P.N. Burrows

DESIGN AND PERFORMANCE OF THE SLD VERTEX DETECTOR : A 307 MPIXEL TRACKING SYSTEM

This paper describes the design, construction, and initial operation of SLDs upgraded vertex detector which comprises 96 two-dimensional charge-coupled devices (CCDs) with a total of 307 Mpixel. Each pixel functions as an independent particle detecting element, providing space point measurements of charged particle tracks with a typical precision of 4 μm in each co-ordinate. The CCDs are arranged in three concentric cylinders just outside the beam-pipe which surrounds the e+e− collision point of the SLAC Linear Collider (SLC). The detector is a powerful tool for distinguishing displaced vertex tracks, produced by decay in flight of heavy flavour hadrons or tau leptons, from tracks produced at the primary event vertex. The requirements for this detector include a very low mass structure (to minimize multiple scattering) both for mechanical support and to provide signal paths for the CCDs; operation at low temperature with a high degree of mechanical stability; and high speed CCD readout, signal processing, and data sparsification. The lessons learned in achieving these goals should be useful for the construction of large arrays of CCDs or active pixel devices in the future in a number of areas of science and technology.

Application of “optimised” perturbation theory to determination of αs(MZ2) from hadronic event shape observables in e+e− annihilation

Abstract We have applied so-called “optimised” perturbation theory to resolve the renormalisation-scale (μ) ambiguity of exact O( α s 2 ) QCD calculations of event shape observables in e + e − → hadrons. We fitted the optimised predictions for 15 observables to hadronic Z 0 decay data from the SLD experiment to determine α s ( M Z 2 ). Comparing with results using the physical scale μ = Mz we found no reduction in the scatter among α s ( M Z 2 ) values from the 15 observables, implying that the O( α s 2 ) predictions with optimised scales are numerically no closer to the exact all-orders results than those with the physical scale.

The SLD VXD3 detector and its initial performance

The SLD collaboration completed construction of a new CCD vertex detector (VXD3) in January 1996 and started data taking in April 1996 with the new system. VXD3 is an upgrade of the original CCD vertex detector, VXD2, which had successfully operated in SLD for three years. VXD3 consists of 96 large area CCDs, each having 3.2 million 20 microns x20 microns pixels. By reducing the detector material and lengthening the lever arm, VXD3 is expected to improve secondary vertex resolution by about a factor of two compared with VXD2. The new three layered structure enables stand alone tracking without any ambiguity and its extended size along the beam direction improves the polar angle coverage to COS THETA {lt}0.85. An overview of this detector system and its initial performance are described.

Measurement of [alpha][sub [ital s]] from energy-energy correlations at the [ital Z][sup 0] resonance

We determine the strong coupling [alpha][sub [ital s]] from a comprehensive study of energy-energy correlations (EECs) and their asymmetry (AEECs) in hadronic decays of [ital Z][sup 0] bosons collected by the SLD experiment at SLAC. The data are compared with all four available predictions of QCD calculated up to [ital O]([alpha][sub [ital s]][sup 2]) in perturbation theory, and also with a resummed calculation matched to all four of these calculations. We find large discrepancies between [alpha][sub [ital s]] values extracted from the different [ital O]([alpha][sub [ital s]][sup 2]) calculations. We also find a large renormalization scale ambiguity in [alpha][sub [ital s]] determined from the EECs using the [ital O]([alpha][sub [ital s]][sup 2]) calculations; this ambiguity is reduced in the case of the AEECs and is very small when the matched calculations are used. Averaging over all calculations, and over the EEC and AEEC results, we obtain [alpha][sub [ital s]]([ital M][sub [ital Z]][sup 2])=0.124[sub [minus]0.004][sup +0.003] (expt.) [plus minus]0.009 (theory).

Performance of the new vertex detector at SLD

During the past year, the SLD collaboration completed the construction and began the operation of a new vertex detector (VXD3) employing 307 million pixels. This detector, based on 96 CCDs of 13 cm/sup 2/ area each, is an upgrade of the original vertex detector of SLD (VXD2), made possible by advances in the technology of CCD detectors. Its improved impact parameter resolution, larger solid angle coverage and virtually error-free track linking will enhance the SLD measurement of the polarization-enhanced forward-backward asymmetry for b and c-quarks, increase the precision of the measurement of the b-fraction in hadronic Z decays, and open the possibility to observe B/sub s//sup 0/-mixing. Full separation of primary, secondary and tertiary vertices is accessible. A description of the mechanics and electronics of VXD3 are presented along with results from the first data.

Application of Padé approximants to determination of αs(MZ2) from hadronic event shape observables in e+e− annihilation

Abstract We have applied Pade approximants to perturbative QCD calculations of event shape observables in e+e− → hadrons. We used the exact O(αs2) prediction and the [ 0 1 ] Pade approximant to estimate the O(αs3) term for 15 observables, and in each case determined αs(MZ2) from comparison with hadronic Z0 decay data from the SLD experiment. We found the scatter among the αs(MZ2) values to be significantly reduced compared with the standard O(αs2) determination, implying that the Pade method provides at least a partial approximation of higher-order perturbative contributions to event shape observables.

Three-jet event orientation in e+e− annihilation: new tests of the Standard Model

Abstract We discuss the orientation of e + e − → qqg events in terms of the polar and azimuthal angles of the event plane w.r.t. the electron beam direction. We define an asymmetry of the azimuthal-angle distribution which, along with the left-right forward-backward polar-angle asymmetry, is sensitive to parity-violating effects in three-jet events; these have yet to be explored experimentally. We have evaluated these observables at O( α s ) in perturbative QCD and present their dependence on longitudinal beam polarisation and c.m. energy. We also define a moments analysis in terms of the orientation angles that allows a new and more detailed test of QCD by isolating the six independent helicity cross-sections.

Measurement of the charged multiplicity of [ital Z][sup 0][r arrow][ital b[bar b]] events

Using an impact parameter tag to select an enriched sample of

Measurement of the charged multiplicity of Z0-->bb-bar events.

Z \rightarrow b \overline{b}

Measurement of [alpha][sub [ital s]] from jet rates at the [ital Z][sup 0] resonance

events, we have measured the difference between the average charged multiplicity of