Bit by bit, physicists are winnowing down the potential masses for hypothetical particles called axions.

If they exist, the subatomic particles could make up dark matter, a mysterious source of mass that pervades the universe. Axions are expected to be extremely lightweight — billionths or trillionths the mass of an electron. But there were no sightings of the elusive particles in a mass range between 2.81 millionths and 3.31 millionths of an electron volt (between about 5.5 trillionths and 6.5 trillionths of an electron’s mass), physicists with the ADMX experiment report in a paper in press in Physical Review Letters.

Scientists expect axions to have masses between a millionth and a thousandth of an electron volt. Previously, ADMX searched a small range of masses, and scientists are now expanding that range (SN: 4/9/18). The new result is “one step on a long road to exploring the whole plausible range,” says physicist Gray Rybka of the University of Washington in Seattle, co-spokesperson of ADMX.

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