Explaining the ttbar Forward-Backward Asymmetry from a GUT-Inspired   Model
  We consider a model that includes light colored scalars from the {\bf 45} and {\bf 50} representations of SU(5) in order to explain the CDF- and D$\O$-reported $t\bar{t}$ forward-backward asymmetry, $A^{t\bar{t}}_{FB}$. These light scalars are, labeled by their charges under the Standard Model gauge groups, the $(6,1)_{4/3}$ and $(\bar{6},3)_{1/3}$ from the {\bf 50} and the $(8,2)_{1/2}$ from the {\bf 45}. When the Yukawa coupling of the {\bf 50} is reasonably chosen and that of the {\bf 45} kept negligible at the scale of $M_Z$, the model yields phenomenologically viable results in agreement with the total $A^{t\bar{t}}_{FB}$ reported by CDF at the 0.7$\sigma$ level and with $A^{t\bar{t}}_{FB} (M_{t\bar{t}} \ge 450 \text{GeV})$ at the 2.2$\sigma$ level. Additionally, the Yukawa coupling of the {\bf 50} remains perturbative to the GUT scale (which defines a "reasonably chosen" value at $M_Z$), and the presence of the light scalar from the {\bf 45} allows for gauge coupling unification at a scale of $M_{GUT} \sim 10^{17}$ GeV. 
David C. Stone and Patipan Uttayarat
hep-ph