Measurement of W-pair production in e+e−e^+ e^- collisions at 189 GeV

The production of W-pairs is analysed in a data samplecollected by ALEPH at a mean centre-of-mass energy of 188.6 GeV,corresponding to an integrated luminosity of 174.2 pb^-1. Crosssections are given for different topologies of W decays intoleptons or hadrons. Combining all final states and assumingStandard Model branching fractions, the total W-pair cross sectionis measured to be 15.71 +- 0.34 (stat) +- 0.18 (syst) pb.Using also the W-pair data samples collected by ALEPH at lowercentre-of-mass energies, the decay branching fraction of the W bosoninto hadrons is measured to be BR (W hadrons) = 66.97+- 0.65 (stat) +- 0.32 (syst) %, allowing a determination of theCKM matrix element |V(cs)|= 0.951 +- 0.030 (stat) +- 0.015 (syst)

Determination of sin2 θeff w using jet charge measurements in hadronic Z decays

The electroweak mixing angle is determined with high precision from measurements of the mean difference between forward and backward hemisphere charges in hadronic decays of the Z. A data sample of 2.5 million hadronic Z decays recorded over the period 1990 to 1994 in the ALEPH detector at LEP is used. The mean charge separation between event hemispheres containing the original quark and antiquark is measured for bb̄ and cc̄ events in subsamples selected by their long lifetimes or using fast D*'s. The corresponding average charge separation for light quarks is measured in an inclusive sample from the anticorrelation between charges of opposite hemispheres and agrees with predictions of hadronisation models with a precision of 2%. It is shown that differences between light quark charge separations and the measured average can be determined using hadronisation models, with systematic uncertainties constrained by measurements of inclusive production of kaons, protons and A's. The separations are used to measure the electroweak mixing angle precisely as sin2 θeff w = 0.2322 ± 0.0008(exp. stat.) ±0.0007(exp. syst.) ± 0.0008(sep.). The first two errors are due to purely experimental sources whereas the third stems from uncertainties in the quark charge separations

Searches for neutral Higgs bosons in e+e−e^{+}e^{-} collisions at centre-of-mass energies from 192 to 202 GeV

Searches for neutral Higgs bosons are performed with the 237 pb^-1 of data collected in 1999 by the ALEPH detector at LEP, for centre-of-mass energies between 191.6 and 201.6 GeV. These searches apply to Higgs bosons within the context of the Standard Model and its minimal supersymmetric extension (MSSM) as well as to invisibly decaying Higgs bosons. No evidence of a signal is seen. A lower limit on the mass of the Standard Model Higgs boson of 107.7 GeV/c^2 at 95% confidence level is set. In the MSSM, lower limits of 91.2 and 91.6 GeV/c^2 are derived for the masses of the neutral Higgs bosons h and A, respectively. For a Higgs boson decaying invisibly and produced with the Standard Model cross section, masses below 106.4 GeV/c^2 are excluded

Measurement of the W mass by direct reconstruction in e+e−e^+ e^- collisions at 172 GeV

The mass of the W boson is obtained from reconstructed invariant mass distributions in W-pair events. The sample of W pairs is selected from 10.65~pb$^{-1}$ collected with the ALEPH detector at a mean centre-of-mass energy of 172.09 \GEV. The invariant mass distribution of simulated events are fitted to the experimental distributions and the following W masses are obtained: $WW \to q\overline{q}q\overline{q } m_W = 81.30 +- 0.47(stat.) +- 0.11(syst.) GeV/c^2$, $WW \to l\nu q\overline{q}(l=e,\mu) m_W = 80.54 +- 0.47(stat.) +- 0.11(syst.) GeV/c^2$, $WW \to \tau\nu q\overline{q} m_W = 79.56 +- 1.08(stat.) +- 0.23(syst.) GeV/C62$. The statistical errors are the expected errors for Monte Carlo samples of the same integrated luminosity as the data. The combination of these measurements gives: $m_W = 80.80 +- 0.11(syst.) +- 0.03(LEP energy) GeV/^2$

Electroweak parameters of the z0 resonance and the standard model

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Measurement of B - anti-B mixing at the Z

From more than 175 000 hadronic Z decays observed with the ALEPH detector at LEP, we select 823 events with pairs of leptons in the final state. From these we measure χ, the probability thata b hadron which is observed to decay originated as a hadron. We find χ=0.132−0.026+0.027

PRODUCTION OF CHARMED MESONS IN Z DECAYS

The production of charmed mesons D0(-), D+/-, and D*+/- is studied in a sample of 478,000 hadronic Z decays. The production rates are measured to be GAMMA(Z --> D*+/- X)/GAMMA(had) = 0.187 +/- 0.015 (exp.) +/- 0.013 (BR), GAMMA(Z --> D+/- X) = 0.251 +/- 0.026 (exp.) +/- 0.025 (BR), GAMMA(Z --> D0(-) X)/GAMMA(had) = 0.518 +/- 0.052 (exp.) +/- 0.035 (BR), where the errors from this analysis are separated from those coming from the D branching ratios (BR). The D *+/momentum distribution is extracted separately for Z --> cc and Z --> bbBAR events with the help of event shape variables. It is consistent with the prediction of the JETSET Monte Carlo program after adjustment of the charm fragmentation function. Constraining the shape of the Z --> bbBAR contribution, the average fraction of the beam energy taken by a D* meson produced in the fragmentation of a charm quark is extracted by a parametric fit to be [X(E)]c = 0.495 +/- 0.011 +/- 0.007. Evidence for D**0 (D1(2420)0 and/or D2*(2460)0) production is found in the D*+/-pi-/+ channel, accounting for a fraction (18 +/- 5 +/- 2)% of all D*+/- production. The relative production of vector and pseudoscalar mesons is dicussed, together with the possible effects of D** production. The c-quark forward-backward Z-pole asymmetry is determined from that of high momentum D*+/- to be A(FB)0,c = (7.7 +/- 4.4)%

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