Abstract
A search for pair-produced vector-like quarks using events with exactly one lepton (
e
or
μ
), at least four jets including at least one
b
-tagged jet, and large missing transverse momentum is presented. Data from proton–proton collisions at a centre-of-mass energy of
s
=
13
TeV
, recorded by the ATLAS detector at the LHC from 2015 to 2018 and corresponding to an integrated luminosity of 139 fb
-
1
, are analysed. Vector-like partners
T
and
B
of the top and bottom quarks are considered, as is a vector-like
X
with charge
+
5
/
3
, assuming their decay into a
W
,
Z
, or Higgs boson and a third-generation quark. No significant deviations from the Standard Model expectation are observed. Upper limits on the production cross-section of
T
and
B
quark pairs as a function of their mass are derived for various decay branching ratio scenarios. The strongest lower limits on the masses are 1.59
TeV
assuming mass-degenerate vector-like quarks and branching ratios corresponding to the weak-isospin doublet model, and 1.47
TeV
(1.46
TeV
) for exclusive
T
→
Z
t
(
B
/
X
→
W
t
) decays. In addition, lower limits on the
T
and
B
quark masses are derived for all possible branching ratios.
