Distributions of event shape variables obtained from 120600 hadronic Z decays measured with the DELPHI detector are compared to the predictions of QCD based event generators. Values of the strong coupling constant c~ s are derived as a function of the renormalization scale from a quantitative analysis of eight hadronic distributions. The final result, c% (Mz) = 0.113 +_ 0.007, is based on second order perturbation theory and uses two hadronization corrections, one computed with a pat ton shower model and the other with a QCD matrix element model.

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Distributions of event shape variables obtained from 120600 hadronicZ decays measured with the DELPHI detector are compared to the predictions of QCD based event generators. Values of the strong coupling constant αs are derived as a function of the renormalization scale from a quantitative analysis of eight hadronic distributions. The final result, αs(M Z), is based on second order perturbation theory and uses two hadronization corrections, one computed with a parton shower model and the other with a QCD matrix element model.

We have searched for pair produced scalar leptoquarks each decaying to a quark and a charged lepton in a sample of 116 000 hadronic Z0 events produced at LEP. No candidate was detected and cross section and branching ratio limits are set for the above process at 95% CL. Mass limits are found to be about 42 GeV/c2 depending only slightly on the models used and a coupling times branching ratio exclusion line is drawn for a scalar leptoquark with a free coupling. We have also probed the mass region above 45 GeV/c2 for a singly produced scalar leptoquark and set limits on the cross section and the coupling λ2/4π up to 60 GeV.

From a sample of about 120 000 hadronic Z0 decays, using a technique based on a separation of the different event categories in the boosted sphericity product, the fraction of bb decays has been measured to be 0.219 ± 0.014 (stat)± 0.019 (syst). Using the DELPHI determination of the hadronic Z0 width, this corresponds to a partial width τbb = 378 ± 42 MeV (in good agreement with the standard model prediction of ∼-380 MeV). Combining this measurement with the determinations based on events with high pt leptons gives an estimate for the branching ratio of b into leptons at LEP of (11.2 ± 1.2)%, consistent with previous determinations.

The pair production of the lightest scalar Higgs boson, h, and a pseudoscalar Higgs boson, A, was searched for in a data sample containing 10 000 hadronic Z0 decays. The search involved both leptonic and purely hadronic decay channels of each Higgs boson. No signal was found, and limits on the Higgs boson masses, in the framework of the minimal supersymmetric extension of the standard model, ar reported up to 35 GeV/c2 at 95% CL, for both tan β1 and tan β<1, where tan β is the ratio of the vacuum expectation values of the two Higgs doublets.

The differential cross section for final state radiation from primary quarks is obtained from a study of isolated energetic photons produced in the reaction e + e ~ Z ~ ~ h a d r o n s + 7 , as measured in the D E L P H I detector at the CERN LEP collider. When combined with the measurement of the total hadronic width of the Z ~ the observed rate determines the electroweak coupling constants of up and down type quarks, i.e., vZ/3+a~/3=l.13+_0.29 and vZ/3+a2/3=l.65+_0.43. No evidence is seen for additional photon production from anomalous decays of the Z ~ or from decays of new particles. This measurement leads to upper limits on the production cross section times branching fraction of (a) the Higgs boson in the reaction e + e ~ Z ~ + 7, H ~ hadrons, (b) an excited quark, q * ~ q + 7, and (c) the contribution of an anomalous decay of the Z ~ into a photon and hadrons. These limits, all at the 95% confidence level, vary from 3 to 10pb as the mass of the intermediate state (H,q* or Z*) varies from 10 GeV/c 2 to 80 GeV/c 2.

Distributions of event shape variables obtained from 120600 hadronic Z decays measured with the DELPHI detector are compared to the predictions of QCD based event generators. Values of the strong coupling constant alpha(s) are derived as a function of the renormalization scale from a quantitative analysis of eight hadronic distributions. The final result, alpha(s) (M(Z)) = 0.113 +/- 0.007, is based on second order perturbation theory and uses two hadronization corrections, one computed with a parton shower model and the other with a QCD matrix element model.

The four LEP experiments have each performed precision measurements of Z0 parameters. A method is described for combining the results of the four experiments, which takes into account the experimental and theoretical systematic errors and their correlations. We apply this method to the 1989 and 1990 LEP data, corresponding to approximately 650 000 Z0 decays into hadrons and charged leptons, to obtain precision values for the Z0 parameters. We use these results to test the standard model and to constrain its parameters.

Using a data sample of 115000 hadronicZ0 decays, a search was performed for the production of excited charged leptons decaying into an ordinary lepton and one photon. Pair production of excited leptons of mass below 45 GeV/c2 is excluded. From an analysis of the lepton-photon mass spectra this experiment is also able to exclude single production of these particles for masses that reach up to 85 GeV/c2, if the compositeness scale is below 1 TeV.

