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We point out that in nonsupersymmetric grand unified theories, in which the $SU(5)$ gauge symmetry is broken down to the standard model gauge group by a 24 Higgs multiplet the Appelquist-Carazzone decoupling is violated. This is because the SU(2)L Higgs triplet contained in the 24 acquires a dimension-full coupling to the SU(2)L Higgs doublets which is proportional to the grand unified symmetry breaking vacuum expectation value. As a result, at one-loop heavy gauge and Higgs fields contribution to tadpoles generates a vacuum expectation value of the triplet which is not suppressed for V→∞ and violates the custodial symmetry.
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Chankowski et al. (2008) studied this question.
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