Caitlin Sikora felt that a research experience would be valuable in deciding on a career path or future area of study so she decided to complete a senior thesis to finish her physics major. She chose to work with Dr. Chen, who offered her a rare opportunity to work on particle theory as an undergraduate, in a very specific and exciting branch of particle physics. In addition to her major in physics, Caitlin has completed a major in dance, dancing and choreographing in many UCI Dance Department performances. Over the next few years she hopes to pursue a dance career before moving on to graduate school in physics.

The origin of the asymmetry between matter and antimatter in the universe remains a great mystery. In 1998, neutrino oscillations were observed in atmospheric neutrinos, shattering assumptions that neutrinos were massless and suggesting a possible violation of change and parity symmetry (CP-symmetry) in the neutrino sector. This suggests that leptogenesis is possible, hypothetically generating leptons in greater quantities than antileptons, potentially explaining the asymmetry between matter and antimatter, which makes existence possible. By deriving the neutrino mixing matrix and expanding it in terms of small deviations about the Tri-bimaximal mixing pattern to the third order, it has been shown that slight variations in these parameters can significantly affect flavor transition probabilities and the possibility of leptogenesis. I examined the dependence of each transition probability on each mixing angle, Dirac CP-violating phase, and mass ordering, identifying the electron to muon and tau to electron flavor channels as the most sensitive to such variations. I also calculated leptogenesis in terms of the Dirac CP-violating phase for a model derived from the double tetrahedral T' group theory, finding it to be nonzero with flavor effects included. These results may guide future experiments and model building in hopes of observing CP-violation and explaining the matter-antimatter asymmetry.

Mu-Chun Chen School of Physical Sciences

The recent discovery of neutrino oscillation—that is, that neutrinos morph from one type to another during their free flight in space—has given solid evidence of the existence of tiny but non-zero neutrino masses. Massive neutrinos may also explain the asymmetry between matter and antimatter in the Universe. Sikora has investigated the phenomenological implications for neutrino oscillation and the matter-antimatter asymmetry in a theoretical model based on the symmetry that also describes the structure of methane. This highly predictive model unifies three of the four fundamental forces in nature into a single grand unified interaction. The model’s predictions for the mixing angles will be tested in upcoming neutrino oscillation experiments.