Neutrinos are elementary particles having zero electric charge that interact with themselves and other particles via the weak interaction. There are three types or “flavours”, which are associated with the charged leptons (electron, muon and tau). Neutrinos are produced in the beta-decay of neutrons and other particles, and play vital roles in both astrophysics and cosmology. Neutrinos stream out of the sun, for example, having been produced during nuclear fusion processes, and constitute cosmic messengers from a diverse range of high-energy astrophysical systems. They participate in the proton-neutron interconversion process during the primordial nucleosynthesis epoch of the early universe, thus helping to set the cosmological hydrogen to helium ratio, which in turn determines the dominant elemental composition of the universe. They exist today as relics from this process, just as the cosmic microwave background photons are a relic from the epoch of neutral atom formation that occurred somewhat later. We cannot understand particle physics, nuclear physics, astrophysics and cosmology without understanding neutrinos.

Yet the neutrinos present a number of deep puzzles. Like their charged lepton and quark cousins, they come in three “generations” or “families”, which appear to be copies of each other. The origin of this threefold replication is a longstanding mystery. In the very successful standard model of particle physics, neutrinos are exactly massless particles, just like photons, but unlike the more closely related charged leptons and quarks. Yet a heroic program of experiments has established that the neutrino flavours oscillate into each other as they travel freely through space, which can only happen if neutrinos are actually massive. This means that the standard model, despite its many successes, is incomplete. The search is on for the origin of neutrino masses, one of the prime drivers of research in particle physics globally and in the Melbourne Theoretical Particle Physics research group. A striking fact about the neutrino masses is that while they are nonzero, they are really tiny, at least a million times smaller than the electron mass, which is itself a small quantity. The suspicion is that neutrinos acquire their masses via a quite different mechanism from the other particles. We do not know what that mechanism is. Researchers at Melbourne are at the forefront of developing standard model extensions that explain the tiny neutrino masses, and exploring how experimental evidence may be obtained to determine which of the theories is correct.

In addition to their role in primordial nucleosynthesis, it is quite possible that neutrinos are responsible for the matter-antimatter asymmetry of the universe. In the early stages of the big bang, the universe was a hot plasma of almost equal numbers of particles and antiparticles. Today, however, all the planets, stars, galaxies and the intervening gas are composed of matter, not antimatter. At some point after the big bang, some process had to have produced slightly more matter than antimatter, so that when all the antimatter annihilated with matter to produce the relic photons and neutrinos, some matter was left over, which is what we are all made of. Several explanations of the origin and smallness of neutrino masses have a major additional feature: a process called “leptogenesis”, during which neutrinos help to induce more leptons than antileptons in the cosmological plasma. This lepton asymmetry then seeds the general preponderance we see of matter over antimatter. Our research group is very active in these and related aspects of the cosmological origin of the matter-antimatter asymmetry of the universe.

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