Scalar diquark in t tbar production and constraints on Yukawa sector of grand unified theories

A colored weak singlet scalar state with hypercharge 4/3 is one of the possible candidates for the explanation of the unexpectedly large forward-backward asymmetry in $t \bar t$ production as measured by the CDF and D0 experiments. We investigate the role of this state in a plethora of flavor changing neutral current processes and precision observables of down-quarks and charged leptons. Our analysis includes tree- and loop-level mediated observables in the K and B systems, the charged lepton sector, as well as the $Z \to b \bar b$ width. We perform a fit of the relevant scalar couplings. This approach can explain the $(g-2)_\mu$ anomaly while tensions among the CP violating observables in the quark sector, most notably the nonstandard CP phase (and width difference) in the $B_s$ system cannot be fully relaxed. The results are interpreted in a class of GUT models which allow for a light colored scalar with a mass below 1 TeV.