Physics Colloquium

Prof. David Nygren, University of Texas at Arlington

"The Matter – Anti-matter Asymmetry of the Universe: Why is there something, rather than nothing?"

4:00pm, The John A. Wheeler Lecture Hall (RLM 4.102). Coffee and cookies will be served at 3:45pm in RLM 4.102

Abstract: Our universe is filled with matter, but antimatter is an ephemera seen only naturally in the debris of high-energy processes and cosmic rays. Yet the standard model of particle physics and cosmic evolution requires that the universe begin in complete symmetry between matter and antimatter, and implies that only about 1 proton or neutron in about 1020 should have escaped annihilation into photons or neutrinos.  The measured ratio, however, of baryons–protons and neutrons–to the photons left over is about 6 x 10-10, ten orders of magnitude greater!  At some very early moment, something broke that initial symmetry, yielding ultimately the universe we inhabit and explore today. How did this happen? One attractive theory–leptogenesis–postulates the existence of very massive unstable neutrinos that distinguish between matter and antimatter, did their work, and then disappeared–but left their faint fingerprints on today’s neutrinos. How might we learn something – anything – about this scenario? We might, by searching for an almost unimaginably rare nuclear decay–double beta decay but without the emission of today’s light neutrinos. Even more strangely, a biochemistry technique might help us succeed.  If we do observe the decay, we learn that that lepton number is not conserved, and also that the neutrino and the anti-neutrino are identical and neither matter nor anti-matter–a unique possibility among spin ½ particles.  Perhaps, an exciting discovery awaits that may indicate how the universe chose to keep a bit of matter!