STUDY OF THE 4-FERMION FINAL-STATE AT THE Z-RESONANCE

The process e+e- --> llBARffBAR, where 1 is a charged or a neutral lepton and f any charged fermion, is analyzed. The study uses the ALEPH data collected at LEP from 1989 to 1993 at centre-of-mass energies between 88 and 95 GeV, corresponding to almost two million hadronic Z decays and to a total integrated luminosity of 79 pb-1. For all channels, the data agree well with the standard model expectation both in shape and normalization. The indication of an excess in the e+e- --> tau+ tau- ffBAR channel, reported by ALEPH in 1991, is not confirmed

STUDY OF THE 4-FERMION FINAL-STATE AT THE Z-RESONANCE

The process e+e- --> llBARffBAR, where 1 is a charged or a neutral lepton and f any charged fermion, is analyzed. The study uses the ALEPH data collected at LEP from 1989 to 1993 at centre-of-mass energies between 88 and 95 GeV, corresponding to almost two million hadronic Z decays and to a total integrated luminosity of 79 pb-1. For all channels, the data agree well with the standard model expectation both in shape and normalization. The indication of an excess in the e+e- --> tau+ tau- ffBAR channel, reported by ALEPH in 1991, is not confirmed

A search for new quarks and leptons from Z0 decay at LEP

A search for Z0 decays into pairs of possible new heavy quarks (t and b′), new heavy charged leptons (L±), stable heavy neutral leptons (νL) and unstable heavy neutral leptons (L0) is performed on data collected by the ALEPH detector corresponding to 11 550 events of Z0→hadrons. The limits on the masses of the heavy quarks are Mt > 45.8 GeV and Mb′ > 46.0 GeV, allowing for both charged-current and flavor-changing neutral-current decays of the b′. If an L± decays into a stable νL, then for MνL MνL. Finally, while the mass of the stable νL is excluded up to 42.7 GeV, the mass of the unstable L0 is excluded up to 45.7 GeV with the mixing parameters |UℓL0|2 down to 10−13 at this mass. For 25.0 GeV < ML0 < 42.7 GeV, all values of |UℓL0|2 are excluded. All limits are given at 95% CL

Measurement of D(s)+ meson production in Z decays and of the anti-B(s)0 lifetime

D-s(+) mesons produced in Z(0) --&gt; b (b) over bar events were separated from the Z(0) --&gt; c (c) over bar component using a lifetime tag. Using a sample of 1.5 million hadronic Z decays collected with the ALEPH detector the (B) over bar(s)(0) and D-s(+) yields have been measured: [GRAPHICS] The (B) over bar(2)(0) lifetime was measured in a (B) over bar(s)(0) enriched sample reconstructing the decay length from the vertex of the D-s(+) with a hadron from the (B) over bar(0)(s) decay, The result obtained is: [GRAPHICS

Measurements of the b baryon lifetime

Using about 1.5 million hadronic Z decays recorded with the aleph detector, the lifetime of the b baryons has been measured using two independent data samples. From a maximum likelihood fit to the impact parameter distribution of leptons in 519 Λℓ− combinations containing a b baryon sample of 290 decays, the measured b baryon lifetime is τb—baryon = 1.05−0.11+0.12(stat)±0.09(syst) ps. The lifetime of the Λb0 baryon from a maximum likelihood fit to the proper time distribution of 58 Λc+ℓ− candidates containing a Λb0 sample of 44 decays, is τΛb0 = 1.02−0.18+0.23(stat) ± 0.06(syst) ps

INCLUSIVE PI(+/-), K+/- AND (P,(P)OVER-BAR) DIFFERENTIAL CROSS-SECTIONS AT THE Z-RESONANCE

Inclusive pi(+/-), K-+/- and (p, (p) over bar) differential cross-sections in hadronic decays of the Z have been measured as a function of z = p(hadron)/p(beam), the scaled momentum. The results are based on approximately 520 000 events measured by the ALEPH detector at LEP during 1992. Charged particles are identified bp their rate of ionization energy loss in the ALEPH Time Projection Chamber. The position, xi*, of the peak in the ln(1/z) distribution is determined, and the evolution of the peak position with centre-of-mass energy is compared with the prediction of QCD

PRODUCTION OF K(0) AND LAMBDA IN HADRONIC Z-DECAYS

Measurements of the inclusive cross-sections for K0 and LAMBDA production in hadronic decays of the Z are presented together with measurements of two-particle correlations within pairs of LAMBDA and K0. The results are compared with predictions from the hadronization models Jetset, based on string fragmentation, and Herwig, based on cluster decays. The K0 spectrum is found to be harder than predicted by both models, while the LAMBDA spectrum is softer than predicted. The correlation measurements are all reproduced well by Jetset, while Herwig misses some of the qualitative features and overestimates the size of the LAMBDALAMBDABAR correlation. Finally, the possibility of Bose-Einstein correlation in the K(S)0K(S)0 system is discussed

Observation of mono - jet events and tentative interpretation

A data sample corresponding to almost two million hadronic Z decays collected by the ALEPH detector at LEP has been searched for monojet events. Three events were found, in agreement with the expectation from the process e+e- --> gamma*nunuBAR, with gamma* --> ffBAR. Two events are hadronic, the third one being an e+e- pair. All monojet masses are in excess of 3 GeV/c2, and two of the events have large transverse momenta: 18.5 and 20.3 GeV/c. These kinematic characteristics are quite unlikely in the process e+e- --> gamma*nunuBAR. The probability of their occurrence increases substantially when processes involving further Z or W exchanges are taken into account, but still remains at the 5% level