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Baryon plus lepton number is the sum of baryon number and lepton number. It is an accidental classical symmetry of the renormalizable Standard Model but has an $SU(2{)}_L$ chiral anomaly, so electroweak instantons and sphalerons violate it.

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 ${\theta }_{\mathrm{EW}}=0$ 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.