Measurement of the average B hadron lifetime in Z0 decays using reconstructed vertices.

We report a measurement of the average B-hadron lifetime using data collected with the SLD detector at the SLC in 1993. An inclusive analysis selected three-dimensional vertices with B-hadron lifetime information in a sample of 50k Z0 decays. A lifetime of 1.564±0.030 (stat)±0.037 (syst) ps was extracted from the decay length distribution of these vertices using a binned maximum-likelihood method. Measurements of the B-hadron lifetime τB are useful in exploring the physics of b-quarks, particularly in determining the weak couplings of the b to lighter quarks. Precise measurements of the average value of τB remain interesting in view of the significant variation in the world average over the past few years (see Refs. [1–3]). Previous determinations of τB have relied on either tight lepton momentum cuts [4] or stringent vertex constraints [5] to isolate B hadron decay vertices. In the method presented here, all decay modes are used with high efficiency. This letter presents a measurement of τB based on inclusive three-dimensional reconstruction of secondary vertices in Z → bb̄ events. The data for this analysis were collected at the SLAC Linear Collider (SLC) with the SLD experiment in 1993. The analysis uses a topological technique to select vertices with lifetime information and relies on Monte Carlo (MC) modeling to extract τB, using a maximum-likelihood method. This yielded a precise measurement of τB, even with a relatively small data sample. During the 1993 run, SLD recorded 1.8 pb−1 of e+e− annihilation data at a center-ofmass energy of 91.26 ± 0.02 GeV. Charged particle tracking was provided by the central drift chamber (CDC) [6] and by the vertex detector (VXD) [7], with a combined impact parameter resolution in rφ(rz) parameterized as σ = 11(38) ⊕ 70/p √ sin θ μm, where p is expressed in GeV/c. The liquid argon calorimeter [8] was used for triggering, thrust axis determination, and jet finding. The 〈 rms 〉xyz profile of the SLC beams was approximately 2.4 × 0.8 × 700 μm at the interaction point (IP). The x and y positions of the IP were continuously measured, using reconstructed tracks from ∼ 30 sequential hadronic Z decays, giving σ xy = 7 ± 2 μm [9]. The z position was measured on an event-by-event basis using the median z position of tracks at their point-of-closest-approach to the IP in the xy plane, with a resolution of σz ≈ 52 μm for Z → bb̄ events [9]. A detailed simulation of the detector and physics processes was used in this analysis. Hadronic Z decays were generated using JETSET 6.3 [10] adjusted to reproduce data from other e+e− experiments. The fragmentation function for band c-quarks followed the Peterson parameterization [11] with 2b = 0.006 and 2c = 0.06, respectively. A detailed description of theB-hadron decay model may be found in Ref. [9]. Beam related backgrounds and detector noise were simulated by overlaying random trigger events that occurred in close time proximity to a Z decay. Detector response was simulated with GEANT 3.15 [12]. Hadronic Z decays were selected by requiring at least seven reconstructed tracks, total track energy greater than 18 GeV, and |cos θthrust| < 0.71 (thrust axis within CDC-VXD acceptance). A sample of 29,400 events was selected with a nonhadronic background estimated to be < 0.1%.