Weak charge
In nuclear physics and atomic physics, weak charge refers to the standard model weak interaction vector coupling of nucleons to the Z boson, namely
where T3 is the weak isospin, Q the charge, and θ the weak mixing angle. Thus, it is approximately -0.99 for a neutron and +0.07 for a proton. Measurements in 2017 give the weak charge of the proton as 0.0719 ± 0.0045
It further affects parity violating electron scattering. The weak charge may be summed in atomic nuclei, so that the predicted weak charge for 133Cs is -73.23, while the experimentally deduced value was -72.58.
A recent study using four even-numbered isotopes of ytterbium was found consistent, to a 0.1% level of accuracy, with the formula weak charge Qw = −0.989 N + 0.071 Z, the latter two variables corresponding to the respective numbers of neutrons and protons in 170Yb, 172Yb, 174Yb, and 176Yb. In the ytterbium system, atoms were excited by laser light in the presence of electric and magnetic fields, and the resulting parity violation was observed. The specific transition observed was the forbidden transition from 6s2 1S0 to 5d6s 3D1. The latter state was mixed, due to weak interaction, with 6s6p 1P1 to a degree proportional to the nuclear weak charge.
