Gauge Coupling Unification in the Exceptional Supersymmetric Standard Model

We consider the renormalisation group flow of gauge couplings within the so-called exceptional supersymmetric standard model (E$_6$SSM) based on the low energy matter content of 27 dimensional representations of the gauge group $E_6$, together with two additional non-Higgs doublets. The two--loop beta functions are computed, and the threshold corrections are studied in the E$_6$SSM. Our results show that gauge coupling unification in the E$_6$SSM can be achieved for phenomenologically acceptable values of $\alpha_3(M_Z)$, consistent with the central measured low energy value, unlike in the minimal supersymmetric standard model (MSSM) which, ignoring the effects of high energy threshold corrections, requires significantly higher values of $\alpha_3(M_Z)$, well above the experimentally measured central value.