engleski

Limits on scalar leptoquark interactions and consequences for GUTs

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 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 $Z\to b \bar b$ decay width. We perform a global 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 in the $B_s$ system cannot be relaxed. The results are interpreted in a class of grand unified models which allow for a light colored scalar with a mass below $1$\, TeV. {; ; We find that the $SU(5)$ scenario can be realized with a very small vacuum expectation value for the Higgs scalar in the $45$-dimensional representation, while in the $SO(10)$ case the viability requires the presence of both the $126$- and $120$-dimensional representations. Conversely, neither $SU(5)$ nor $SO(10)$ unification with a purely symmetric Yukawa sector are compatible with our global fit.

14.80.Sv; 12.15.Ji; 12.10.-g; 12.10.Kt

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