Data Sample & Event Selection

The analysis is performed on a sample corresponding to an integrated luminosity of 179 $\pm$ 11 pb$^{-1}$ collected with the CDF II detector ([2]) between February 2002 and September 2003. Data were collected by the hadronic $B$ trigger, designed to select events containing track pairs exiting from a common displaced vertex. The Level 1 trigger is based on the eXtremely Fast Tracker which reconstructs tracks with $p_T >$ 1.5 GeV/c in the transverse plane of the drift chamber. The trigger selection requires two opposite charge tracks with $p_T >$ 2.0 GeV/c and azimuthal opening angle smaller than 135$^{\circ}$. At Level 2 the Silicon Vertex Tracker performs a high-resolution online track reconstruction in the transverse plane within the silicon vertex detector and allows to select pairs of tracks with impact parameters larger than 100 $\mu$m, and transverse decay length larger than 200 $\mu$m, which are compatible with the hypothesis of a two-body decay of a long-lived particle. At Level 3, full event reconstruction is performed by standard reconstruction software running on a farm of commercial CPUs'. Cuts applied at Level 1 and Level 2 are confirmed using offline-quality quantities, and in addition the invariant mass of the track pair is required to be 4.0 GeV/c$^2< m(pair) <$ 6.0 GeV/c$^2$.