Orateur
Nejc Kosnik
Description
The experimental results on the $t \bar t$ production cross section at the Tevatron are well described by the QCD contributions within the standard model, while the recent measurement of the forward-backward asymmetry cannot be accounted for within this framework. This discrepancy can be explained by an exchange of a colored weak singlet scalar in the $u$-channel. Such state $\Delta$ couples to up-quark pairs antisymmetrically in flavor space, independently of an underlying model, and appears naturally in some of the grand unified theories. We find that both the $t \bar t$ production cross section and the forward-backward asymmetry can be accommodated simultaneously if mass of $\Delta$ is around 400 GeV. The additional constraints on the relevant up-quark Yukawa couplings come from di-jet and single top production measurements at the Tevatron as well as from $D^0-\bar D^0$ oscillations. In a particular $SU(5)$ GUT model, this scalar state stems from the 45-dimensional Higgs representation and is allowed to be lighter than 1 TeV without violating limits from proton decay lifetime. We demonstrate how the obtained information enables to constrain the Yukawa couplings of the up-quarks at the GUT scale. Additional constraints from processes involving down-quarks and leptons indicate that this $SU(5)$ scenario should be realized with very small vacuum expectation value for the $45$-dimensional Higgs. We analyze experimental signatures and find that $\Delta$ associated top production could be probed in the $t \bar t$ + jets final states at Tevatron and the LHC.
References:
arXiv:0912.0972
arXiv:1007:2604
arXiv:1107.5393
