\Like Dustmaids Down a Drafty Hall": Neutrinos at the Sudbury Neutrino Observatory

Even with my oÆ e door partly losed, I an hear my olleagues down the hall arguing. Ea h is riti izing the other's latest theory of the mi ros opi Universe. Ba king his opponent into a orner, one a uses the other of trying to reate a theory whi h violates a fundamental symmetry of Nature. I smile to myself, be ause I know what the response will be: in this theory, there is a new parti le whi h ensures that this symmetry is preserved. Not a bad week, I think|that will make three new parti les predi ted, and it is still only Tuesday. In 1930, the situation was very di erent. The Universe was made up of just a handful of parti les whose intera tions with one another were almost entirely understood. Physi ists believed there were only a few details left to understand before we had a omplete theory des ribing the fundamental makeup of the Universe|the eld of parti le physi s was nearing its end. One of these details was the surprising behavior of the ele trons emitted when a radioa tive nu leus underwent de ay. In these de ays, a heavy nu leus be ame lighter by emitting an ele tron (a ` parti le'). When a nu leus de ays into two parti les (in this ase, the lighter nu leus and the ele tron) the energy available is the same ea h time|some of the mass of the nu leus was onverted into the kineti energy of both the ele tron and the re oiling lighter nu leus (as des ribed by Einstein's famous formula E = m 2 ). With just two parti les in the nal state, onservation of momentum and energy requires that they ea h have the same energy every time. The problem was that measurements showed the ele trons had energies all the way from zero to the total energy they should have had. The problem of the `missing energy' in de ay was so serious, physi ists as prominent as Niels Bohr were making radi al suggestions, su h as a violation of energy onservation