Limits on scalar leptoquark interactions and consequences for GUTs

Abstract

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 tt̄ production as measured by the CDF and DØ experiments. We investigate the role of this state in a plethora of avor 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 → bb̄ decay width. We perform a global fit of the relevant scalar couplings. This approach can explain the (g-2)μ anomaly while tensions among the CP violating observables in the quark sector, most notably the nonstandard CP phase (and width difference) in the Bs system cannot be fully relaxed. The results are interpreted in a class of grand unified models which allow for a light colored scalar with a mass below 1TeV. We find that the renormalizable SU(5) scenario is not compatible with our global fit, while in the SO(10) case the viability requires the presence of both the 126- and 120-dimensional representations.