Electroweak measurements in electron–positron collisions at w-boson-pair energies at lep

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Determination of sin2 \u3b8 w eff 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 forbbar b andcbar c events in subsamples selected by their long lifetimes or using fast D*\u2019s. 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 \u39b\u2019s. The separations are used to measure the electroweak mixing angle precisely as sin2 \u3d1 {w/eff}=0.2322\ub10.0008(exp.stat.) \ub10.0007(exp.syst.)\ub10.0008(sep.). The first two errors are due to purely experimental sources whereas the third stems from uncertainties in the quark charge separations

Three-prong \u3c4 decays with charged kaons

Final states with charged kaons in three-prong \u3c4 decays are studied by exploiting the particle identification from the dE/dx measurement. The results are based on a sample of about 1.6 7 105 detected \u3c4 pairs collected with the ALEPH detector between 1991 and 1995 around the Z peak. The following branching ratios have been measured: B(\u3c4- \u2192 K 12K+\u3c0 12\u3bd\u3c4) = (1.63 \ub1 0.21 \ub1 0.17) 7 10 123, B(\u3c4 12 \u2192 K 12\u3c0+\u3c0 12\u3bd\u3c4) = (2.14 \ub1 0.37 \ub1 0.29) 7 10 123, B(\u3c4 12 \u2192 K 12K+\u3c0 12\u3c00\u3bd\u3c4) = (0.75 \ub1 0.29 \ub1 0.15) 7 10 123, and B(\u3c4 12 \u2192 K 12\u3c0+\u3c0 12\u3c00\u3bd\u3c4) = (0.61 \ub1 0.39 \ub1 0.18) 7 10 123. The first two measurements are more precise than the current world averages, while the last two channels are investigated for the first time. The 95% C.L. upper limit on the branching ratio for the decay \u3c4 12 \u2192 K 12K+K 12\u3bd\u3c4 is 0.19 7 10 123. A study of intermediate states occurring in the K 12K+\u3c0 12\u3bd\u3c4 and K 12\u3c0+\u3c0 12\u3bd\u3c4 decays is also presented

Search for the standard model Higgs boson at the LEP2 Collider near GeV

During 1997 the ALEPH experiment at LEP gathered 57 pb-1 of data at centre-of-mass energies near 183 GeV. These data are used to look for possible signals from the production of the Standard Model Higgs boson in the reaction e+e--->HZ. No evidence of a signal is found in the data; seven events are selected, in agreement with the expectation of 7.2 events from background processes. This observation results in an improved lower limit on the mass of the Higgs boson: mH>87.9 GeV/c2 at 95% confidence level

Measurement of triple gauge boson couplings at 172-GeV

The triple gauge-boson couplings, αWΦ, αW and αBΦ, have been measured using 34 semileptonically and 54 hadronically decaying W+W− candidate events. The events were selected in the data recorded during 1996 with the ALEPH detector at 172 GeV, corresponding to an integrated luminosity of 10.65 pb−1. The triple gauge-boson couplings have been measured using optimal observables constructed from kinematic information of W+W− events. The results are in agreement with the Standard Model expectation

A Measurement of A(b)(FB) in lifetime tagged heavy flavor Z decays

A new measurement of the forward-backward asymmetry in Z -> b bbar 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 sin^2(theta(w)eff), which is determined to be 0.2315 +/- 0.0016 (stat.) +/- 0.0009 (syst.), corresponding to an A(FB)b of 0.00992 +/- 0.0084 (stat.) +/- 0.0046 (stat.)

Charm counting in b decays

The inclusive production of charmed particles in decays has been measured from the yield of D0, D+, Ds+ and Λc+ decays in a sample of events with high b purity collected with the ALEPH detector from 1992 to 1995. From these measurements, adding the charmonia production rate and an estimate of the charmed strange baryon contribution, the average number of charm quarks per b decay is determined to be nc = 1.230 ± 0.036 ± 0.038 ± 0.053, where the uncertainties are due to statistics, systematic effects and branching ratios, respectively

Search for single top production in e+ e- collisions at s**(1/2) = 189-GeV - 202-GeV

The single top production via flavour changing neutral currents in the reactions ee -> t c/u is searched for in approximately 411 pb-1 of data collected by ALEPH at centre-of-mass energies in the range between 189 and 202 GeV. In total, 58 events are selected in the data to be compared with 50.3 expected from Standard Model backgrounds. No deviation from the Standard Model expectation is observed. A 95\%~CL upper limit of 0.72 pb on the single top production cross section at 202 GeV is derived assuming a top mass of 174 GeV/c^2 and a 100% branching ratio of the top decay into bW. A model dependent limit on the flavour-changing couplings kZ and kg is obtained by combining all centre-of-mass energies

Measurement of Λ polarization from Z decays

The polarization of Λ baryons from Z decays is studied with the Aleph apparatus. Evidence of longitudinal polarization of s quarks from Z decay is observed for the first time. The measured longitudinal Λ polarization is PLΛ = −0.32 ± 0.07 for . This agrees with the prediction of −0.39 ± 0.08 from the standard model and the constituent quark model, where the error is due to uncertainties in the mechanism for Λ production. The observed Λ polarization is diluted with respect to the primary s quark polarization by Λ baryons without a primary s quark. Measurements of the Λ forward-backward asymmetry and of the correlation between back-to-back Λ pairs are used to check this dilution. In addition the transverse Λ polarization is measured. An indication of transverse polarization, more than two standard deviations away from zero, is found along the normal to the plane defined by the thrust axis and the Λ direction

One prong tau decays into charged kaons

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