First direct limit on the top quark lifetime
We select tt-bar events by identifying an electron/muon and a secondary vertex (secvtx) tagged jet in events with >= 3 jets and large missing energy. In such events, the impact parameter (d0) of the lepton should be typically very small. Its distribution can be predicted from (1) the resolution of d0 measurements for high momentum lepton tracks, (2) the characteristics of standard model top quark decays, and (3) the magnitude and d0 distribution of non tt-bar backgrounds. Inconsistency between the predicted distribution and our observation could be indicative of a new, long-lived background to tt-bar, anomalous top production by a long-lived parent particle, or a large t lifetime.
Using electron and muon tracks produced in Z0/γ -> e+e-/μ+μ- events to determine the detector resolution, we show that the lepton d0 distribution in top candidate events from 318 pb-1 of CDF data is consistent with expectations, and derive a limit on the top quark lifetime:
cτt < 52.5 μm @ 95% C.L.
cτt < 43.5 μm @ 90% C.L.
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