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Possibility of Dark Bosons Entices Physicists

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Physicists are on the hunt for dark forces. These forces are not as ominous as they sound: “dark” simply refers to the fact that no one has observed them before. In this case, they would act between neutrons and electrons.

One path to investigating dark forces involves using lasers to make precision measurements of isotopes (atoms of an element possessing different numbers of neutrons). If there is a dark force working behind the scenes, it could affect an isotope’s energy levels—discrete regions around the atom’s nucleus where its electrons exist.

Now two teams have independently performed the most precise measurements of this type. Their findings, reported this month in Physical Review Letters, are mixed: One group, led by researchers at Aarhus University in Denmark, analyzed calcium isotopes and saw no deviation from predictions. But the other team, led by scientists at the Massachusetts Institute of Technology, used ytterbium isotopes and found a deviation in the electron energy levels with “three sigma” significance—that is, assuming no dark forces or other factors, the result would happen once every 370 times because of random chance.

If there is a dark force at work here, physicists believe it would be carried by a force-carrying particle: a boson. “‘Dark boson’ is not well-defined,” says Elina Fuchs, a physicist at the University of Chicago and a member of the Aarhus-led team. “It’s just a very feebly interacting particle that can be connected to matter.” These efforts are searches for a dark force, not dark matter—the mysterious stuff that composes 85 percent of the matter in the universe. Such a dark boson could be an important component of dark matter, or it could simply be part of a larger “dark sector” of particles.

The Aarhus-led study’s results do not rule out dark forces, but the M.I.T.-led team’s