Search for Pair Produced Heavy Quarks Decaying into a Z Boson and a Light Generation Jet

We present here our results from the search for the pair production of new massive quarks using $20\ifb$ data collected by the ATLAS detector in 2012 proton-proton collisions at $\rts=$8TeV centre of mass energy. There are many models extending beyond the well established Standard Model (SM) predicting additional massive quarks on top of the existing quark list. One of them is the grand unified theory (GUT) having the exceptional $E_{6}$ as its symmetry group. This model predicts an additional iso-singlet down type quark for each existing SM family with possible decay channels involving $\Wpm,\Zboson$ and $H$. Assuming a similar mass structure, the lightest of these quarks denoted by the letter $D$ would be the first one to be discovered at ATLAS. Our search focuses on the decay signature of $D$ quark decays via a $\Zboson$ boson which would further decay into two leptons. The other heavy quark searches at ATLAS are carried out with the assumption that the new quarks should couple to the heavy generations $t$,$b$ only with final states involving $b$-quarks. Our search differs by vetoing the $b$-quarks in the final state. No excess events have been observed beyond the backgrounds of the SM. Our measurements are consistent with the SM expectations without a significant deviation. We exclude the heavy quarks with mass less than 530 GeV with 95\% confidence level using the branching ratios predicted by the $E_{6}$ model. A higher mass limit of 730 GeV is put with the assumption that quarks decay in the $Z$ channel with 100\% branching ratio.