King plot spectroscopy detects new physics by analyzing isotope shifts in atomic spectra.King plot spectroscopy is a precision technique that compares the frequencies of atomic transitions across different isotopes to search for deviations from standard physics. When an atom's nucleus changes (by adding or removing neutrons), the electron energy levels shift slightly—an effect called the isotope shift. By plotting these shifts for two different transitions against each other, one gets a nearly straight line (the King plot). Any tiny nonlinearity in this plot can signal new physics, such as a new force mediated by a hypothetical particle or a variation in fundamental constants.
The technique was introduced by W. H. King in 1963 as a way to separate mass and field shift contributions in isotope shifts. Its use as a probe for new physics gained prominence in the 2010s with ultra-precise measurements in ytterbium and other elements.
The method relies on measuring the isotope shifts for at least two transitions in an atom with high precision, typically using laser spectroscopy. For each isotope, the shift is recorded, and the data are plotted with one transition's shift on the x-axis and the other's on the y-axis. In the standard model, the relationship is linear because the shifts are dominated by two nuclear properties (mass and charge radius). A deviation from linearity—beyond known nuclear effects—indicates an additional contribution, possibly from a new boson coupling to electrons and neutrons.
Primarily a research tool in fundamental physics, used to test the standard model and search for dark matter candidates or new forces.
King plot spectroscopy is so sensitive that it can detect a hypothetical new particle even if it interacts with matter a billion times more weakly than the electron does.
