Isotopic variation of parity violation in atomic ytterbium

The weak force is the only fundamental interaction known to violate the symmetry with respect to spatial inversion (parity). This parity violation can be used to isolate the effects of the weak interaction in atomic systems, providing a unique, low-energy test of the standard model (see, for example, reviews1,2,3). These experiments are primarily sensitive to the weak force between the valence electrons and the nucleus, mediated by the neutral Z0 boson and dependent on the weak charge of the nucleus, Qw. The standard model parameter Qw was most precisely determined in caesium4,5 and has provided a stringent test of the standard model at low energy. The standard model also predicts a variation of Qw with the number of neutrons in the nucleus, an effect whose direct observation we are reporting here. Our studies, made on a chain of ytterbium isotopes, provide a measurement of isotopic variation in atomic parity violation, confirm the predicted standard model Qw scaling and offer information about an additional Z′ boson