Tanβ Bounds in Renormalizable SU(5) Grand Unified Theories

In this article non-supersymmetric, renormalizable grand unified theories based on the Special Unitary Group of degree 5 (SU(5)) gauge group are considered. These models extend the Georgi-Glashow paradigm by the presence of a Higgs field transforming as the 45 representation. The presence of a second Higgs doublet in this representation ensures realistic mass matrices for the charged fermions of the Standard Model. Perturbativity of the Yukawa couplings of down-type quarks and charged leptons up to the unification scale require the tanβ parameter, which is proportional to the ratio of the vacuum expectation values of the neutral components of the two Higgs doublets, to be constrained as follows: 3.4×10^(-3)≤tanβ≤1.2×10^2. A unification scenario in which the physical second Higgs doublet is at 1 TeV, the weak doublet scalar gluons are decoupled from the theory below the unification scale, and the unification is achieved at 1.9×10^14 GeV is presented, too. Whereas the weak and color triplet scalar leptoquark must have slightly suppressed couplings to up-type quarks, because it has a mass of 1.5×10^10 GeV.