Weak charge
Type of weak Interaction in nuclear and atomic physics
In nuclear physics and atomic physics, weak charge, or rarely neutral weak charge, refers to the Standard Model weak interaction coupling of a particle to the Z boson; it is named by analogy to electric charge, which measures coupling to the photon of electromagnetism. For example, for any given nuclear isotope, the total weak charge is approximately −0.99 per neutron, and +0.07 per proton. It also shows an effect of parity violation during electron scattering.
This same term is sometimes also used to refer to other, different quantities, such as weak isospin or weak hypercharge; this article concerns the use of weak charge for a quantity that measures the degree of vector coupling of a fermion to the Z boson (i.e. the coupling strength of weak neutral currents).
Empirical formulas
Measurements in 2017 give the weak charge of the proton as 0.0719±0.0045 .
The weak charge may be summed in atomic nuclei, so that the predicted weak charge for 133Cs (55 protons, 78 neutrons) is 55×(+0.0719) + 78×(−0.989) = −73.19, while the value determined experimentally, from measurements of parity violating electron scattering, was −72.58 .
A recent study used four even-numbered isotopes of ytterbium to test the formula Qw = −0.989 N + 0.071 Z , for weak charge, with N corresponding to the number of neutrons and Z to the number of protons. The formula was found consistent to 0.1% accuracy using the 170Yb, 172Yb, 174Yb, and 176Yb isotopes of ytterbium.
In the ytterbium test, atoms were excited by laser light in the presence of electric and magnetic fields, and the resulting parity violation was observed.
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