be insufficient to address the missing-matter dilemma, could muon and tauon neutrinos offer enough bulk to do the job?
These burning questions motivated the construction of the largest neutrino detector to date, the Sudbury Neutrino Observatory (SNO) in Ontario. A converted nickel mine, more than a mile and a quarter beneath the Canadian soil, houses a gargantuan acrylic vessel filled with 1,000 tons of ultrapure heavy water (with deuterium instead of hydrogen) surrounded, in turn, by a reservoir of ordinary water. Thousands of photomultiplier tubes (high-precision light sensors), arranged like sentries around the tank, stand guard for the unique flashes of neutrino collisions. Each type of neutrino, as it slams into a deuterium atom, produces a characteristic signature. These signals are collected and statistically analyzed, offering a sample of the Sun’s varied output.
In 2001 the SNO collaboration—a team of Canadian, American, and British scientists headed by Art McDonald of Queen’s University—announced the first results. In a stunning breakthrough, they found enough events to resolve the solar neutrino problem and prove that these particles come in three varieties. This finding, along with results by the Liquid Scintillator Neutrino Detector experiment at Los Alamos, helped establish the mass differences between each of the types.
Based on these and other critical results from around the world, today scientists believe that the neutrino trio constitutes a segment, but not a major component, of the dark matter in the universe. Even tallying neutrinos along with MACHOS yields far too little mass to fill the gap. Attention has shifted to some of the other candidates— particularly axions and WIMPs.
Like the Mixmaster universe, axions are whimsically named after a commercial product—in this case a brand of laundry detergent. Particle physicist Frank Wilczek couldn’t resist the opportunity to