The Standard Model (SM) of particle physics has shaped our understanding of the universe at the subatomic scale with unprecedented experimental precision over the past decades, yet it leaves its deepest mysteries unexplained; for example, it remains silent on gravity, fails to identify a particle candidate for dark matter, and cannot explain cosmic matter-antimatter asymmetry. Searching for cosmic secrets requires finding where this framework breaks down; physicists look toward "forbidden" processes—phenomena the SM predicts should virtually never happen. One such process is Charged Lepton Flavour Violation (CLFV). The observation of atmospheric and solar neutrino oscillations by Super-Kamiokande and Sudbury Neutrino Observatory confirms the first direct evidence of lepton flavour violation (LFV) in the neutral lepton sector. These observations raise the question of whether LFV also occurs in the charged-lepton sector. The SM, when extended to include neutrino masses, allows charged lepton flavour violation (CLFV) processes only through loop diagrams, which are highly suppressed by the small neutrino masses and occur at rates beyond current and planned experiments. Any experimental observation of CLFV is therefore unambiguous evidence for physics beyond the SM, and its exceptional sensitivity makes CLFV searches powerful indirect probes of new physics. In this lecture, we will explore the theoretical landscape of the lepton sector and review current experiments that are pushing technical limits to hunt for these rare transformations. Finally, we will discuss how these subatomic searches complement high-energy colliders, providing a critical path forward in our quest to decode the laws of nature.
Dr. Ahmed is an experimental particle physicist and Associate Professor of Physics at St. Francis Xavier (StFX) University. He was a W.F. James Research Scholar in the Pure and Applied Sciences from 2020 to 2022 at StFX. Dr. Ahmed earned his PhD from the University of Alberta, where he contributed to the ATLAS experiment at CERN, focusing on Jet Energy Scale and Top Quark mass measurements. He then joined the University of Victoria as a postdoctoral research fellow and began work on the BaBar experiment at the SLAC National Accelerator Laboratory, where he remains an active collaborator. His ongoing research focuses on precision measurements in particle physics, including Boost Vectors, Integrated Luminosity, Lepton Universality, Semi- leptonic Bottom-quark Decay, and Charged Lepton Flavour Violation. In 2019, Dr. Ahmed joined the Belle II experiment at KEK, the High Energy Accelerator Research Organization in Japan — making StFX the only university in the Maritime provinces where undergraduate students can work directly on high-energy collider data.