Search for pair production of new heavy quarks that decay to a $\mathbf{Z}$ boson and a third generation quark in $\mathbf{pp}$ collisions at $\mathbf{\sqrt{s}=8}$ TeV with the ATLAS detector

A search is presented for the production of a new heavy quark with its antiparticle, assuming the new quark has a significant branching ratio to decay into a $Z$ boson and a third generation Standard Model quark. In the case of a new charge $+2/3$ quark ($T$) the decay targeted is $T \rightarrow Zt$, while for a new charge $-1/3$ quark ($B$) the decay targeted is $B \rightarrow Zb$. The search uses a dataset corresponding to $14.3~\mathrm{fb}^{-1}$ of $pp$ collisions at $\sqrt{s}=8$ TeV recorded in 2012 with the ATLAS detector at the CERN Large Hadron Collider. Selected events contain a high transverse momentum $Z$ boson candidate reconstructed from a pair of oppositely charged electrons or muons. Additionally, the presence of at least two jets possessing properties consistent with the decay of a $b$ hadron is required, as well as large total transverse momentum of all central jets in the event. No significant excess of events above the Standard Model expectation is observed, and upper limits are derived for vector-like quarks of various masses in a two-dimensional plane of branching ratios. Under branching ratio assumptions corresponding to a weak-isospin singlet scenario, a $T$ ($B$) quark with mass lower than 585 (645) GeV is excluded at the 95% confidence level. Under branching ratio assumptions corresponding to a particular weak-isospin doublet scenario, a $T$ ($B$) quark with mass lower than 680 (725) GeV is excluded at the 95% confidence level.