Colloquia This Term

This year marks two decades since the theoretical discovery of the quantum spin Hall effect. It is exhibited in two-dimensional insulators with a topologically non-trivial band structure, which leads to the existence of helical edge states. The theoretical proposals were soon followed by an experimental discovery of the quantum spin Hall effect in HgTe/CdTe heterostructures. Since then, thousands of papers have been written on the subject, yet very few (perhaps only two) relevant materials have been discovered, and very few theoretical works have claimed to quantitatively explain experimental data. One such material, now established to exhibit the quantum spin Hall effect, is the monolayer tungsten ditelluride. In addition to the quantum spin Hall state, it hosts a correlated insulator ground state at low doping, as well as superconductivity at high enough electronic density. This material will be the main focus of the talk.

After a brief of the research pursued in my group, I will describe our efforts to explain experimentally observed singular linear and nonlinear magnetotransport on the helical edge of the monolayer tungsten ditelluride. I will discuss a model of bulk midgap states "side-coupled" to the edge which appears to account for many experimental features, particularly the dependence of various transport coefficients on the direction of the external magnetic field, as well as their singular dependence on its magnitude.

Gravitational waves are distortions in the fabric of space and time that are a feature of general relativity, and which were detected one-hundred years after their prediction. The Advanced LIGO and Virgo detectors have observed gravitational waves from nearly one-hundred collisions of black holes and neutron stars, and they are detecting a comparable number during their ongoing observing run. In this talk, I will review the status of these gravitational-wave detections and discuss their implications for understanding fundamental physics through two examples. The first case pertains to the gravitational-wave memory effect: a lasting change in the gravitational-wave strain that is closely connected to the infrared properties of gravitational scattering. LIGO and Virgo will be able to make a statistical measurement of the memory effect towards the end of this decade. I will also discuss new generalizations of the memory effect and their gravitational-wave signatures. The second case relates to using gravitational waves to study dark matter. When a stellar-mass black hole inspirals into a massive black hole surrounded by a high dark-matter density, the dark matter will influence the inspiral of lighter black hole and the gravitational waves emitted from such a binary. I will show how the planned space-based gravitational-wave detector LISA can detect these gravitational waves and measure the presence of dark matter in these binaries.

The Higgs Boson discovery in 2012 required us to think differently about planning for the future of Particle Physics twice now, since that summer 12 years ago. While the decades-long confirmation of the Standard Model itself is a historic episode – as a guide to the future,  it’s not as helpful as one would like because the Standard Model is complete! I will describe Particle Physics planning in the US, review the features of the Standard Model that make it superb, point out why it’s frustrating, and describe hints that continue to motivate us for the coming decades.

Despite its unparalleled successes, the Standard Model (SM) of elementary particles and their interactions is known to be incomplete. Additional particles and phenomena must exist. I will report on a few precision experiments which my group participated in. Their common goal is to search for physics beyond the Standard Model (SM). The Nab experiment just started to take neutron beta decay data, with goals to (a) confirm or refute current evidence for the violation of unitarity of the quark mixing matrix, and (b) to search for scalar and tensor interactions. The whispering gallery experiment is investigating quantum states of neutrons in close vicinity of a curved mirror that are sensitive to new short-range interactions in the range of nanometers which could be mediated by light scalar or pseudoscalar bosons. The MUonE experiment investigates the apparent failure of the SM to correctly predict the magnetic moment of the muon by observing the running of the fine structure constant in muon-electron scattering.

The SpinQuest experiment (Fermilab E1039) is designed to explore one of the fundamental questions in hadronic physics: the contribution of sea quarks to the nucleon’s spin. Utilizing a 120 GeV unpolarized proton beam directed at a polarized proton and deuteron target, SpinQuest aims to measure the Sivers function, which describes the correlation between the momentum direction of the struck quark and the spin of its parent nucleon. By focusing on both Drell-Yan and J/ψ production processes, SpinQuest seeks to provide critical insights into the spin and momentum structure of the nucleon, particularly the contribution of antiquarks to its overall spin. I will present the motivation behind the experiment, including the importance of the Sivers function and how SpinQuest can advance our understanding of spin-dependent phenomena in QCD. I will also discuss the experimental design and progress, highlighting recent milestones such as the successful commissioning of the polarized target system and the initial polarized data collection.  I will review ongoing analysis and the roadmap toward extracting the Sivers asymmetry. Additionally, I will outline the experimental challenges faced, future plans for data collection, and the expected contributions of SpinQuest to the global understanding of nucleon structure.

