Masses of Third Family Vector-like Quarks and Leptons in Yukawa-Unified   $E_6$
  In supersymmetric $E_6$ the masses of the third family quarks and charged lepton, $t-b-\tau$, as well as the masses of the vector-like quarks and leptons, $D-\bar{D}$ and $ L-\bar{L}$, may arise from the coupling $27_3$ x $27_3$ x $27_H$, where $27_3$ and $27_H$ denote the third family matter and Higgs multiplets respectively. We assume that the SO(10) singlet component in $27_H$ acquires a TeV scale VEV which spontaneously breaks U(1)$_\psi$ and provides masses to the vector-like particles in $27_3$, while the MSSM doublets in $27_H$ provide masses to $ t, b$ and $\tau$. Imposing Yukawa coupling unification $h_t=h_b=h_{\tau}=h_D=h_L$ at $M_{GUT}$ and employing the ATLAS and CMS constraints on the $Z'_\psi$ boson mass, we estimate the lower bounds on the third family vector-like particles $D-\bar{D}$ and $L-\bar{L}$ masses to be around 5.85 TeV and 2.9 TeV respectively. These bounds apply in the supersymmetric limit. 
Aditya Hebbar, George K. Leontaris, Qaisar Shafi
hep-ph