Search for pair-produced vectorlike quarks coupling to light quarks in the lepton plus jets final state using 13 TeV $pp$ collisions with the ATLAS detector

A search is presented for the pair production of heavy vectorlike quarks (VLQs) that each decay into a $W$ boson and a light quark. This study focuses on events where one $W$ boson decays into leptons and the other into hadrons. The search analyzed $140\text{}\text{}{\mathrm{fb}}^{-1}$ of $pp$ collision data with $\sqrt{s}=13\text{}\text{}\mathrm{TeV}$, recorded by the ATLAS detector from 2015 to 2018 during run 2 of the Large Hadron Collider. The final state is characterized by a high-transverse-momentum isolated electron or muon, large missing transverse momentum, multiple small-radius jets, and a single large-radius jet identified as originating from the hadronic decay of a boosted $W$ boson. With higher center-of-mass energy and integrated luminosity than in the run 1 search, and improved analysis tools, this analysis excludes VLQs ($Q$) with masses below 1530 GeV at 95% confidence level for the branching ratio $B\left(Q\to Wq\right)=1$, an improvement of 840 GeV on the previous ATLAS limit.