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What Are Neutrinos?

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A few weeks ago, the 2015 Nobel Prize in Physics was awarded to Takaaki Kajita and Arthur B. McDonald for their discovery that neutrinos have mass. This is the fourth Nobel Prize associated with the measurement of Neutrinos. Simply put, neutrinos are elementary, subatomic particles, and they interact very rarely with normal matter. They have no charge and are produced by the decay of radioactive elements. They are also incomprehensibly small. Fredrick Reines, who won the 1995 Nobel Prize in Physics for his official discovery of the neutrino, said they are "the most tiny quality of reality ever imagined by a human being". Neutrinos were predicted in 1930, by Wolfgang Pauli, but Reines was the first to experimentally observe them in 1956. The reason they are so difficult to find is that they almost always pass through matter without stopping. More than six trillion neutrinos pass through your body every second, coming from the core of the sun. On average, neutrinos can travel more than one thousand light years until they interact with a particle even once. The fact that we can detect them at all is a miracle in itself, and shows human ingenuity. Because the laws of quantum physics are based on probability, while most neutrinos will pass right through Earth, some will interact with the earth. If a detector is big enough, it can observe one of these interactions. The first one was built in the 1960s, deep in a South Dakota mine. An area of the mines was filled with a hundred thousand gallons of cleaning fluid, and on average one neutrino a day would interact with it. A neutrino would interact with a chlorine molecule and create an argon atom. Amazingly, Raymond Davis Jr. the physicist in charge of the machine, was able to figure out how to detect these few argon atoms. Because they interact so rarely, neutrinos can travel vast distances, and provide windows into places that we could never see otherwise. We've been able to detect neutrinos coming from an exploding star, more than one-hundred thousand light years away. Neutrinos allow us to see the universe at its smallest scale, which is why they are so important. When Davis experimented with the solar neutrinos, he found only a third of what he thought he would. This phenomenon was dubbed "the neutrino deficit". Most thought that this was due to poor knowledge of physics inside the sun, yet as solar models became better, the neutrino deficit remained. Physicists then began to think that perhaps the problems had to do with a lack of knowledge of neutrinos. According to the prevailing model of particle physics, neutrinos are massless. But some physicists argued neutrinos do have mass, and that mass accounted for the missing neutrinos in the detectors. This theory was called the theory of neutrino oscillations: there are three different types of neutrinos and if it has a small mass, a neutrino can convert from one type to another, as it travels through space. The three types are electron, muon, and tau, each being



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