Theory and phenomenology of exotic isosinglet quarks and squarks

TLDR

Extensions of the MSSM predict vectorlike fermions and scalar superpartners that are chiral under additional gauge symmetries. This study investigates the production and decay of a heavy isosinglet –1/3 quark and its scalar partner, and examines how their collider signatures depend on mass hierarchies with the lightest neutralino. We analyze the exotic quark and squark decay modes—mixing with d, s, or b quarks, leptoquark or diquark couplings, or higher‑dimension operators—under various symmetry assumptions, and assess existing and projected collider, indirect, proton‑decay, and BBN constraints. Depending on low‑energy symmetries, the exotic particles may decay via mixing with SM quarks, leptoquark/diquark couplings, or remain stable against renormalizable interactions but decay through higher‑dimension operators on cosmological timescales.

Abstract

Extensions of the minimal supersymmetric standard model often predict the existence of new fermions and their scalar superpartners which are vectorlike with respect to the standard model gauge group but may be chiral under additional gauge factors. In this paper we explore the production and decay of an important example, i.e., a heavy isosinglet charge $\ensuremath{-}1/3$ quark and its scalar partner, using the charge assignments of a $\mathbf{27}$-plet of ${E}_{6}$ for illustration. We emphasize that, depending on the symmetries of the low-energy theory, such exotic particles may decay by the mixing of the fermion with the $d$, $s$, or $b$ quarks; may decay by leptoquark or diquark couplings (which may nevertheless preserve a form of $R$ parity); or may be stable with respect to renormalizable couplings but decay by higher-dimension operators on cosmological times scales. We discuss the latter two possibilities in detail for various assumptions concerning the relative masses of the exotic fermions, scalars, and the lightest neutralino, and emphasize the necessity of considering the collider signatures in conjunction with the normal minimal supersymmetric standard model processes. Existing and projected constraints from colliders, indirect experiments, proton decay, and big bang nucleosynthesis are considered.