Ytterbium’s Broken Symmetry: Largest Parity Violations Ever Observed In An Atom
Yacov Zel’dovich proposed that the weak force induces electrical currents in the nucleus, which flow like currents in a tokamak. This anapole moment has been detected in nuclear valence protons but not yet in valence neutrons. (Credit: Image courtesy of DOE/Lawrence Berkeley National Laboratory)
The elemernt Ytterbium was discovered in 1878, but until it recently became useful in atomic clocks, the soft metal rarely made the news. Now ytterbium has a new claim to scientific fame. Measurements with ytterbium-174, an isotope with 70 protons and 104 neutrons, have shown the largest effects of parity violation in an atom ever observed – a hundred times larger than the most precise measurements made so far, with the element cesium.
“Parity” assumes that, on the atomic scale, nature behaves identically when left and right are reversed: interactions that are otherwise the same but whose spatial configurations are switched, as if seen in a mirror, ought to be indistinguishable. Sounds like common sense but, remarkably, this isn’t always the case.
“It’s the weak force that allows parity violation,” says Dmitry Budker, who led the research team. Budker is a member of the Nuclear Science Division at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory and a professor of physics at the University of California at Berkeley.
Of the four forces of nature – strong, electromagnetic, weak, and gravitational – the extremely short-range weak force was the last to be discovered. Neutrinos, having no electric charge, are immune to electromagnetism and only interact through the weak force. The weak force also has the startling ability to change the flavor of quarks, and to change protons into neutrons and vice versa.