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“Can it be explained by a sterile neutrino of mass around 1 eV? The answer is no.”

To detect dark sector particles with masses as low as 1/100,000th that of a proton, the material would need to unleash an electron when pinged by a photon of just 0.03 eV.

Researchers have long had indirect evidence that the particles should be lighter than 1 eV, but this is the first time that this has been shown in a direct measurement.

Previous measurements put the upper bound at 2 eV.

From other particle physics experiments, scientists also know the average mass cannot be less than 0.02 eV.