Atomic electrons interact with the nucleus with not only the electromagnetic interaction (photon exchange) but also with neutral current weak interactions (Z exchange). The latter contribution is extremely small but the interference of the two gives detectable parity violation effects. The main contribution comes from the product of the axial electron current and the quarks vectors currents. Notice that the Z-charge of the nucleus is the sum of those of its nucleons and the latter are the sums of those of their quarks (consider only valence quarks in the following).

(a) Evaluate the axial Z-charges of the proton and of the neutron.

(b) Evaluate the axial Z-charge of a nucleus with Z protons and N neutrons. Is the larger contribution due to protons or neutrons?

(c) Explain why the product of the axial electron current and quarks vector currents is less important.