Abstract
A search is presented for the pair production of heavy vectorlike quarks, πβ’Β―π or π΅β’Β―π΅, that decay into final states with jets and no reconstructed leptons. Jets in the final state are classified using a deep neural network as arising from hadronically decaying π/π bosons, Higgs bosons, top quarks, or background. The analysis uses data from the ATLAS experiment corresponding to 36.1ββfbβ1 of proton-proton collisions with a center-of-mass energy of βπ =13ββTeV delivered by the Large Hadron Collider in 2015 and 2016. No significant deviation from the Standard Model expectation is observed. Results are interpreted assuming the vectorlike quarks decay into a Standard Model boson and a third-generation-quark, π βπβ’π,π»β‘π‘,πβ’π‘ or π΅ βπβ’π‘,π»β‘π,πβ’π, for a variety of branching ratios. At 95% confidence level, the observed (expected) lower limit on the vectorlike π΅-quark mass for a weak-isospin doublet (π΅, π) is 950 (890) GeV, and the lower limits on the masses for the pure decays π΅ βπ»β‘π and π βπ»β‘π‘, where these results are strongest, are 1010 (970) GeV and 1010 (1010) GeV, respectively.