (LDMX) is a planned electron-beam fixed-target missing-momentum experiment that has unique sensitivity to light DM in the sub-GeV range. I will give an overview of the theoretical motivation, the main experimental challenges and how they are addressed, as well as projected sensitivities in comparison to other experiments.