Özet:
We present here our results from the search for the pair production of new massive quarks using 20fb{u100000}1 data collected by the ATLAS detector in 2012 protonproton collisions at p s =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 E6 as its symmetry group. This model predicts an additional iso-singlet down type quark for each existing SM family with possible decay channels involving W ;Z and H. Assuming a similar mass structure, the lightest of these quarks denoted by the letter D would be the rst one to be discovered at ATLAS. Our search focuses on the decay signature of D quark decays via a Z 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 nal states involving b-quarks. Our search di ers by vetoing the b-quarks in the nal state. No excess events have been observed beyond the backgrounds of the SM. Our measurements are consistent with the SM expectations without a signi cant deviation. We exclude the heavy quarks with mass less than 530 GeV with 95% con dence level using the branching ratios predicted by the E6 model. A higher mass limit of 730 GeV is put with the assumption that quarks decay in the Z channel with 100% branching ratio.
