The cobalt-60 apparatus is set up with the nuclear spins polarised along a vertical…
PHYSICS · Multi-part · Fermi University Physics · Quantum Mechanics · Particle Physics & Quantum Fields · Symmetry, groups and the quark model
Particle Physics, Ch Parity, charge conjugation, time reversal and CPT
Problem
The cobalt-60 apparatus is set up with the nuclear spins polarised along a vertical axis (call it the z-axis) by an external magnetic field, and in the actual experiment electrons were counted preferentially emitted opposite to the spin direction. Consider a mirror placed as a horizontal plane, perpendicular to the spin axis, and its effect on the picture.
((a)) State what this mirror does to (i) the electron momentum component along z and (ii) the nuclear spin direction along z.
((b)) Using this, write down what counting-rate pattern (relative to the spin direction) the mirror image of the actual apparatus predicts, for the same spin-up orientation.
((c)) Explain, in terms of the sign of ⟨S⋅p⟩, why the fact that only one of these two patterns (original or mirror) is ever observed in nature demonstrates that parity is not a symmetry of the interaction responsible.