First measurements of the mass and width of the Z 0 performed at the newly commissioned LEP Collider by the DELPHI Collaboration are presented. The measuements are derived from the study of multihadronic final states produced in e + e − annihilations at several energies around the Z 0 mass. The values found for the mass and width are M (Z 0 )=91.06±0.09 (stat) ±0.045 (syst.) GeV and Γ (Z 0 )=2.42±0.21 (stat.) GeV respectively, froma three-parameter fit to the line shape. A two-parameter fit in the framework of the standard model yields for the number of light neutrino species N ν =2.4±0.4 (stat.) ±0.5 (syst.).
Measurements of charged particle multiplicity distributions in the central rapidity region in p-p and p-α, and α-α collisions are reported. They are better fitted to the “wounded nucleon” than to the “gluon string” model. The average transverse momenta, for all three reactions, are identical (and almost independent of multiplicity) up to very high multiplicities.
The total and differential cross-sections for the reaction e + e − → γγ ( γ ) are measured at centre of mass energies around 91 GeV using an integrated luminosity of 4.7 pb −1 . The aggreement with QED prediction is good. Consequently there is no evidence for non-standard channels which would have the same experimental signature. The lower limits on the QED cuttoff parameters are Λ + > 113 GeV and Λ − > 95 GeV. An upper limit on the effective coupling between a possible excited electron and the gamma is derived. At 95% confidence level the branching ratios for Z 0 decay into π 0 γ, ηψ and γγγ are below 1.5 × 10 −4 , 2.8 × 10 −4 and 1.4 × 10 −4 respectively.
A determination of the partial width Γ c c ̄ of the Z 0 boson into charm quark pairs is presented, based on a total sample of 36 900 Z 0 hadronic decays measured with the DELPHI detector at the LEP collider. The production rate of cc̄ events is derived from the inclusive analysis of charged pions coming from the decay of charmed meson D ∗+ → D 0 π + and D ∗− → D ̄ 0 π − where the π ± is constrained by kinematics to have a low p T with respect to the axis. The probability to produce these π ± from D ∗± decay in cc̄ events is taken to be 0.31 ±_0.05 as measured at √ s =10.55 GeV. The measured relative partial width Γ c c ̄ Γ h = 0.162± 0.030 ( stat. ) ±0.050 ( syst. ) is in good agreement with the standard moel value of 0.171. Together with our previous measurement of the total hadronic width Γ h this implies Γ c c ̄ = 282±53 ( stat. )±88( syst. ) MeV .
A measurement of the cross section for e + e - → hadrons using 11 000 hadronic decays of the Z boson at ten different center-of-mass energies is presented. A three-parameter fit gives the following values for the Z mass M z , the total width Γ z , the product of the electronic and hadronic partial widths Γ e Γ h , and the unfolded pole cross section σ 0 : M Z =91.171±0.030(stat)±0.030 (beam) GeV, Γ Z =2.511±0.065 GeV, Γ e Γ h =0.148±0.006 (stat.)±0.004 (syst.) GeV 2 , σ 0 =41.6±0.7(stat.)±1.1 (syst.) nb,
We have studied the energy-energy angular correlations in hadronic final states from Z 0 decay using the DELPHI detector at LEP. From a comparison with Monte Carlo calculations based on the exact second order QCD matrix element and string fragmentation we find that Λ (5) MS =104 +25 -20 ( stat. ) +25 -20( syst. ) +30 00 ) theor. ) . MeV, which corresponds to α s (91 GeV)=0.106±0.003(stat.)±0.003(syst.) +0.003 -0.000 (theor). The theoretical error stems from different choices for the renormalization scale of α s . In the Monte Carlo simulation the scale of α s as well as the fragmentation parameters have been optimized to described reasonably well all aspects of multihadron production.
Hadronic decays of Z 0 bosons are studied in the Delphi detector. Global event variables and singel particles inclusive distributions are compared with QCD-based predictions. The mean charged multiplicity is found to be 20.6±1.0 (stat+syst). The mean values of the sphericity, aplanarity, thrust, minor value, p in T and p out T are compared with values found at lower energy e + e − colliders.
The hadronic fragmentation functions of the various quark flavours and of gluons are measured in a study of the inclusive hadron production from Z 0 decays with the DELPHI detector and are compared with the fragmentation functions measured elsewhere at energies between 14 GeV and 91 GeV. A large scaling violation is observed, which is used to extract the strong coupling constant from a fit using a numerical integration of the second order DGLAP evolution equations. The result is α s ( M Z ) = 0.124 −0.007 +0.006 (exp) ± 0.009(theory) where the first error represents the experimental uncertainty and the second error is due to the factorization and renormalization scale dependence.
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