Event Time:
Sunday, May 25, 2025 | 1:00 pm - 3:30 pm
Event Location:
HENN 318
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2025-05-25T13:00:00
2025-05-25T15:30:00
Zero-energy Modes in Quantum Field Theories
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Abstract:
We discuss three instances where zero-energy or soft modes appear in quantum field theory.
First, we examine massless fermions in a 2+ 1 dimensional system with a spatial boundary, specifically graphene in half-space. Two boundary conditions and their interplay with the discrete and continuous symmetries of the system are analyzed. For doubled fermions, we identify a special case that respects CP T symmetry but breaks Lorentz and conformal symmetry, featuring fermion zero mode edge states. These edge states lead to unconventional representations of scale, phase, and translation symmetries, and enforcing symmetry constraints results in edge ferromagnetism.
Second, we investigate the infrared structure of a massless scalar theory coupled to fermions. We demonstrate the existence of a field theory containing massless scalar particles that mirrors the infrared structure of quantum electrodynamics and perturbative quantum gravity but lacks gauge invariance, internal symmetries, or apparent asymptotic symmetry. Unlike soft photons and gravitons, soft scalars do not decouple from dressed states and are generally produced during interactions of hard dressed particles, though their entanglement is minimal.
Lastly, we develop a novel method to calculate changes in an operator’s expectation value at asymptotic times, relevant to gravitational wave observations, by exploiting its soft limit. We derive a formula for asymptotic in-in observables from the soft limit of five-point amputated response functions. Using this, we re-derive the KMOC formulas for linear impulse and radiated momentum during scattering and provide an unambiguous calculation of radiated angular momentum at leading order. We introduce a causal method of computing classical observables using the Schwinger-Keldysh formalism.
Event Location:
HENN 318
Event Time:
Tuesday, May 27, 2025 | 8:00 am - 10:00 am
Event Location:
BC Cancer Research Agency (675 W 10th Ave, Vancouver, BC V5Z 0B4), Boardroom first floor
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2025-05-27T08:00:00
2025-05-27T10:00:00
Advancing Quantitative Dosimetry SPECT with Open-Source Image Reconstruction, Uncertainty Estimation, and Image Generation Optimization
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Abstract:
Over the past decade, radiopharmaceutical therapies have demonstrated considerable potential in cancer treatment. Notably, the success of the NETTER-1 and VISION clinical trials led to FDA approval of Lu-177, a beta-emitting isotope, for treating neuroendocrine tumors in 2018 and prostate cancer in 2022. Coinciding with these advancements, there has been growing interest in exploring treatment outcomes using alternative isotopes like the alpha-emitter Ac-225, which may offer enhanced therapeutic benefits. Many therapeutic isotopes also emit photons that, while not directly contributing to therapy, can be detected using SPECT imaging. This enables concurrent delivery and evaluation of patient absorbed dose: a practice that is well-established in the field of external beam radiotherapy. Although current radiopharmaceutical treatment protocols use a standard "one-size-fits-all" approach whereby all patients receive the same injected activity, it is conjectured that image-based dosimetry can be used to tailor dosimetry on an individual basis and consequently improve treatment outcome. One of the major challenges of dosimetry is minimizing and accounting for the presence of bias and uncertainty in acquired SPECT images.
This thesis contains a collection of studies aimed at improving SPECT image quality and interpretability via improvements and modifications to existing image reconstruction protocols. Chapter 2 of the work describes the development of the open-source medical imaging software PyTomography, which enabled the subsequent innovations of this work. Chapter 3 derives a collimator detector response model for SPECT reconstruction of high energy photons, such as those emitted by the daughters of Ac-225. Chapter 4 outlines a modification to existing reconstruction algorithms to permit uncertainty estimation in medical images and subsequently in image-based dosimetry. Chapter 5 explores the optimal image acquisition and reconstruction parameters for Ac-225 imaging, and Chapter 6 explores Monte Carlo based reconstruction techniques to further improve image quality.
Event Location:
BC Cancer Research Agency (675 W 10th Ave, Vancouver, BC V5Z 0B4), Boardroom first floor
Event Time:
Friday, May 30, 2025 | 12:00 pm - 2:30 pm
Event Location:
HEBB 116
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2025-05-30T12:00:00
2025-05-30T14:30:00
Searching for Millisecond and Slow Pulsars with CHIME
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Abstract:Born in core-collapse supernovae, pulsars are highly-magnetized, spinning neutron stars, which emit highly directional electromagnetic radiation in beams above their magnetic poles. This produces a lighthouse effect: we see a pulse of radiation as the beam crosses our line-of-sight, repeating with each rotation of the neutron star.
These periodic pulses can reveal a wealth of information about the neutron star, its environment, material the signal encounters on its way to Earth, and even the behaviour of spacetime. There are many remaining mysteries about these objects and, thus far, we have only discovered a small fraction of them. New surveys to discover more pulsars, therefore, have great scientific potential.In this thesis I use the Canadian Hydrogen Intensity Mapping Experiment (CHIME) telescope to perform two distinct pulsar surveys on the same small patch of sky. These serve as pilots for larger-scale surveys over the entire CHIME-visible sky.
The CHIME All-sky Multi-day Pulsar Stack Search (CHAMPSS) survey uses the data stream from the CHIME/FRB instrument, taking data from the whole Northern sky as it passes overhead and combining data from multiple days to detect fainter pulsars. However, it is not sensitive to the fastest pulsars with periods below ∼ 10 ms.
The other survey uses the CHIME/Pulsar system to take multiple observations of the same point on the sky, correcting for different amounts of material between the Earth and potential pulsars. This scheme lets it detect the fastest pulsars further out into the Galaxy. However, the CHIME/Pulsar survey does not combine multiple days’ data and so will not detect the faintest pulsars found by CHAMPSS. The two surveys are thus sensitive to different, but overlapping, sections of the pulsar population, and further demonstrate the collaborative nature of CHIME which allows multiple experiments to run simultaneously.
In this thesis I describe the software pipeline for each survey. The CHIME All-sky Multi-day Pulsar Stack Search (CHAMPSS) pipeline I developed as part of a group; the CHIME/Pulsar pipeline was an individual project. I also present PSR J2108+5001, a newly discovered pulsar in the pilot survey area, and J1629+4636, J2100+4711, J2151+5128, and J2319+4919 which were discovered during a subsequent CHAMPSS commissioning survey.
Event Location:
HEBB 116