Searching for a New Force
Among the proposed add-ons to the standard model of particle physics is a fifth force coupling electrons to neutrons. Although the bosons mediating the force could be lighter than a neutrino or heavier than a top quark, the force itself is evidently puny. Now an international collaboration of research groups in Germany, Switzerland, and Australia has placed new upper limits on the force’s strength for bosons ranging in mass from 10 to 107 eV/c2 [1]. The researchers did so by measuring tiny differences in optical transition frequencies among calcium isotopes.
An atomic transition varies in frequency from one isotope to another because each isotope’s nucleus differs in mass and charge distribution. The shifts are minuscule but are sensitive to a hypothetical force between neutrons and electrons. To boost their chances of detecting an anomalous shift, the researchers made high-precision measurements of nuclear masses and of two narrow transitions in different charge states of calcium. Calcium has five stable isotopes and therefore four separate shifts with respect to the most abundant isotope, calcium-40 (40Ca), for each transition.
Plotting one transition’s shifts against the others’ yields a so-called King plot. Absent a fifth force or other anomaly, the shifts for 42Ca , 44Ca , 46Ca , and 48Ca would fall on a straight line. Thanks to the sensitivity and precision of their experiment, the researchers recorded tiny deviations from linearity for each isotope. Some or all of the nonlinearity could be attributed to effects within the standard model. Despite the wiggle room, the new upper limits on the fifth force are more stringent than previous values derived from King plots.
–Charles Day
Charles Day is a Senior Editor for Physics Magazine.
References
- A. Wilzewski et al., “Nonlinear calcium King plot constrains new bosons and nuclear properties,” Phys. Rev. Lett. 134, 233002 (2025).