This Shape-Shifter Could Tell Us Why Matter Exists

Neutrinos can flit between states effortlessly, hinting at deep new physics.
BY MAGGIE MCKEE

Neutrino physics is full of unusual characters. There was Ettore Majorana, who disappeared in 1938 without a trace, taking his savings with him. No record of him has ever been found, though there have been numerous disputed sightings of him throughout the years.

Then there was Bruno Pontecorvo. Suspected of slipping nuclear secrets out of England, he vanished while on vacation in Italy in 1950 and reappeared five years later singing the praises of his new homeland: the Soviet Union.

Strangest of all, though, is the neutrino itself. It is electrically neutral, making it invisible to particle detectors, and bizarrely lightweight, at most 0.0004 percent the weight of the next-lightest particle, the electron. Although it is the most numerous massive particle in the universe, it is so slippery that it can pass through a light year of lead as if it wasn’t there. And then there is the matter of the shape-shifting.

Neutrinos come in three flavors: electron, muon, and tau, each named for the charged particle with which it is associated. But the flavors are not pure essences—each is made up of a different combination (or superposition) of three ingredients, or mass states.

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