Dead ringer. In the water-filled Super-Kamiokande detector (sketch, above), an electron neutrino triggers an interaction that produces a high-energy electron. The electron then radiates a telltale circle of light (event display, below

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U.S. neutrino experiment's first result tantalizes

The United States' newest neutrino experiment has collected only one-thirteenth of its expected data, but early results suggest it could achieve its main goal: ranking the mysterious particles by their weights. Neutrinos are nearly massless and barely interact with other matter; they come in three "flavors"—electron, muon, and tau—that can morph into one another. Two of the neutrinos are close in mass and one is different, but physicists don't know whether there are two light ones and one heavy one (the so-called normal hierarchy) or the other way around (the inverted hierarchy). Physicists with the $278 million NOνA experiment at Fermi National Accelerator Laboratory (Fermilab) in Batavia, Illinois, hope to determine the ordering by firing muon neutrinos to a 14,000-tonne detector (above) in northern Minnesota and looking for electron neutrinos emerging in the beam. The first year's data yield between six and 11 conversions—somewhat more than expected—and hint that the hierarchy is normal, NOνA researchers reported at the lab today. The result suggests that NOνA will eventually deliver a definitive answer—which is not a sure bet. Other experiments to test whether the neutrino is, weirdly, its own antiparticle may be feasible only if the hierarchy is inverted.