The vectorlike top partners T are predicted in many extensions of the Standard Model (SM). In a simplified model including a single vectorlike T quark with charge 2/3, we investigate the process pp → T j induced by the couplings between the top partner with the first and the third generation quarks at the LHC. We find that the mixing with the first generation can enhance the production cross section. We further study the observability of the single heavy top partner through the process pp → T (→ th)j → t(→ bℓν ℓ )h(→ γγ)j at the high-luminosity (HL)-LHC (a 14 TeV pp collider with an integrated luminosity of 3 ab −1 ). For three typical heavy T quark masses m T = 600, 800 and 1000 GeV, the 3σ exclusion limits, as well as the 5σ discovery reach in the parameter plane of the two variables g * − R L , are respectively obtained at the HL-LHC.

We explore the phenomenology of models containing one Vector-Like Quark (VLQ), t , which can decay into the Standard Model (SM) top quark, t, and a new spin-0 neutral boson, S, the latter being either a scalar or pseudoscalar state. We parametrise the underlying interactions in terms of a simplified model which enables us to capture possible Beyond the SM (BSM) scenarios. We discuss in particular three such scenarios: one where the SM state is supplemented by an additional scalar, one which builds upon a 2-Higgs Doublet Model (2HDM) framework and another which realises a Composite Higgs Model (CHM) through partial compositeness. Such exotic decays of the t can be competitive with decays into SM particles, leading to new possible discovery channels at the Large Hadron Collider (LHC). Assuming t pair production via strong interactions, we design signal regions optimised for one t → S t transition (while being inclusive on the othert decay, and vice versa), followed by the decay of S into the two very clean experimental signatures S → γ γ and S → Z(→ + −)γ. We perform a dedicated signalto-background analysis in both channels, by using Monte Carlo (MC) event simulations modelling the dynamics from the proton-proton to the detector level. Under the assumption of BR(t → S t) = 100%, we are therefore able to realistically quantify the sensitivity of the LHC to both the t and S masses, assuming both current and foreseen luminosities. This approach paves the way for the LHC experiments to surpass current VLQ search strategies based solely on t decays into SM bosons (W ± , Z, h).

Motivated by the latest LHC Higgs data, we calculate the new physics contributions to the Higgs decay channels of h → γγ, Zγ, τ τ, W W * and ZZ * in the left-right twin Higgs (LRTH) model, induced by the loops involving the heavy T-quark, the W H and φ ± bosons appeared in the LRTH model. We find that (a) for a SM-like Higgs boson around 125.5 GeV, the signal rates normalized to the corresponding standard model (SM) predictions are always suppressed when new physics contributions are taken into account and approach the SM predictions for a large scalar parameter f ; and (b) the LRTH prediction for R γγ agree well with the CMS measurement R γγ = 0.77 ± 0.27 at 1σ level, but differ with the ATLAS result. The forthcoming precision measurement of the diphoton signal at the LHC can be a sensitive probe for the LRTH model.