Can Electro-Weak $\h$-Term be Observable ?

We rederive and discuss the result of the previous paper that in the standard model $θ$-term related to $W$-boson field can not be induced by weak instantons. This follows from the existence of the fermion zero mode in the instanton field even when Yukawa couplings are switched on and there are no massless particles. We consider the new index theorem connecting the topological charge of the weak gauge field with the number of fermion zero modes of a certain differential operator which depends not only on gauge but also on Higgs fields. The possible generalizations of the standard model are discussed which lead to nonvanishing weak $θ$-term. In $SU(2)_L \times SU(2)_R$ model the $θ$ dependence of the vacuum energy is computed.

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