Boundary localized terms in universal extra-dimensional models through a dark matter perspective

Abstract

In universal extra dimension (UED) models with one compactified extra dimension, a ${\mathbf{Z}}_{2}$ symmetry, termed KK parity, ensures the stability of the lightest Kaluza-Klein particle (LKP) which could be a viable dark matter candidate. This symmetry leads to two fixed points. In nonminimal versions of UED boundary-localized (kinetic or mass) terms (BLT) for different fields are included at these fixed points and KK parity may be violated. However, BLTs with same strength at both points induce a new ${\mathbf{Z}}_{2}$ symmetry which restores the stability of the LKP. We show that the BLTs serve to relax the bounds set on the compactification scale in UED by the observed dark matter relic density. At the same time, the precision of the dark matter measurements severely correlates the BLT parameters of gauge bosons and fermions. Depending on the parameter values, the LKP can be chosen to be the level-1 photon, which is essentially the ${B}^{(1)}$, or the level-1 $Z$ boson, basically the ${W}_{3}^{(1)}$. We find that in the latter case the relic density is too small if the ${W}_{3}^{(1)}$ has a mass $\ensuremath{\sim}1\text{ }\text{ }\mathrm{TeV}$. We also explore the prospects of direct detection of an LKP which matches the observed dark matter relic density.