Abstract
Vector-like quarks (VLQ) that are partners of the heavy top and bottom quarks are predicted in many extensions of the standard model (SM). We explore the possibility that these states could explain not only the long-standing anomaly in the forward–backward asymmetry in b-quark production at LEP, A_mathrm{FB}^b , but also the more recent sim 2sigma deviation of the cross section for the associated Higgs production with top-quark pairs at the LHC, sigma (pprightarrow tbar{t} H). Introducing three illustrative models for VLQs with different representations under the SM gauge group, we show that the two anomalies can be resolved while satisfying all other theoretical and experimental constraints. In this case, the three different models predict VLQ states in the 1–2 TeV mass range that can be soon probed at the LHC. In a second step, we discuss the sensitivity on the VLQ masses and couplings that could be obtained by means of a percent level accuracy in the measurement of ratios of partial Higgs decay widths, in particular Gamma (H rightarrow gamma gamma )/Gamma (H rightarrow ZZ^*) and Gamma (H rightarrow bbar{b})/Gamma (H rightarrow WW^*). We show that top and bottom VL partners with masses up to sim 5 TeV and exotic VLQs with masses in the 10 TeV range can be probed at the high-luminosity LHC.
Highlights
At the Large Hadron Collider (LHC), direct experimental searches have imposed the model independent bound mVLQ 800 GeV [49,50,51] on Vector-like quarks (VLQ) masses from pair-production through strong interactions, almost independently of the electric charge
A 5 % error in the measurement of Dbb can be sensitive to the presence of bottom-like VLQs with masses up to ∼5 TeV
Dγ γ is more sensitive to the VLQs with higher electric charge that occur in models B and C
Summary
At the LHC, direct experimental searches have imposed the model independent bound mVLQ 800 GeV [49,50,51] on VLQ masses from pair-production through strong interactions, almost independently of the electric charge. Third generation VLQs alter the properties of the heavy top and bottom quarks through fermion mixing and strong constraints can be e.g. obtained from the Z -boson decay into bottom quarks, Z → bb, as measured at the LEP e+e− collider at energies close to the Z -resonance [54,55]. Indirect constraints on VLQs come from the data collected on the 125 GeV Higgs particle that has been observed at the LHC [61,62,63] These new quarks contribute to the loop-induced Higgs couplings to pairs of gluons and photons, either through their additional exchange in the triangular loops or when altering the important top quark loop contribution by mixing [65,66,67,68,69]. The Higgs decay channels in the various final states detected so far by the ATLAS and CMS collaborations, namely the H → γ γ , Z Z , W W and eventually τ +τ − final states with the Higgs state dominantly
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