A search for single sneutrino production under the assumption that R-parity is violated via a single dominant LLĒ coupling is presented. This search considers the process eγ → ν̃ℓ and is performed using the data collected by the ALEPH detector at centre-of-mass energies from 189 GeV up to 209 GeV corresponding to an integrated luminosity of 637.1 pb-1. The numbers of observed candidate events are in agreement with Standard Model expectations and 95% confidence level upper limits on five of the LLĒ couplings are given as a function of the assumed sneutrino mass.

Inclusive branching ratios involving b → τ transitions are measured in approximately four million hadronic Z decays collected by the ALEPH detector at LEP. The fully-inclusive branching ratio BR(b → τ-ν̄τX) and the semi-inclusive branching ratio BR(b → τ-ν̄τD*±X) are measured to be (2.43 ± 0.20 ± 0.25)% and (0.88 ± 0.31 ± 0.28)%, in agreement with the standard model predictions. Upper limits on the branching fractions BR(B- → τ-v̄τ) and BR(b → sνν̄) are set to 8.3 × 10-4 and 6.4 × 10-4 at the 90% C.L. These results allow a 90% C.L. lower limit of 0.40 (GeV/c2)-1 to be set on the tan β/mH± ratio, in the framework of type-II two-Higgs-doublet models.

A measurement of the Michel parameters and the average vτ helicity in τ lepton decays is described. The data was collected with the ALEPH detector at LEP during the years 1991 to 1995. A total integrated luminosity of 155 pb-1 is analysed. The Michel parameters ρl, ξl, (ξδ)l (l = e, μ), and ημ are determined for the leptonic decays, and the chirality parameters ξπ, ξρ, and ξa1 for the hadronic final states. Under the assumptions of e-μ universality and ξπ = ξρ = ξa1, the values ρl = 0.742 ± 0.016, ηl = 0.012 ± 0.026, (ξδ)l = 0.776 ± 0.051, ξl = 0.986 ± 0.074, and ξh = 0.992 ± 0.011 are obtained. No significant deviation is observed from the Standard Model assumption of the V - A structure of the charged weak interaction.

The production rates and the inclusive cross sections of the isovector meson π0, the isoscalar mesons η and η′ (958), the strange meson K0 s and the A baryon have been measured as functions of scaled energy in hadronic events, two-jet events and each jet of three-jet events from hadronic Z decays and compared to Monte Carlo models. The analysis is based on 3.7 million hadronic events collected with the ALEPH detector at LEP at a centre-of-mass energy of √s = 91.2 GeV. The JETSET modelling of the gluon fragmentation into isoscalar mesons is found to be in agreement with the experimental results. HERWIG fails to describe the K0 g spectra in gluon-enriched jets and the Λ spectra in quark jets.

An analysis based on 124 000 selected τ pairs recorded by the ALEPH detector at LEP provides the vector (V) and axial-vector (A) spectral functions of hadronic τ decays together with their total widths. This allows the evaluation of finite energy chiral sum rules that are weighted integrals over the (V - A) spectral functions. In addition, a precise measurement of αs, along with a determination of non-perturbative contributions at the τ mass scale is performed. The experimentally and theoretically most robust determination of αs(Mτ 2) is obtained from the (V + A) fit that yields αs (Mτ 2) = 0.334 ±0.022 giving αs (MZ 2) = 0.1202 ± 0.0027 after the extrapolation to the mass of the Z boson. The approach of the Operator Product Expansion (OPE) is tested experimentally studying the evolution of the τ hadronic widths to masses smaller than the τ mass.

Using a sample of four million hadronic Z events collected in ALEPH from 1991 to 1995, the decays D8+ --> D(0)pi(s)(+), with D-0 decaying to K- pi(+) or to K+ pi(-), are studied. The relative branching ratio B(D-0 --> K+ pi(-))/B(D-0 --> K- pi(+)) is measured to be (1.84 +/- 0.59(stat.) +/- 0.34(syst.))%. The two possible contributions to the D-0 --> K+ pi(-) decay, doubly Cabibbo-suppressed decays and DO-DO mixing, are disentangled by measuring the proper-time distribution of the reconstructed DO's. Assuming no interference between the two processes, the upper limit obtained on the mixing rate is 0.92% at 95% CL. The possible effect of interference between the two amplitudes is also assessed. (C) 1998 Elsevier Science B.V. All rights reserved.

The data collected by the ALEPH experiment at LEP at centre-of-mass energies around 183 GeV are analysed to search for sleptons, the partners of leptons in supersymmetric theories. The previously published search for acoplanar leptons and missing energy has been updated. New searches have been developed to cover a wider range of slepton signals. These include single electrons, acoplanar leptons accompanied by two photons plus missing energy as well as particles with lifetime. No evidence for the production of any such particles is found. Slepton mass limits are reported within gravity mediated and gauge mediated SUSY breaking scenarios. © 1998 Elsevier Science B.V. All rights reserved.

Inclusive π±, K± and (p, p̄) production is investigated using data recorded by the ALEPH detector between 1992 and 1994. The momentum spectra and multiplicities are measured separately in Z → bb̄, Z → cc̄ and Z → uū, dd̄, ss̄ decays. The number of protons found in b-hadron decays is used to estimate the fraction of b-baryons in b events to be (10.2 ± 0.7 ± 2.7)% assuming BR(b-baryon→ pX) = (58 ± 6)%. From an additional study of proton-lepton correlations in b events, the branching ratio BR(b-baryon → plv̄X) = (4.63 ± 0.72 ± 0.98)% is obtained. The ratio BR(b-baryon → plv̄X)/BR(b-baryon → pX) is found to be 0.080 ± 0.012 ± 0.014.

A lower limit on the oscillation frequency of the B0 SB0 S system is obtained from approximately four million hadronic Z decays accumulated using the ALEPH detector at LEP from 1991 to 1995. Leptons are combined with opposite sign D- S candidates reconstructed in seven different decay modes as evidence of semileptonic B0 S decays. Criteria designed to ensure precise proper time reconstruction select 277 D- Sl+ combinations. The initial state of these B0 S candidates is determined using an algorithm optimized to efficiently utilise the tagging information available for each event. The limit at 95% confidence level on the B0 SB0 S oscillation frequency is ��ms > 6.6 ps-1. The same data is used to update the measurement of the B0 S lifetime, ��s = 1.54 +0.14 -0.13, (stat) �� 0.04 (syst) ps.