Electron scattering has been a tool of choice for the precision study of nucleon and nuclear structure since the pioneering, Nobel Prize-winning investigations by Robert Hofstadter and collaborators at Stanford during the 1950s. Over the ensuing decades, as accelerator capabilities increased, such experiments generated a wealth of knowledge of the rich and complicated structure of nucleons and nuclei, answering many questions and raising many more. Owing largely to the understanding gained from electron scattering, nucleons are now understood to consist of elementary quarks and gluons confined by strong interactions, described within the Standard Model by Quantum Chromodynamics (QCD). The Continuous Electron Beam Accelerator Facility at Jefferson Lab (JLab), with its medium energy, its unrivaled intensity, duty cycle and polarization performance, and its four experimental halls with complementary target and detector capabilities, is uniquely suited to the investigation of strong interaction physics at the luminosity frontier. Among the most famous results in the history of JLab is the surprising discovery of the rapid decrease with four-momentum-transfer Q2 of the proton’s electromagnetic form factor ratio G p E /Gp M using the polarization transfer method. These measurements upended long-held notions about the proton’s internal structure, revealed the importance of quark orbital angular momentum in understanding the origins of the proton’s spin, and exposed the limits of applicability of the one-photon-exchange approximation in the interpretation of electron scattering data. The technical challenges involved in extending the Q2 reach and precision of these measurements motivated the development of medium-acceptance, open-geometry spectrometers capable of precision charged-particle tracking in an environment of extremely high luminosity and background rate. The detector capabilities required to measure the proton form factor ratio at very large Q2 enable a large, ambitious physics program including high-Q2 proton and neutron form factor measurements, single-spin asymmetries in semi-inclusive deep inelastic scattering, measurements of the pion structure function using tagged deep inelastic scattering, and more. The collection of apparatus developed to carry out this ambitious program is collectively referred to as the Super BigBite Spectrometer (SBS). After 14 years of development, the SBS program started in JLab’s Hall A in 2021. Three major experiments measuring neutron form factors at large Q2 have already been completed, while the “flagship” measurement of G p E /Gp M is scheduled to run in 2025. In this colloquium, I will give an overview of the SBS apparatus, science program, and technical achievements, the data analysis status of completed experiments, and the outlook for completing the remaining approved physics program. Time permitting, I will also discuss some potential future opportunities taking advantage of the unique capabilities of SBS.

TBA

Black holes are utterly simple to describe---regions of space from which nothing can escape---but nevertheless have profound implications across widely disparate fields of physics.  The study of quantum effects near black holes challenges our most basic notions of quantum information, while observational astronomy demands black holes as an energizing power source for a huge variety of spectacular displays.  After reviewing the history of the black hole idea and describing their role in modern astronomy and quantum gravity, I will turn to two recent developments in the two fields.  On the astronomy side, I will discuss black hole imaging and a proposed space mission, the Black Hole Explorer, that will measure light that has orbited the black hole before arriving at the detector.  On the fundamental physics side, I will discuss the recent discovery that black holes decohere all quantum superpositions, even those held far outside the horizon.  These topics are united by the remarkable conceptual depth that emerges from the simple underlying description of a black hole.

Over the last few decades physicists have delved deeply into student learning and applied this understanding to create curricula and environments that dramatically improve student outcomes that: increase student mastery of the subject, broaden who participates in and succeeds in physics, and advance pathways to success for our major.  More recently, we have built on these successes of knowing what to do by developing novel tools, technologies, and models for how we might create sustainable and scalable approaches. In this talk, I discuss examples of educational innovations that have dramatic impacts, the promises and pitfalls of new technologies (including generative artificial intelligence), and how we, at the departmental scale, might systematically build these improvements into our programs. 

TBA

Do you want to have a career in physics?  Do you want to help others find a career in physics?  Do you want to be a better mentee or mentor?  If you answered “yes” to one of those questions, this presentation is for you! Physics degree holders are highly employable in both the private and public sectors. However, students, early career scientists, and even senior faculty are often unaware of the available career paths. This talk will provide data on the number of physics degree holders and where they typically find employment, give examples of common career paths, and highlight career resources useful for exploring options and searching for and applying for jobs. The importance of mentorship along the career trajectory will be highlighted.  Tips will also be provided on finding mentors and serving as a mentor.  The speaker will also discuss his career and mentee/mentorship path.  Undergraduates are encouraged to attend.