GUT Color-Triplet Around the Corner: The Story of an Overlooked Lightness

Abstract

In traditional approach to the doublet-triplet splitting problem in grand unified theories (GUTs) it is assumed that color-triplet partner of the Higgs doublet must be superheavy in order not to mediate proton decay. However, it has been proposed [1] that the splitting, instead of the mass sector, can occur among the Yukawa couplings. In this scenario the color-triplet emerges as a long-lived $T$-particle with the mass around the TeV scale. This framework provides a direct low-energy window into GUT physics. In the present paper, we study an explicit supersymmetric $SO(10)$ GUT which realizes this mechanism, including effects of gravity-mediated supersymmetry breaking. We show how the shift of the heavy fields, triggered by supersymmetry-breaking, generates a tiny coupling between the $T$-particle and the Standard Model fields and explore the resulting phenomenology. Depending on the order of gravitino mass in which the coupling emerges, the lifetime of the $T$-particle is either around a second or exceeds the age of the Universe. In our analysis, we validate some earlier constraints on (super)gravity vacua imposed by the $S$-matrix: based on the Goldstino Argument, we show that none of the GUT vacua have positive energy densities, whereas the Minkowski vacua are safe against tunneling. We also highlight the role of the species scale cutoff in sensitivity of the low energy observables towards the GUT physics.