Neutrinos have always been good for surprises. The postulate of their existence by Wolfgang Pauli in 1930 was already revolutionary. Later, physicists learned that neutrinos oscillate, meaning that the three known neutrino “flavors” (electron, muon, and tau) periodically convert into one another as they travel through space—a neutrino born in the muon flavor, for instance, may later be detected as an electron neutrino or tau neutrino. The discovery of neutrino oscillations implied that neutrinos have nonzero mass, which required a modification of the standard model of particle physics. Adding another surprise, the parameters that govern neutrino oscillations turned out to be vastly different from theoretical expectations.

Now, the MiniBooNE experiment at Fermilab in Illinois has reignited excitement about neutrinos on yet another front. Data from the experiment suggest that muon neutrinos convert into electron neutrinos over distances that are too short for conventional neutrino oscillations to occur [1]. This finding is all the more intriguing when considering that an earlier experiment—the Liquid Scintillator Neutrino Detector (LSND) at Los Alamos—already observed a similar signal in the late 1990s [2]. The reason for excitement is that these signals could be beacons of sterile neutrinos, particles that only interact through gravity and aren’t foreseen in the standard model. The existence of sterile neutrinos could lead us to answers to some of the most pressing puzzles in physics—from the nature of dark matter to matter asymmetry in the Universe.

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