Abstract
We obtain constraints on the mixing of vector-like quarks coupling predominantly to the third generation. We consider all (seven) relevant types of vector-like quarks, individually. The constraints are derived from oblique corrections and Z -> b bbar measurements at LEP and SLC. We investigate the implications of these constraints on LHC phenomenology, concerning the decays of the heavy quarks and their single production. We also explore indirect effects of heavy quark mixing in top and bottom couplings. A remarkable effect is the possibility of explaining the anomalous forward-backward asymmetry in Z -> b bbar at LEP, with a hypercharge -5/6 doublet. We also study the impact of the new quarks on single Higgs production at the LHC and Higgs decay.
Highlights
Vectorlike quarks are hypothetical spin 1=2 particles that transform as triplets under the color gauge group and whose left- and right-handed components have the same color and electroweak quantum numbers
We find that a quark doublet with hypercharge À5=6 has relatively weak bounds and can improve significantly the electroweak fit by reconciling, with just one free parameter, the predictions for Rb and the forward-backward (FB) asymmetry in eþeÀ ! Z ! bb", AbFB, with their observed values at the Large ElectronPositron (LEP) Collider and the Stanford Linear Collider (SLC) [40]
New vectorlike quarks can naturally have masses above the electroweak symmetry breaking scale. They are being searched for at the Large Hadron Collider (LHC), with lower limits on their masses in the range of 600–800 GeV, at present. These limits typically imply a small mixing with the standard model (SM) quarks, in order to fulfill indirect constraints from oblique corrections and Z ! bb" data
Summary
Vectorlike quarks are hypothetical spin 1=2 particles that transform as triplets under the color gauge group and whose left- and right-handed components have the same color and electroweak quantum numbers. The addition of vectorlike quarks to the SM is the simplest way of breaking the Glashow-Iliopoulos-Maiani [7] mechanism giving rise for example to tree-level flavor-changing neutral currents [8,9] and potentially striking new effects in low energy physics, none of which have been observed, . We find limits on the mixings from electroweak precision data at the Z pole, and use them to extract the allowed values for the mass splittings of the new quarks, their decay branching ratios, the rates of single production and the deviations in. As we have mentioned above, this pattern of mixing is well motivated by the large Yukawa coupling of the top quark, which suggests a close connection of the top quark (and the lefthanded component of the bottom quark) with any new physics related to electroweak symmetry breaking or to the fermion mass hierarchy. Appendix A collects the analytical expressions for all the couplings of light and heavy quarks to the gauge bosons and the Higgs, and Appendix B the partial widths for the different heavy quark decay modes
Sign-in/Register to view highlights & summary 
Published Version (
Free)
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call