PGSS 2025

PGSS Invited Faculty Lecture: Liquid-noble detectors as tools for dark matter and neutrino physics

Scott Haselschwardt

Assistant Professor, Department of Physics

Understanding the particle nature of dark matter is one of the most pressing tasks of modern science, motivating numerous theoretical dark matter candidates and experimental efforts to detect them. Chief among these are direct-detection experiments which use liquified noble gasses as their detection medium. In this talk I will describe direct searches for weak scale dark matter particles, highlighting in particular the LUX ZEPLIN (LZ) experiment, which uses liquid xenon as its detection medium. I will then discuss future efforts aimed at detecting dark matter lighter than the proton, including the development of a low threshold detector based on quasiparticle excitations of a superfluid helium-4 target (HeRALD). Taken together, these experiments directly probe an entire class of extremely well-motivated models known as “thermal relics”: dark matter particles that were once in thermal equilibrium with ordinary matter in the early Universe.

Cancelled (as of 6/5/25)

A Computationalist's P.O.V.: Hierarchical and Dynamical Control of Colloidal Systems

Kate Jensen

Second Year, Department of Physics

Biological materials possess remarkable capabilities unmatched by most synthetic systems, including the ability to dynamically alter their function, shape, or color in response to external stimuli such as temperature, chemical gradients, mechanical forces, or electrical signals. Replicating these adaptive behaviors in engineered materials holds tremendous potential for creating programmable, reconfigurable, and hierarchically structured systems with applications spanning photonics, display technologies, and soft robotics. For the sake of this talk, we will limit our discussion to our ability to mimic four major desirable properties of biological materials that lead to this type of desirable phenomenon: (1) responsiveness to external stimuli, (2) intrinsically hierarchy, (3) complex collective behavior, and (4) use dynamical processes.

This presentation highlights two ongoing computational research efforts, conducted in close collaboration with experimental partners, to prototype synthetic materials that exhibit biologically inspired behaviors at the colloidal scale. The first, the “Oleg MURI Reconfiguration” project, focuses on coarse-grained simulations of reconfigurable, hierarchical DNA lattices. The second, the “Flexicle” project, centers on modeling the mechanics and dynamics of cell-like synthetic structures that mimic key features of biological membranes. In both projects, theory and simulation are deeply integrated with experimental design and validation, enabling iterative development of materials that capture the complexity of biological systems. While the systems of interest for both projects are vastly different, each exemplifies the four core characteristics of adaptive biological materials and contributes to the advancement of tunable, multifunctional soft robotic matter at the colloidal scale.

June 5th, 2025 --- 12:00 PM Eastern --- West Hall 335

Optically Manipulating the Interaction Between Electron and Nuclear Spins in Solid State Systems

Estefanio Kesto

Third Year, Department of Physics

Everything around us is made up of compounds – but remember, compounds are made of elements, and elements are made of atoms! When you zoom all the way into the guts of an atom, you’ll find they’re made up of electrons energetically racing around a nucleus. In our group, we’re interested in how the electrons and nuclei interact in a semiconducting compound known as gallium arsenide (GaAs). Our lab has the capability of exciting, manipulating, and interrogating the electrons’ behavior using a pulsed laser system. What we specifically excite is something known as electron spin polarization, and it turns out that we can tailor our optical scheme to force the electrons to interact with the nuclei through a process known as dynamic nuclear polarization (DNP). Conventionally, electron and nuclear spins interacting through DNP by a pulsed laser have periodic properties. However, in a recent experiment, we have found that this periodicity can be manipulated using unconventional methods. In this talk, I’ll first introduce some semiconductor concepts in addition to the conventional optical techniques utilized to generate these effects. Following the introduction, we will jump into the unconventional way our lab recently used to manipulate the electron-nuclear spin interactions. 

Together, we will be exploring the relationship between optically excited electron and nuclear spins!

June 12th, 2025 --- 12:00 PM Eastern --- West Hall 335

No seminar on June 19th, 2025 in observance of Juneteenth

Effective Field Theory of Modified Gravity: Initial Conditions and Cosmological Constraints 

Jiaming Pan

Third Year, Department of Physics

Recent cosmological observations — including data from DESI baryon acoustic oscillation and the cosmic microwave background — suggest that dark energy is not behaving as a cosmological constant but may be evolving with time. In this talk, I will show how these phenomena can be interpreted as a hint for modifications to Einstein’s theory of gravity on cosmological scales. In particular, I will present a ~3 sigma indication that gravity is non-minimally coupled to matter in the latest observations using model-independent analysis. This result represents a test of modified gravity in the late universe.

