Building these behemoth experiments consumes inordinate amounts of time and money, so researchers have been looking for alternative ways to probe for new physics. Traditionally, these searches have occurred at higher energies using particle accelerators such as the Large Hadron Collider at CERN near Geneva to create new particles in the wreckage of extreme subatomic smashups. Hints, such as the existence of dark matter, the mystery of why neutrinos have mass and the relative weakness of gravity, suggest that there may be particles and forces beyond what physicists have accounted for in the Standard Model. The hunt for dark forces is part of a broader search for new physics. “Most likely, we are measuring new nuclear physics, but there is the possibility of something else going on.” “We’re not claiming to have discovered anything like a new particle,” says Vladan Vuletić, a physicist at M.I.T. Most likely, the deviation is instead because of some as yet uncalculated nuclear force-nothing outside the Standard Model, the theory that governs known particles and forces, minus gravity. The M.I.T.-led researchers are quick to clarify that they are skeptical their result is linked to dark forces.
Typically, physicists will not assert a discovery has been made until a result’s statistical significance reaches five sigma (in this case, a one-in-1.7-million chance). The Aarhus-led study’s results do not rule out dark forces, but the M.I.T.-led team’s findings do not confirm them. 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. “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. “‘Dark boson’ is not well-defined,” says Elina Fuchs, a physicist at Fermi National Accelerator Laboratory near Chicago and a member of the Aarhus-led team. If there is a dark force at work here, physicists believe it would be carried by a force-carrying particle: a boson. 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. 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.
Now two teams have independently performed the most precise measurements of this type. 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. One path to investigating dark forces involves using lasers to make precision measurements of isotopes (atoms of an element possessing different numbers of neutrons). In this case, they would act between neutrons and electrons.
These forces are not as ominous as they sound: “dark” simply refers to the fact that no one has observed them before. Physicists are on the hunt for dark forces.