Search for single production of vector-like quarks decaying into a W-boson and a b-quark at 13 TeV

A search for singly-produced vector-like quarks $Q$, where $Q$ can be either a $T$ quark with charge $2/3$ or a $Y$ quark with charge $-4/3$, is performed in $3.2$ fb$^{-1}$ of proton-proton collision data at a centre-of-mass energy of 13 TeV recorded with the ATLAS detector at the LHC. The analysis targets $Q \to Wb$ decays where the $W$ boson decays leptonically. No significant excess over Standard Model backgrounds is observed and upper limits on the $Q \to Wb$ cross-section times branching ratio are set as a function of the vector-like quark mass. For a $QWb$ coupling strength of $\sqrt{(c_{L}^{Wb})^2+(c_{L}^{Wb})^2}=1/\sqrt{2}$, the observed (expected) 95% confidence level lower limit on the $Y$-quark mass is 1.44 TeV (1.45 TeV). The results are also interpreted as limits on the $QWb$ coupling strength and the mixing with the Standard Model sector for a singlet $T$ quark or a $Y$ quark from a $(B,Y)$ doublet. The smallest excluded coupling-strength values are obtained for vector-like quark masses around 1000 GeV; they are as small as $|c_{L}^{Wb}|=0.45$ for a $T$ quark and $\sqrt{(c_{L}^{Wb})^2+(c_{L}^{Wb})^2}=0.33$ for a $Y$ quark. Within a $(B,Y)$ doublet model, the excluded mixing parameter, $|\sin{\theta_{R}}|$, in this mass region is comparable to the exclusion limits from electroweak precision observables.