To extend such tests to the early universe, however, it is essential to model cosmological initial conditions accurately. I will show that, in modified gravity models, the standard initial conditions derived from general relativity may no longer be valid, and using them can lead to significant biases in cosmological predictions, including the cosmic microwave background. I will present how to derive and implement consistent initial conditions for modified gravity models, and illustrate their impact on cosmological observables.

June 26th, 2025 --- 12:00 PM Eastern --- West Hall 335

Measuring the Missing Energy of Neutrinos Due to Nuclear Interactions 

Cassandra Little

Fifth Year, Department of Physics

As modern neutrino experiments become increasingly precise, a better understanding of neutrino interaction physics becomes a necessity for controlling uncertainties. Kaon decay at rest (KDAR; $K^+\rightarrow\mu^+\nu_\mu$) produces a monoenergetic neutrino with a known energy (235.5 MeV) which could be used to observe the energy lost in a neutrino-nucleus interaction. The J-PARC Sterile Neutrino Search at the J-PARC Spallation Neutron Source experiment (JSNS²) observes the monoenergetic neutrino interaction $\nu_\mu^{12}C\rightarrow\mu^{- 12}N$ or $\nu_\mu n\rightarrow\mu^- p$ and defines the missing neutrino energy as the energy that is transferred to the nucleus minus the kinetic energy of the outgoing proton. Naively, this missing energy is expected to be zero, but nuclear interactions and processes (e.g. nucleon separation energy, Fermi momenta, and final-state interactions) are uniquely reflected. The shape only, differential cross section is presented from 621 events, based on a 77 ± 3 % pure double coincidence KDAR signal. This measurement provides detailed insight into the neutrino-nucleus interaction, even allowing the nuclear orbital shell of the struck nucleon to be inferred. This measurement provides an important benchmark for models and event generators in the hundreds of MeV region, a difficult to model area that transitions between neutrino-nucleus and neutrino-nucleon scattering. It additionally benefits neutrino oscillation measurements, nuclear physics, and Type-II supernova studies.

July 3rd, 2025 --- 12:00 PM Eastern --- West Hall 335

Collinear Multidimensional Coherent Spectroscopy with Imaging on Atomically Thin Transition Metal Dichalcogenides

Blake Hipsley

Sixth Year, Department of Physics

Transition-metal dichalcogenides (TMDs) have promising for applications in atomically thin photonics and quantum information due to their efficient light-matter coupling and tightly bound excitons (bound electron-hole pairs) that dominate the optical processes of semiconductors. Understanding the fundamental physics behind exciton dynamics and how to control their interactions is essential for engineering quantum devices. To study these materials, I utilize multidimensional coherent spectroscopy (MDCS), a form of nonlinear spectroscopy that correlates different features of a material’s optical response and reveals information to the dephasing rate of excitons. After introducing the material and technique, I will show the results and capabilities of MDCS, especially when combined with a laser-scanning microscope to produce nonlinear images. To finish, I will be reporting the effects of electron-doping on excitons in monolayer MoSe2 and show in a doped regime there is a clear saturation, quenching, and broadening of the exciton resonances with increasing excitation density, indicating doping modifies the exciton interactions.

July 10th, 2025 --- 12:00 PM Eastern --- West Hall 335

Cosmology with Gravitational Wave Standard Sirens: A Multi-Messenger View of Stellar Mergers

Simran Kaur

Third Year, Department of Physics

Gravitational-wave standard sirens are compact binary mergers that offer a direct measurement of luminosity distance, providing a powerful and independent method to probe cosmological parameters, most notably the Hubble constant (H₀). This talk will introduce the methodology of cosmological inference using both bright sirens (with identified electromagnetic counterparts) and dark sirens (statistically associated with galaxy catalogs), and discuss how they contribute to precision cosmology.

The success of this approach relies on a coordinated effort between gravitational wave detectors (such as LIGO and Virgo) and wide-field optical survey telescopes like the Dark Energy Camera (DECam) and the upcoming Vera C. Rubin Observatory. The Dark Energy Survey–Gravitational Waves (DESGW) program exemplifies such coordination by enabling timely electromagnetic follow-up of GW events and contributing to early standard siren measurements.

