Inclusive π±, K± and\((p,\bar p)\) differential cross-sections in hadronic decays of the Z have been measured as a function ofz=Phadron/Pbeam, 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 by their rate of ionization energy loss in the ALEPH Time Projection Chamber. The position, ξ*, 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.

Inclusive π±, K± and <img src="/fulltext-image.asp?format=htmlnonpaginated&src=U062552J33V46M85_html\10052_2005_Article_BF01556360_TeX2GIFIE2.gif" border="0" alt=" $$(p,\bar p)$$ " /> differential cross-sections in hadronic decays of the Z have been measured as a function ofz=Phadron/Pbeam, 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 by their rate of ionization energy loss in the ALEPH Time Projection Chamber. The position, ξ*, 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.

Inclusive ��, K�� and {Mathematical expression} differential cross-sections in hadronic decays of the Z have been measured as a function of z=Phadron/Pbeam, 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 by their rate of ionization energy loss in the ALEPH Time Projection Chamber. The position, ��*, 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. ? 1995 Springer-Verlag.

The process e+e−→, where l 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\(e^ + e^ - \to \tau ^ + \tau ^ - f\bar f\) channel, reported by ALEPH in 1991, is not confirmed.

The performance of the ALEPH detector at the LEP e+e− collider is reviewed. The accuracy of the tracking detectors to measure the impact parameter and momentum of charged tracks is specified. Calorimeters are used to measure photons and neutral hadrons, and the accuracy obtained in energy and angle is given. An essential property of the detector is its ability to identify particles; the performance in identification of electrons, muons, neutrinos (from missing energy), charged hadrons, π0's and V0's is described.

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−→γ*vv¯, with γ*→ff¯. 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 event have large transverse momenta: 18.5 and 20.3 GeV/c. These kinematic characteristics are quite unlikely in the process e+e−→γ*vv¯. The probability of their occurence increases substantially when processes involving further Z or W exchanges are taken into account, but still remains at the 5% level.

Measurements of the inclusive cross-sections forK0 and ? production in hadronic decays of the Z are presented together with measurements of two-particle correlations within pairs of ? andK0. The results are compared with predictions from the hadronization models Jetset, based on string fragmentation, and Herwig, based on cluster decays. TheK0 spectrum is found to be harder than predicted by both models, while the ? 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 <img src="/fulltext-image.asp?format=htmlnonpaginated&src=P67L0P53564715H4_html\10052_2005_Article_BF01560096_TeX2GIFIE1.gif" border="0" alt=" $$\Lambda \bar \Lambda $$ " /> correlation. Finally, the possibility of Bose-Einstein correlation in theKS0KS0 system is discussed.

B0B0 oscillation is studied using almost a million hadronic Z decays collected by the ALEPH experiment at LEP. Events are selected with two leptons present, on opposite sides of the event and with high transverse momentum. The leptons are expected to be dominantly from b decays; a topological vertexing technique is applied to measure the decay length of the b hadrons, and their momentum is determined using an energy-flow method. The fraction of events in which the leptons have the same charge is studied as a function of the measured decay time, and clear evidence is seen for the time-dependent nature of Bd0Bd0 mixing. The frequency measured for the oscillation corresponds to a mass difference for the Bd0 mass eigenstates Δmd = (3.3−0.4+0.5 ± 0.7) × 10−4 eV/c2. Allowing a second frequency component for the Bs0 a high value for Δms is favoured, leading to the limit Δms > 12 × 10−4 eV/c2 (95% CL), from which (Δm/Γ)s > 2.0 is derived.

Using data collected with the ALEPH detector at LEP correlations between the decay products of the tau(+) and tau- produced in the decay of the Z have been measured. The measurements performed in the decays tau --> pi nu and tau --> p nu place limits on deviations from the Standard Model. These are given by the parameter xi with xi = -1 for the Standard Model. The measured values are xi(pi) = -0.95 +/- 0.11 +/- 0.05 and xi(p) = -1.03 +/- 0.11 +/- 0.05. For models which predict xi(pi) = xi(p) = xi - interpreted as the average tau neutrino helicity - the measurement is xi = -0.99 +/- 0.07 +/- 0.04.

In a sample of 190 000 hadronic Z decays, three signals of charm production are observed: two from the exclusive decays D° →K−π+andD∗+→D°π+→K−π+π+ and one in the transverse-momentum distribution of soft hadrons relative to the nearest jet. The features of these signals are in good agreement with expectations based on the standard model and previous measurements of the branching fractions. The number of D∗±→K±π±π± per hadronic decay of the Z is measured to be (5.11±0.34) × 10−3, and the branching ratio B(D0 → K−π+) is (3.62 ± 0.34 ± 0.44)%. Charm hadronization has been studied. The average fraction of the beam energy carried by the D∗ meson is found to be 〈XE〉c = 0.504−0.017+0.013±0.008, and implications of the measurements on the pseudoscalar-to-vector production ratio of charmed mesons are discussed.

