Using an impact-parameter tag to select an enriched sample of Z0bb» events, we have measured the difference between the average charged multiplicity of bb» and all hadronic Z0 decays to be 2.11.8(stat) 0.6(syst) tracks per event. The resulting total (nonleading) charged multiplicity for Z0 bb» events is 23.1(12.0)1.8 0.6 tracks. A comparison of this nonleading multiplicity to hadronic multiplicity data in the range of 10 to 60 GeV supports the hypothesis of flavor-independent hadronic fragmentation, and yields a measurement of the average energy fraction of bottom hadrons in Z0 decays of xEb=0.620.100.04. © 1992 The American Physical Society.

A search for pair production of stable charged particles from Z decay has been performed with the Mark II detector at the SLAC Linear Collider. Particle masses are determined from momentum, ionization energy loss, and time-of-flight measurements. A limit excluding pair production of stable fourth-generation charged leptons and stable mirror fermions with masses between the muon mass and 36.3 GeV/c2 is set at the 95% confidence level. Pair production of stable supersymmetric scalar leptons with masses between the muon mass and 32.6 GeV/c2 is also excluded. © 1990 The American Physical Society.

We have measured the fraction of bb» events in hadronic Z0 decays, Rbb», using the vertex detector system of the Mark II detector at the SLAC Linear Collider. We tag bb» events by requiring the coincidence of three or more tracks with significant impact parameters. This tag is 50% efficient and results in a sample of 85% purity. We find Rbb»=0.251+0.049+0.030, in good agreement with other measurements and the standard model prediction. © 1991 The American Physical Society.

Using isolated leptons reconstructed in the Mark II detector to tag bb events, we measure the fraction of bb events in hadronic Z0 decays to be 0.23-0.09+0.11, in good agreement with the standard-model prediction of 0.22. We find (Zbb)=0.43-0.17+0.21 GeV. © 1990 The American Physical Society.

We present measurements by the Mark II experiment of the ratios of the leptonic partial widths of the Z boson to the hadronic partial width. The results are ee/had=0.037-0.012+0.016, &=0.053-0.0150.020, and /had=0.066-0.017+0.021, in good agreement with the standard-model prediction of 0.048. From the average leptonic width result, ll/had=0.053-0.009+0.010, we derive had=1.56-0.24+0.28 GeV. We find for the vector coupling constants of the tau and muon v2=0.310.31-0.30+0.43 and v2=0.050.30-0.23+0.34. © 1989 The American Physical Society.

We measured the differential jet-multiplicity distribution in e+e- annihilation with the Mark II detector. This distribution is compared with the second-order QCD prediction and s is determined to be 0.1230.0090.005 at ssSMZ (at the SLAC Linear Collider) and 0.1490.0020.007 at ss=29 GeV (at the SLAC storage ring PEP). The running of s between these two center-of-mass energies is consistent with the QCD prediction. © 1990 The American Physical Society.

We have searched for supersymmetric particles in 528 Z decays with the Mark II detector at the SLAC Linear Collider. We place 95%-confidence-level lower mass limits on degenerate squarks, nondegenerate up-type squarks, nondegenerate down-type squarks, charginos, pair-produced unstable neutralinos, and neutralinos from associated production. © 1990 The American Physical Society.

We have searched for events with new-particle topologies in 390 hadronic Z decays with the Mark II detector at the SLAC Linear Collider. We place 95%-confidence-level lower limits of 40.7 GeV/c2 for the top-quark mass, 42.0 GeV/c2 for the mass of a fourth-generation charge -(1/3 quark, and 41.3 GeV/c2 for the mass of an unstable Dirac neutral lepton. © 1989 The American Physical Society.

Using the sample of neutral vector bosons (Z's) produced at the SLAC Linear Collider and detected with the Mark II detector, we search for the decay of the Z to a pair of particles, one of which decays to two charged particles. The observed number of Z decays with this signature excludes, at a confidence level greater than 95%, the decay of the Z to a pair of fourth-generation Dirac neutrinos with mass between 2.5 and 22 GeV, a decay length less than about 1 cm, and coupling to any of the first three generations of charged leptons. This is the first time the existence of such a lepton coupling to the ± has been excluded by a direct search. © 1990 The American Physical Society.

We describe a search for the decay of the Z boson into pairs of doubly charged Higgs bosons with the Mark II detector operating at the SLAC Linear Collider. Each Higgs boson is assumed to decay into a same-sign leptonic pair. No event candidates are found in a sample of 528 Z decays. At the 95% confidence level, this result excludes the region of leptonic coupling gll3×10-7 and Higgs-boson mass 6.5MH 36.5 GeV/c2 for isotriplet (left-handed) Higgs bosons. Isosinglet (right-handed) Higgs bosons are excluded in the same gll interval and in the mass interval 7.3 MH 34.3 GeV/c2. © 1990 The American Physical Society.