Neutrinos and Experimental Physics with Art McDonald, 2015 Nobel laureate in physics

Art McDonald, the former director of SNO and 2015 Nobel Prize in physics laureate, gives a really nice talk at the Perimeter Institute. I was expecting a rather dull review of his pathway to the SNO neutrino mass measurement papers in 2002-2004 (for which the Nobel was awarded) — but he has been busy! He is now an emeritus professor at Queen’s, but still involved in SNO+/SNOlab, with DEAP, an experiment aiming at direct-detection of dark matter.

The introduction mentioned Art’s generosity towards students. I’d like to add to the point. Unusually for a PI for such a large collaboration, Art insisted that any one of us who participated in a material way to the production of SNO results should be on the collaboration authors’ list. I’ve not experienced anything like it anywhere else. Academics fight tooth-and-nail to appear as authors on important papers, but on the SNO authors’ list, you’ll see the names of senior professors, research scientists, postdocs, PhD and even lowly MSc students (like I was), along with professional engineers who helped to build the machine, as well as a short but crucial list of scientists who tragically died before the results were released. Furthermore, Art won the Gerhard Herzberg Canada Gold Medal for Science and Engineering in 2003, and quite a bit of money, which he used to set up a postgraduate prize in honour of a young postdoc who succumbed to cancer, and funded all SNO graduate students at Queen’s to travel to Berkeley, California, for the annual SNO collaboration meeting at Lawrence Berkeley National Labs.

McDonald talks about the “solar neutrino problem” (the discrepancy between solar thermonuclear fusion theory, that produced an abundance of electron neutrinos from beta-decay of boron-8 in the complex process that fuses 2 protons into helium, and the experimental measurement made by Ray Davis a half-century ago, which detected only ~1/3 of the expected neutrino flux at earth), the importance of neutrinos in cosmology (e.g., thermal equilibrium of the cosmic microwave background), the production of elements heavier than iron (without neutrinos it would be tough for a supernova to dump the energy rapidly building as a stellar core collapses), as a good chunk of dark matter, and improving our understanding of the Standard Model (SM, which presently may be summarized with an unsatisfactory-tasting “it’s just so”).

That last point is a really interesting one, worthy of another Nobel Prize: if neutrinoless double-beta decay is observed (as one of the experiments in SNOlab is trying to do), it would mean that the neutrino is its own antiparticle (a so-called Majorana-particle). This would break the symmetry of the SM, now a convenient grid-shape with quarks, and antiquarks (the building blocks of a class of massive particles called hadrons, such as protons and neutrons); “heavy” leptons, like electrons, muons, tauons, and their positrons, antimuons, and antitauons, and their associated neutrinos, electron-, muon-, and tau-neutrinos; the force-carrying gauge bosons (electromagnetism-associated photons, strong nuclear force-associated gluons, and weak nuclear force-associated W/Z-bosons); and finally the Higgs boson. There already has a place for the graviton, an as yet undiscovered spin 2 boson responsible for quantum gravity, if that’s a thing. Aside from symmetry arguments (e.g., they tend to appear in threes, etc.) there is no satisfactory mathematical underpinning of the SM from first-principles, so it’s hard to probe around its edges for new physics. If neutrinos are Majorana particles, they would certainly be different from quarks and the other leptons, like electrons; and they may also be the reason that there is a curious imbalance of matter to antimatter in the universe. (Except for a special case in which antimatter and matter are somehow well segregated from one anther throughout space, the fact that we don’t see bright, high-energy photons streaming in from all around us is good evidence that matter-antimatter annihilation is uncommon. The best cosmogenic models predict an equivalent production of matter and antimatter. So where did all the antimatter go?)

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