Looking ahead, simulation efforts are underway to forecast the cosmological power of the Rubin Observatory’s LSST, including expected constraints on H₀. These projections highlight the exciting potential of next-generation surveys to transform gravitational-wave cosmology in the coming decade.

July 17th, 2025 --- 12:00 PM Eastern --- West Hall 335

Statistical physics of inferring network structure

Max Jerdee

Fifth Year, Department of Physics

Many systems across science can be meaningfully represented as networks of simple interactions. Within networks of metabolic pathways, transportation links, neuronal connections, outcomes of sports matches, and social ties, the patterns and directions of data often exhibit collective behavior. In this talk we discuss how these statistical structures can be understood from a physical perspective. We focus on characterizing two common features of these networks: community structure and hierarchy. We demonstrate how the Ising model can be used to find friend groups, and define a "temperature" of hierarchies that measures their strictness. From these perspectives, we find larger variation in group structures than assumed in existing models and observe that social hierarchies tend to be more stratified than those found in sports and games.

July 24th, 2025 --- 12:00 PM Eastern --- West Hall 335

ATP-Dependent Free Energy Dissipation Governs Mitotic Oscillator Precision

Liam Yourston

Fourth Year, Department of Biophysics

The precise regulation of the cell cycle during early embryonic development relies on a tightly controlled mitotic clock centered around the cyclinB-Cdk1 complex. Like information processing and many other biological processes functioning in spite of noise, the precision of a far-from-equilibrium oscillatory system like the cell cycle is energetically costly. Theoretical studies have suggested a quantitative relationship between oscillator precision and energy dissipation. Using cell-sized, water-in-oil microemulsion droplets to encapsulate Xenopus Laevis egg extracts with fully intact mitotic network components we observe self-sustained oscillations. By systematically manipulating ATP levels in each droplet, we discovered a non-monotonic response in the oscillator's speed and an increase in oscillator precision. These insights shed light on the complex coupling between the free energy budget and the mitotic oscillator performance, suggesting that an optimal energy budget is required to promote fast, yet precise mitotic cycles while constrained by energetic efficiency.

July 31st, 2025 --- 12:00 PM Eastern --- West Hall 340

Measurement of the Xenon-129 EDM

Henry Sottrel

Second Year, Department of Physics

A nonzero atomic Electric Dipole Moment (EDM) would be a signal of CP-violating physics beyond the standard model. Xenon-129 is a promising isotope for a possible EDM discovery due to its large atomic number and the ease of polarizing samples of 129-Xe gas. The 129Xe EDM collaboration at Los Alamos National Laboratory aims to conduct the most accurate measurement of the 129Xe EDM. To conduct the measurement, Xenon-129 and Helium-3 are polarized via spin exchange with a Rubidium vapor, and the Xenon precession frequency is measured under a large electric field. Improving the sensitivity of the measurement requires the development of several innovations, including a system for circulating polarized gas during measurement and a systematic study of the process of electrical breakdown within our gas mixture.

August 7th, 2025 --- 12:00 PM Eastern --- West Hall 335

Renormalization of Complex Networks

Connie Sheeran

First Year, Department of Physics

A longstanding challenge in statistical physics and complex systems is extending renormalization group (RG) methods beyond regular lattices to the richly heterogeneous topologies of real-world networks. Traditional RG techniques rely on translational invariance and uniform coarse-graining, leaving critical questions about phase transitions, universality, and multiscale dynamics in (e.g. neural) networks unanswered. This talk introduces a unified, exact real-space RG framework for the Ising model on arbitrary graphs, bridging the gap between microscopic interactions and emergent macroscopic behavior. Monte Carlo simulations validate the derived flow equations on one-dimensional chains, Bethe lattices, random regular graphs and Erdős-Rényi graphs. The resulting toolkit establishes a foundation for discovering novel universality classes and for controlling dynamical processes on complex networks. Future work is also presented.

August 14th, 2025 --- 12:00 PM Eastern --- West Hall 335

Interested in signing up to give a PGSS talk? Applications for 2025 are closed, but be on the lookout for the 2026 application next spring!

(Note: You must be a graduate student at the University of Michigan Ann Arbor to be eligible)