From a sample of about 75000 τ decays identified with the ALEPH detector, K0 production in 1-prong hadronic decays is investigated by tagging the KL0 component in a hadronic calorimeter. Results are given for the final states ντh−K0 and ντh−π0K0 where the h− is separated into π and K contributions by means of the dE/dx measurement in in the central detector. The resulting branching ratios are: (Bτ → ντπ−K0) = (0.88±0.14±0.09)%, (Bτ → ντK−K0) = (0.29±0.12±0.03)%, (Bτ → ντπ−π0K0) = (0.33±0.14±0.07)% aand (Bτ → ντK−π0K0) = (0.05±0.05±0.01)%. The K∗ decay rate in the K0π channel agrees with that in the Kπ0 mode: the combined value for the branching ratio is (Bτ → ντK∗−) = (1.45±0.13±0.11)%.

A new measurement of the forward-backward asymmetry in Z→ bb decays is presented. Hadrons from b decays are tagged using their long lifetimes. The b quark charge and direction are reconstructed with a hemisphere charge algorithm. The asymmetry and reconstructed b hemisphere charge are measured in the 69 pb−1 of data collected by ALEPH during 1991, 1992 and 1993. They are used to extract sin2θWeff, which is determined to be 0.2315 ± 0.0016 (stat.) ± 0.0009 (syst.), corresponding to an AFBb of 0.0992 ± 0.0084 (stat.) ± 0.0046 (syst.).

Form a sample of about 75000 τ decays measured in the ALEPH detector, 1-prong charged kaon decays are identified by the dE/dx measurement in the central detector. The resulting branching ratios for the inclusive and exclusive modes are: B(τ → ντK− ≥ 0π0 ≥ 0K0) = (1.60±0.07±0.12)%, B(τ → ντK− = (0.64±0.05±0.05)%, B(τ → ντ−π0 = (0.53±0.05±0.07)% and B(τ → ντK−π0π0) = (0.04±0.03±0.02)%. Exclusive modes are corrected for measured KL0 production. The rate for τ → ντK− agrees well with the prediction based on τ - μ universality.

Measurements of the inclusive cross-sections forK0 and Λ production in hadronic decays of the Z are presented together with measurements of two-particle correlations within pairs of Λ andK0. The results are compared with predictions from the hadronization models Jetset, based on string fragmentation, and Herwig, based on cluster decays. TheK0 spectrum is found to be harder than predicted by both models, while the Λ 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\(\Lambda \bar \Lambda \) correlation. Finally, the possibility of Bose-Einstein correlation in theK S 0 K S 0 system is discussed.

In 431 000 hadronicZ decays recorded with the ALEPH detector at LEP, the yields of electrons and muons in events with one or more prompt leptons have been analysed to give information on the production and decay of heavy quarks. The fractions of\(b\bar b\) and\(c\bar c\) events are measured to be 0.219±0.006±0.005 and 0.165±0.005±0.020, and the corresponding forward-backward asymmetries at theZ mass are measured to be 0.090±0.013±0.003 and 0.111±0.021±0.018, after QED and QCD corrections. Measurements for the semileptonic branching ratios BR\((b \to \ell ^ - \bar vX)\) and BR (b→cℓ+vX) yield 0.114±0.003±0.004 and 0.082±0.003±0.012, respectively. The dilepton events enable measurement of theb mixing parameter, Χ=0.114±0.014±0.008. Results are also presented for the energy variation of the\(b\bar b\) asymmetry and the parameters required to describe heavy quark fragmentation. From the asymmetry measurements, the effective electroweak mixing angle is sin2θ W eff =0.2333±0.0022.

Precise values of theZ resonance cross sections and lepton pair forward-backward asymmetries are measured from a sample of 1.3 millionZ decays into hadrons and charged leptons taken with the ALEPH detector at LEP. A silicon-tungsten luminosity calorimeter was installed in ALEPH in 1992, permitting the determination of the absolute luminosity to a systematic precision of 0.15% (experimental) and 0.25% (theoretical). This is combined with a better understanding of the hadronic event selection and an improved tau pair selection. Including the ALEPH results of the past three years one obtains σ h 0 =(41.60±0.16) nb, andR l =20.69±0.09. The corresponding number of light neutrino species isN v =2.983±0.034. The lepton pair forward-backward asymmetry is used to derive the ratio of vector to axial-vector couplings of leptons:g V 2 (M Z 2 )/g A 2 (M Z 2 )=0.0058 ±0.0011. An application of these results to constrain one model with extraZ bosons is presented.