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Abstract:

Infrared and collinear safe event shape distributions and their mean values are determined using the data taken at five different centre of mass energies above MZ with the DELPHI detector at LEP. From the event shapes, the strong coupling αS is extracted in θ(αS2), NLLA and a combined scheme using hadronisation corrections evaluated with fragmentation model generators as well as using an analytical power ansatz. Comparing these measurements to those obtained at MZ, the energy dependence (running) of αS is accessible. The logarithmic energy slope of the inverse strong coupling is measured to be d αS-1/d log (Ecm) = 1.39 + 0.34(stat) + 0.17(syst), in good agreement with the QCD expectation of 1.27. © 1999 Published by Elsevier Science B.V. All rights reserved.

Abstract:

Inclusive charged hadron distributions as obtained from the DELPHI measurements at 130, 136, 161, 172 and 183 GeV are presented as a function of the variables rapidity, ξp, p and transversal momenta. Data are compared with event generators and with MLLA calculations, in order to examine the hypothesis of local parton hadron duality. The differential momentum spectra show an indication for coherence effects in the production of soft particles. The relation between the energy dependence of the charged multiplicity and the rapidity distribution is examined. © 1999 Elsevier Science B.V. All rights reserved.

Abstract:

Knowledge of the centre-of-mass energy at LEP2 is of primary importance to set the absolute energy scale for the measurement of the W-boson mass. The beam energy above 80 GeV is derived from continuous measurements of the magnetic bending field by 16 NMR probes situated in a number of the LEP dipoles. The relationship between the fields measured by the probes and the beam energy is calibrated against precise measurements of the average beam energy between 41 and 55GeV made using the resonant depolarisation technique. The linearity of the relationship is tested by comparing the fields measured by the probes with the total bending field measured by a flux loop. This test results in the largest contribution to the systematic uncertainty. Several further corrections are applied to derive the centre-of-mass energies at each interaction point. In addition, the centre-of-mass energy spread is evaluated. The beam energy has been determined with a precision of 25 MeV for the data taken in 1997, corresponding to a relative precision of 2.7 × 10-4. This is small in comparison to the present uncertainty on the W mass measurement at LEP. However, the ultimate statistical precision on the W mass with the full LEP2 data sample should be around 25 MeV, and a smaller uncertainty on the beam energy is desirable. Prospects for improvements are outlined.

Abstract:

The data collected with the DELPHI detector at centre-of-mass energies between 130 and 172 GeV, during LEP operation in 1995 and 1996, have been used to determine the hadronic and leptonic cross-sections and leptonic forward-backward asymmetries. In addition, the cross-section ratios and forward-backward asymmetries for flavour-tagged samples of light (uds), c and b quarks have been measured. No significant deviations from the Standard Model expectations are found. The results are interpreted by performing S-matrix fits to these data and to the data collected previously at the energies near the Z° resonance peak (88-93 GeV). The results are also interpreted in terms of physics beyond the Standard Model: contact interactions, R-parity violating SUSY particle exchange and of possible Z′ bosons.

Abstract:

The ²ú²úÌ„ forward-backward asymmetry has been determined from the average charge flow measured in a sample of 3,500,000 hadronic Z decays collected with the DELPHI detector in 1992 - 1995. The measurement is performed in an enriched ²ú²úÌ„ sample selected using an impact parameter tag and results in the following values for the ²ú²úÌ„ forward-backward asymmetry: A²ú²úÌ„FB (89.55 GeV) = 0.068 ± 0.018 (stat.) ± 0.0013(syst.) A²ú²úÌ„FB (91.26 GeV) = 0.0982 ± 0.0047(stat.) ± 0.0016(syst.) A²ú²úÌ„FB (92.94 GeV) = 0.123 ± 0.016 (stat.) ± 0.0027(syst.) The ²ú²úÌ„ charge separation required for this analysis is directly measured in the b tagged sample, while the other charge separations are obtained from a fragmentation model precisely calibrated to data. The effective weak mixing angle is deduced from the measurement to be: sin2θ1eff = 0.23186 ± 0.00083.