The unbearable lightness of neutrinos

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Science  30 Jun 2017:
Vol. 356, Issue 6345, pp. 1322-1326
DOI: 10.1126/science.356.6345.1322

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The silvery vacuum chamber resembles a zeppelin, the vaguely Art Deco lines of the welds between its stainless steel panels looking at once futuristic and old-fashioned. Although earthbound, the blimp-sized chamber at the Karlsruhe Institute of Technology in Germany has an ethereal purpose: weighing the most elusive and mysterious of subatomic particles, the neutrino. Physicists dreamed up the Karlsruhe Tritium Neutrino (KATRIN) experiment in 2001. Now, the pieces of the €60 million project are finally coming together, and the roughly 140 members of the KATRIN team plan to start taking data early next year. It might seem absurd that physicists don't know how much neutrinos weigh, given that the universe contains more of them than any other type of matter particle. However, it's impossible to capture neutrinos, so there's no simple way to measure their mass. Instead, for 70 years, physicists have tried to infer the neutrino's mass by studying a particular nuclear decay from which the particle emerges—the beta decay of tritium. Time and again, these experiments have set only upper limits on the neutrino's mass. In KATRIN, physicists are pushing that classic experiment to its ultimate limits, in what may be their last, best hope to weigh the neutrino.