Search for pair-produced vector-like quarks of charge -1/3 in dilepton+jets final state in pp collisions at sqrt(s) = 8 TeV

A search is performed for the pair-production of a heavy vector-like quark b' of electric charge $-1/3$ and its anti-particle, one of which decays into a Z boson and a bottom quark, using data collected by the CMS experiment, from the LHC pp collisions at centre-of-mass energy of 8 TeV and corresponding to integrated luminosity of 19.6 fb$^{-1}$. The invariant mass distribution of the b' candidates, reconstructed from Z bosons, identified via Z$\rightarrow$e$^{+}$e$^{-}$ or Z$\rightarrow \mu^{+}\mu^{-}$, and b-tagged jets, is examined for an excess of events and is found to be consistent with the standard model expectations. The expected lower limit on the mass of the b' quark is 680 GeV$/c^{2}$, while the observed limit is placed at 700 GeV$/c^{2}$, at $95\%$ confidence level, for b'$\rightarrow$bZ branching ratio of $100\%$. Furthermore, limits on the b'$\rightarrow$bZ branching ratio are quoted as a function of the mass of the b' quark when the b' decay to both bZ and tW are possible.