Search for Pair Production of a New $b'$ Quark that Decays into a $Z$ Boson and a Bottom Quark with the ATLAS Detector at the LHC

A search with the ATLAS detector for the pair production of a new $b'$ quark in the $Z$ boson plus $b$ quark decay channel is reported. The $pp$ collision data comprise $4.7\ \mathrm{fb}^{-1}$ of integrated luminosity at $\sqrt{s}=7\ \mathrm{TeV}$ collected at the CERN Large Hadron Collider in 2011. This search complements recent searches for a $b'$ decaying to a $W$ boson and a top quark, and it is particularly important for new physics models containing vector-like quarks. From events containing both a $Z$ boson reconstructed from electrons and a $b$ tagged jet, any potential signal is enriched by selecting a subsample with large $b'$ candidate $p_T$. No evidence for a $b'$ signal is found in this subsample; the invariant mass spectrum of the $b'$ candidate agrees well with the Standard Model prediction. For the case of a $b'$ that decays only to $Z+b$, masses $m_{b'} < 466\ \mathrm{GeV}$ are excluded at the 95% confidence level. Given the branching ratios of a vector-like singlet $b'$ coupling only to the third Standard Model generation, masses $m_{b'} < 421\ \mathrm{GeV}$ are excluded.