Refined measurement of theB-hadron lifetime

We report a new measurement of the average lifetime of hadrons containing bottom quarks. The B hadron decays are tagged by identifying leptons at high transverse momentum. From a fit to the lepton impact parameter distribution, the average B hadron lifetime is found to be (0.98 f 0.12 f 0.13) x lo-l2 sec. The lifetime of hadrons containing bottom quarks is a measure of the strength of the weak transitions between the bottom quark and the charm and up quarks. In terms of the 3 x 3 quark mixing matrix proposed by Kobayashi and Maskawaf the B hadron lifetime depends on the magnitude of the matrix elements Vub and &b. Studies of B semileptonic decay’ have shown that Iv&l is small compared to I&l, and therefore the B lifetime essentially measures II&l and limits lvUbl. The data used in this measurement were collected with the Mark II detector at the e+estorage ring PEP (EC, = 29 GeV). We have previously reported a B lifetime measurement3 based on a data sample of 80 pb-l. The present work: based on a data sample of 204 pb-l, includes the previous data and supersedes our earlier analysis. We use the same procedure of measuring the impact parameters of leptons produced in B decay. However, we have improved upon the previous result through direct measurement of the experimental resolution function, a more precise determination of the B production point, and a comprehensive analysis of inclusive lepton production. These improvements combined with the increased statistics make this measurement of the B lifetime the most precise from any experiment to date. The Mark II detector has been described in detail elsewhere.5 A high resolution drift chamber, known as the vertex chamber, is situated inside the main tracking chamber. The two drift chambers are immersed in a solenoidal magnetic field of 2.3 kG. Particle trajectories are measured with high precision in the (2, y) plane perpendicular to th e b earns, and the impact parameter is accurately determined in that plane. Electrons are identified over 64 % of the solid angle with a lead-liquid-argon calorimeter. Muons are identified over 44 % of the solid angle by-a system of hadron absorbers and proportional tubes.