Gauge coupling unification without supersymmetry

Abstract

We investigate the prospects to achieve unification of the gauge couplings in models without supersymmetry. We restrict our discussion to SU (5), SO(10) and E 6 models that mimic the structure of the Standard Model as much as possible ("conservative models"). One possible reason for the non-unification of the standard model gauge couplings are threshold corrections which are necessary when the masses of the superheavy fields are not exactly degenerate. We calculate the threshold corrections in conservative models with a Grand Desert between the electroweak and the unification scale. We argue that only in conservative E 6 models the corrections can be sufficiently large to explain the mismatch and, at the same time, yield a long-enough proton lifetime. A second possible reason for the mismatch are particles at an intermediate scale. We therefore also study systematically the impact of additional light scalars, gauge bosons and fermions on the running of the gauge coupling. We argue that for each of these possibilities there is a viable scenario with just one intermediate scale.

Unification, proton decay, and topological defects in non-SUSY GUTs with thresholds

“…In the absence of these threshold corrections, the detailed matching conditions for different scenarios are discussed in [7]. These conditions get modified in the presence of threshold corrections [62,[89][90][91][92][93][94][95][96][97].…”

Threshold effects in SO(10) models with one intermediate breaking scale

“…In the case of no intermediate symmetry, the impact of the threshold effects on the matching conditions is such that they can compensate for the difference between the gauge couplings and therefore allow gauge coupling unification [41,42]. That is, since threshold corrections are meant to account for the failure of gauge coupling unification, we can compare the difference of the gauge couplings with the size of the threshold corrections.…”

“…This can in turn modify the value of M GUT and thereby save some of the models that were previously disfavored [27,[32][33][34][35][36][37][38][39]. Furthermore, since threshold corrections modify the matching conditions of the gauge couplings, they can allow for unification in models where the gauge couplings do not unify [40][41][42]. The thresh-0123456789().…”

“…We do not take into account any restrictions on M I from neutrino masses in order to refrain from making too many assumptions about the models. This work differs significantly from recent works dealing with threshold correction [36,39,42,43] in that we consider all possible breaking chains with at most one intermediate step and treat all models in the same way. This, together with our comprehensive numerical analysis of the effect of threshold corrections, allows a simple and quantitative comparison between the plausibility of the different breaking chains.…”

Road map through the desert: unification with vector-like fermions

“…The issue of gauge coupling unification in the presence of VL fermions is not a new idea and it received a lot of attention in the literarture, both before the launch of the LHC [14,15,16,17,18,19,20,21,22,23], and after [24,25,26,27,28,10]. Similar analyses within the SUSY framework were performed as well [29,30,31,32,33,34,35,36].…”