= Solution
The $SU(2)_L$ <electroweak theta angle> can be shifted by an anomalous global rephasing generated by <baryon plus lepton number>. Because the renormalizable Standard Model has no operator that explicitly breaks this global phase in a way that fixes it, one may choose the rephasing to set $\theta_{\rm EW}=0$ without changing any other physical parameter. The weak theta angle is therefore unobservable in the renormalizable Standard Model; it could enter a physical invariant only after adding suitable explicit violation of <baryon plus lepton number>.
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