Search for Anomalous Missing ET in tt Events

We present a search for anomalous missing transverse energy in the lepton+jets tt final state. Such a phenomenon can arise from a number of Standard Model extensions, but we focus here on a search for a pair-produced top partner decaying to a top quark and a long lived neutral particle which escapes undetected. In a sample of 35.3 pb−1, we establish the first limits from the LHC on the mass of such a particle, excluding m(T)<300 GeV for a neutral particle mass of 10 GeV and m(T)<275 GeV for a neutral particle mass of 50 GeV with 95% confidence.