The mass range for hypothetical dark matter axions is narrowing

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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.

Another team of physicists searched for axions with masses around 6.7 millionths of an electron volt, according to
another paper in press in Physical Review
Letters
. Those researchers, part of the CAPP-8TB experiment, likewise found
no signs of the particles, although their experiment is not yet sensitive
enough to exclude the possibility that axions might be present in that mass
range.

Both experiments search for signals
produced when axions convert into particles of light, photons, in a magnetic
field. The detectors are tuned to search for certain frequencies of light,
which correspond to different masses of axions.

Most previous dark matter experiments
have searched for a different type of hypothetical particle called a WIMP, but
have turned up nothing, leading some to pin their hopes for finding dark matter on axions (SN: 10/25/16).

With each search of a new mass range, there’s
a chance for spotting axions, says Rybka. “We could get lucky.”



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