One-loop normalized hypercharge unification (${\alpha }_Y^{\mathrm{GUT}}=(5/3){\alpha }_Y$)

The normalization of an Abelian gauge coupling matters in testing gauge coupling unification. If $\mu d{g}_i/d\mu =b_i{g}_i^3$, then ${\alpha }_i^{-1}(\mu )={\alpha }_i^{-1}({\mu }_0)-8\pi b_i\log (\mu /{\mu }_0)$. For normalized hypercharge ${\alpha }_Y^{\mathrm{GUT}}=(5/3){\alpha }_Y$, the inverse slope is $(3/5)b_Y$. Subtracting pairs of inverse couplings at the common scale and eliminating its logarithm gives a low-energy consistency relation. Division by the difference of two slopes requires that difference to be nonzero; a scale above ${\mu }_0$ additionally requires a positive inferred logarithm.