Is Vtb=1 ?
J. Alwall, R. Frederix, J. -M. Gerard, A. Giammanco, M. Herquet, S. Kalinin, E. Kou, V. Lemaitre, F. Maltoni
TL;DR
The paper questions the assumption |V_{tb}| oughly 1$ by exploring minimal SM extensions with extra heavy quarks that relax CKM unitarity. It analyzes a vector-like t' quark that mixes with the top, effectively rescaling CKM elements through a mixing angle θ and potentially lowering |V_{tb}|, and also discusses a fourth generation, outlining the resulting FCNCs and modified Higgs couplings. EW precision data and FCNC constraints are shown to limit the allowed parameter space, while direct searches and single-top measurements at the Tevatron and LHC offer practical probes of the scenarios. The authors emphasize strategies to distinguish a vector-like t' from a genuine fourth generation through kinematic signatures and heavy-quark production channels.
Abstract
The strongest constraint on Vtb presently comes from the 3 x 3 unitarity of the CKM matrix, which fixes Vtb to be very close to one. If the unitarity is relaxed, current information from top production at Tevatron still leaves open the possibility that Vtb is sizably smaller than one. In minimal extensions of the standard model with extra heavy quarks, the unitarity constraints are much weaker and the EW precision parameters entail the strongest bounds on Vtb. We discuss the experimental perspectives of discovering and identifying such new physics models at the Tevatron and the LHC, through a precise measurement of Vtb from the single top cross sections and by the study of processes where the extra heavy quarks are produced.