A search for new phenomena in t (t) over bar events with large missing transverse momentum in proton-proton collisions at a center-of-mass energy of 7 TeV is presented. The measurement is based on 1: 04 fb(-1) of data collected with the ATLAS detector at the LHC. Contributions to this final state may arise from a number of standard model extensions. The results are interpreted in terms of a model where new top-quark partners are pair produced and each decay to an on-shell top (or antitop) quark and a long-lived undetected neutral particle. The data are found to be consistent with standard model expectations. A limit at 95% confidence level is set excluding a cross section times branching ratio of 1.1 pb for a top-partner mass of 420 GeVand a neutral particle mass less than 10 GeV. In a model of exotic fourth generation quarks, toppartner masses are excluded up to 420 GeV and neutral particle masses up to 140 GeV. RI della Volpe, Domenico/B-4482-2012; Wolter, Marcin/A-7412-2012; Perrino, Roberto/B-4633-2010; valente, paolo/A-6640-2010; O'Shea, Val/G-1279-2010; Doyle, Anthony/C-5889-2009; Ferrando, James/A-9192-2012; Li, Xuefei/C-3861-2012; Buttar, Craig/D-3706-2011; Pina, Joao /C-4391-2012; Takai, Helio/C-3301-2012; St.Denis, Richard/C-8997-2012; Jones, Roger/H-5578-2011; Robson, Aidan/G-1087-2011; Britton, David/F-2602-2010; Moorhead, Gareth/B-6634-2009; Fazio, Salvatore /G-5156-2010; Smirnova, Lidia/D-8089-2012; Sivoklokov, Sergey/D-8150-2012; Smirnov, Sergei/F-1014-2011; Gladilin, Leonid/B-5226-2011; Barreiro, Fernando/D-9808-2012; Kramarenko, Victor/E-1781-2012; Prokoshin, Fedor/E-2795-2012; Alexa, Calin/F-6345-2010; Gutierrez, Phillip/C-1161-2011; Pacheco Pages, Andres/C-5353-2011; Livan, Michele/D-7531-2012; Petrucci, Fabrizio/G-8348-2012; Wemans, Andre/A-6738-2012; Fabbri, Laura/H-3442-2012; Kurashige, Hisaya/H-4916-2012; Villa, Mauro/C-9883-2009; Kuzhir, Polina/H-8653-2012; Delmastro, Marco/I-5599-2012

Search for New Phenomena in t(t)overbar Events with Large Missing Transverse Momentum in Proton-Proton Collisions at root s=7 TeV with the ATLAS Detector