First came decades of searching by many experiments, with important hints to encourage the chase. In the end, nature provided, and experimentalists discovered, supported by calculations from theorists. Not by neutrino physicists, though – we held out hope. This transformation requires three things: that neutrino masses are nonzero, are different for different types, and that neutrinos of definite flavor are quantum combinations of neutrinos of definite mass (this is called “neutrino mixing”).įor decades, it was generally expected that none of these conditions would be met. They come in three types, called flavors – electron, muon and tauon neutrinos, corresponding to the three charged particles they pair with – and all of these seem to be stable, unlike the heavy cousins of the electron.īecause the three flavors of neutrinos are almost identical, there is the theoretical possibility that they could change into each other, which is another unusual aspect of these particles, one that can reveal new physics. Because they so rarely interact, it’s almost impossible to observe them, and you certainly don’t feel them. There are hundreds per every cubic centimeter left over from the Big Bang. Though neutrinos are not constituents of ordinary matter, they are everywhere around us – a trillion from the sun pass through your eyes every second. We do know why they’re almost noninteracting, though: They don’t feel the electromagnetic or strong forces that bind nuclei and atoms, only the aptly named weak force (and gravity, but barely, because their masses are small). It’s a mystery why neutrinos are almost, but not quite, massless. Those features are different, though often conflated (don’t take advice about neutrinos from a poet, even it is John Updike). How is this elementary particle – the neutrino – different from all other elementary particles? It’s unique in that it’s both almost massless and almost noninteracting. Elementary particles, of which neutrinos are one kind. For example, for electrons these are the muon and tauon. Interestingly, there are many heavy cousins of familiar particles that exist only for fractions of a second, and thus are not part of ordinary matter. ![]() These are elementary particles, forming the basic constituents of ordinary matter: the Lego bricks of the universe. We haven’t been able to take apart electrons or quarks. Second, the higher the accelerator’s beam energy, the more finely we can resolve composite structures, just as we can see smaller things with X-rays than with visible light. First, because of E = mc 2, the energy in the collision can be converted into the mass of particles. ![]() Particle colliders, which accelerate particles to near the speed of light and smash them together, help us discover new elementary particles. An atom is a diffuse cloud of electrons surrounding a tiny, dense nucleus composed of protons and neutrons, which can be broken into up and down quarks. There’s no such thing as half a neutrino.Ītoms, despite the Greek name (“cannot be cut”), are not elementary particles, meaning they can be disassembled. They’re not called elementary because they’re easy to understand – they aren’t – but because they are seemingly point-like in size, and we can’t break them down into smaller constituents. I could begin by telling you that neutrinos are elementary particles, but that sounds condescending. Ironically, these near-undetectable particles can reveal things that cannot be seen any other way. They’re worth it, and the announcement of the 2015 Nobel Prize in Physics recognizes that, following related prizes in 1988, 19. ![]() Kamioka Observatory, ICRR (Institute for Cosmic Ray Research), The University of Tokyo Neutrinos, we’re looking for you! Japan’s Super-Kamiokande detector.
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