G. Ambrosi

The L3 silicon microvertex detector

Abstract The design and construction of the silicon strip microvertex detector (SMD) of the L3 experiment at LEP are described. We present the sensors, readout electronics, data acquisition system, mechanical assembly and support, displacement monitoring systems and radiation monitoring system of the recently installed double-sided, double-layered SMD. This detector utilizes novel and sophisticated techniques for its readout.

Results from the L3 experiment at LEP

In this report we summarize the major results from the first three years of running of the L3 detector at the Large Electron Positron collider (LEP). The L3 detector was designed to measure electrons, photons and muons with high precision; it has achieved ( ΔE / E 0 ≃ 1.4% for electrons and photons at E = 45 GeV and ( Δp / p ) ≃ 2.5% for muons at p = 45 GeV . Most of the experimental results presented are based on approximately 500 000 Z events. The data provide foue independent checks of the Standard Model weak neutral currents to an accuracy of ≃ 1%. These are: the measurement of Z parameters; the measurement of the forward-backward charge asymmetry of ;epton pairs; the measurement of asymmetry parameters from Z→bb decays; and the measurement of the gp lepton polarization asymmetry. The data also information on weak charged currents from measurements of b hadrons such as decay width, lifetime, and B 0 −B 0 mixing, and from measurements of τ leptons such as their lifetime and branching ratios.The large number of Z events and the resoltuion of the L3 detector allow presicion tests of QED and QCD, including the production of π 0 , η and hard photons.Leptonic final states with hard isolated photons are analyzed and high mass photon pairs are observed. Higgs boson production has not been observed and limits are given in the context of the standard Model and its supersymmetric extensions. We also present upper limits on rare Z decays such as Z → γγγ and on flavor violating decays such as Z → τ, μτ. Finally, upper limits obtained in the search for various new particles and new interactions are reported.

Determination of alpha-S from hadronic event shapes measured on the Z0 resonance

Abstract We present a study of the global event shape variables thrust and heavy jet mass, of energy-energy correlations and of jet multiplicities based on 250 000 hadronic Z 0 decays. The data are compared to new QCD calculations including resummation of leading and next-to-leading logarithms to all orders. We determine the strong coupling constant α s (91.2 GeV) = 0.125±0.003 (exp) ± 0.008 (theor). The first error is the experimental uncertainty. The second error is due to hadronization uncertainties and approximations in the calculations of the higher order corrections.

High mass photon pairs in ℓ+ℓ−γγ events at LEP

Abstract From the analysis of the reactions e + e − → l + l − ( nγ ) ( l = e , μ , τ ) we observe four events, one e + e − γγ and three μ + μ − γγ , with the invariant mass of the photon pairs close to 60 GeV. These events were selected from a data sample collected in the L3 detector corresponding to 950 000 produced Z 0 s. More data are necessary to ascertain the origin of these events.

Isolated Hard Photon-Emission in Hadronic Z0 Decays

Abstract We report on a study of energetic, isolated photons in a sample of ∼ 320 000 Z 0 hadronic decays. Energetic isolated photons probe the short-distance structure of QCD. We compare our data with the prediction of several QCD-based calculations. A search for new processes with one or two photons in the hadronic final state is also presented. No evidence for physics beyond the standard model is found.

A search for the neutral Higgs boson at LEP

We determine the relative rates of short GRBs in cluster and field early-type galaxies as a function of the age probability distribution of their progenitors, P(\tau) \propto \tau^n. This analysis takes advantage of the difference in the growth of stellar mass in clusters and in the field, which arises from the combined effects of the galaxy stellar mass function, the early-type fraction, and the dependence of star formation history on mass and environment. This approach complements the use of the early- to late-type host galaxy ratio, with the added benefit that the star formation histories of early-type galaxies are simpler than those of late-type galaxies, and any systematic differences between progenitors in early- and late-type galaxies are removed. We find that the ratio varies from R(cluster)/R(field) ~ 0.5 for n = -2 to ~ 3 for n = 2. Current observations indicate a ratio of about 2, corresponding to n ~ 0 - 1. This is similar to the value inferred from the ratio of short GRBs in early- and late-type hosts, but it differs from the value of n ~ -1 for NS binaries in the Milky Way. We stress that this general approach can be easily modified with improved knowledge of the effects of environment and mass on the build-up of stellar mass, as well as the effect of globular clusters on the short GRB rate. It can also be used to assess the age distribution of Type Ia supernova progenitors.

Search for a Z' at the Z resonance

The search for an additional heavy gauge boson Z′ is described. The models considered are based on either a superstring-motivated E6 or on a left-right symmetry and assume a minimal Higgs sector. Cross sections and asymmetries measured with the L3 detector in the vicinity of the Z resonance during the 1990 and 1991 running periods are used to determine limits on the Z-Z′ gauge boson mixing angle and on the Z′ mass. For Z′ masses above the direct limits, we obtain the following allowed ranges of the mixing angle, θM at the 95% confidence level: −0.004 ⪕ θM ⪕ 0.015 for the χ model, −0.003 ⪕ θM ⪕ 0.020 for the ψ model, −0.029 ⪕ θM ⪕ 0.010 for the η model, −0.002 ⪕ θM ⪕ 0.020 for the LR model,

Searches for Nonminimal Higgs Bosons in Z0 Decays

Abstract We report on a general search for neutral and charged Higgs bosons with no assumption that the Higgs sector consists of a single doublet as in the minimal standard model. No signal inconsistent with background is observed in any of the decay channels analysed. From the results of direct searches, model independent limits on Higgs bremsstrahlung and on Higgs pair production from the Z 0 are presented. We interpret the bremsstrahlung limits in the general two-doublet model. Z 0 lineshape measurements further restrict the parameter space available in the two-doublet model. Finally, the results are interpreted in the framework of the minimal supersymmetric extension of the standard model.

A Measurement of Tau Polarization in Z0 Decays

Abstract The polarization of τ leptons produced in e+e− → τ+τ−(γ) is measured using a sample of 8977 τ+τ − pairs collected near the peak of the Z0 resonance. A polarization of −0.132±0.026(stat)±0.021(syst.) is determined. This corresponds to a ratio of the vector to the axial-vector coupling constants of the τ lepton to the weak neutral current of gVτ/gAeffτ = 0.067±0.017. This leads to a value of the effective sin2 θw at the Z0 resonance of sin2 θeff = 0.2326±0.0043.

Measurement of Gamma (b anti-b) / Gamma (had) from hadronic decays of the Z

We report a measurement of R b = Γ bb Γ had from Z → qq events at LEP. View the Z → qq events are identified using a multidimensional analysis based on a neural network approach. We obtain 60% sample purity with an efficiency of 35%. Our measured value of R b is 0.222±0.003±0.007.