That’s odd: Missing neutrinos may be shapeshifters

 作者:茅郯玮     |      日期:2019-03-15 10:19:00
Roy Kaltschmidt, Lawrence Berkeley National Laboratory By Michael Brooks Study the standard model of particle physics at any length, and you might find yourself musing about the quasi-mystical significance of the number three (see diagram). Among the matter particles, there are three quarks – up, charm and top – with the same electric charge, differing only in mass. Then another group of three quarks – down, strange and bottom – for which the same pattern holds. Then three charged leptons, the electron, the muon and the tau: again, same charge, different mass. And finally, three distinct “flavours” of chargeless, near-massless neutrinos Except maybe not that last one. Neutrinos are particles that interact only via the weak nuclear force that governs radioactive decay. Trillions of the lightest type, electron neutrinos, are generated in reactions in the sun and pass through you every second. They will also pass through Earth without a blip. Fiddly measurements over decades have revealed that a sizable portion of this flux is going missing. This “solar neutrino problem” was resolved conclusively only in 2001, when electron neutrinos were shown to be shape-shifting, or “oscillating”, into muon and tau neutrinos on their way to us. That breakthrough was awarded last year’s Nobel prize in physics. At that point we might all have lived happily ever after, had not a series of detectors, starting with the Liquid Scintillator Neutrino Detector at Los Alamos Laboratory in New Mexico in the 1990s, begun to see more experimental blips. The blips suggested that there were not merely three types of neutrino,