An analysis of the production of strange particles from the decays of the Z0 boson into multihadronic final states is presented. The analysis is based on about 90 000 selected hadronic Z0 decays collected by the DELPHI detector at LEP in 1990. Ks0, K∗±, Λ(Λ) and Ξ− (Ξ+) have been identified by their characteristic decays. The measured production cross sections are compared with predictions of the Lund Monte Carlo tuned to data at PEP/PETRA energies.

The average lifetime ofB hadrons produced in hadronicZ0 decays has been measured with the DELPHI detector at LEP. The measurement is based on the analysis of the impact parameter distributions of highp t muons and hadrons. The resulting meanB lifetimes are τ B =(1.30±0.10±0.08)ps and τ B =(1.27±0.04±0.12)ps respectively, giving a combined value of τ B =(1.28±0.10)ps. The hadronic sample was also used to measure the partialZ0 width\({{\Gamma _{b\bar b} } \mathord{\left/ {\vphantom {{\Gamma _{b\bar b} } {\Gamma _h }}} \right. \kern-\nulldelimiterspace} {\Gamma _h }}\) and gave a value of 0.222 −0.031 +0.033 ±0.017.

The weak mixing angle has been measured from the charge asymmetry of hadronic events with two different approaches using the DELPHI detector at LEP. Both methods are based on a momentum-weighted charge sum to determine the jet charge in both event hemispheres. In a data sample of 247 300 multihadronic Z0 decays a charge asymmetry of 〈QF〉 − 〈QB〉 = −0.0076±0.0012(stat.)±0.0005(exp. syst.)±0.0014(frag.) and a raw forward-backward asymmetry of ArawFB = −0.0109±0.0020(stat.)±0.0010(exp. syst.)±0.0017(frag.) have been measured. This result corresponds to a value of sinθeff=0.2345±0.0030(exp.)±0.0027(frag.), sin2θMS=0.2341±0.0030(exp.)±0.0027(frag.) and to sin2θW=1−m2W/m2Z=0.2299± 0.0030(exp.)±0.0027(frag.)±0.0028(theor.). The experimental error is the quadratic sum of the statistical and the experimental systematic error and the theoretical error originates from a value of mt=130±40 GeV/c2 and a range of mH from 45 GeV/c2 to 1000 GeV/c2.

The forward-backward asymmetry of bottom quarks is measured with statistics of approximately 80 000 hadronic Z0 decays produced in e+e− collisions at a centre of mass energy of √s≈Mz. The tagging of b quark events has been performed using the semileptonic decay channel b→X+μ. Because the asymmetry depends on the weak coupling, this leads to a precise measurement of the electroweak mixing angle sin2θw. The experimental result is AFBb = 0.115±0.043(stat.)±0.013(syst.). After correcting the value for the B0B0 mixing this becomes AFBb=0.161±0.060(stat.)±0.021(syst.) corresponding to sin2θWMS=0.221±0.011(stat.)±0.004(syst.).

The study of the orientation of three-jet events from e+e− → Z0 → multi-hadrons is presented, in particular the polar angle distributions of the thrust axis and of the normal to the three-jet plane, and the azimuthal correlations between the hadron plane and the one defined by the beam and thrust axes. The data are compared with results at lower energy and with QCD predictions. Good agreement with QCD predictions is observed. The scalar gluon theory is excluded by the data.

A strong increase of the mean transverse momentum 〈pt〉 with the number of charged particles nch is observed in e+e − annihilations into hadrons at LEP energies. The effect resembles correlations observed in hadron-hadron interactions. In e+e− annihilations the 〈Pt〉 and nch correlations can be accounted for by gluon radiation.

We have searched for possible fourth family heavy neutrinos, pair produced in Z0 decays, in a sample of about 112 000 hadronic Z0 final states collected with the DELPHI detector. For all mixing matrix elements we exclude a new Dirac neutrino lighter than 44.5 GeV at a 95% confidence level, if the neutrino couples to the electron or muon family, and lighter than 44.0 GeV, if the neutrino couples to the tau family. Depending on the values of the mixing element and to which lepton family the neutrino couples, we obtain mass limits up to 46.2 GeV. For all mixing matrix elements we exclude a new Majorana neutrino lighter than 39.0 GeV, if it couples to the electron or the muon family, and lighter than 38.2 GeV, if it couples to the tau family. Depending on the values of the mixing matrix element and to which lepton family the neutrino couples, we obtain mass limits up to 44.7 GeV. We also exclude stable new Dirac neutrinos lighter than 45.0 GeV and new Majorana neutrinos lighter than 39.5 GeV.

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 Z0 decay into π0γ, ηψ and γγγ are below 1.5 × 10−4, 2.8 × 10−4 and 1.4 × 10−4 respectively.

A search for light Higgs bosons was performed using the data sample collected in 1990 by the DELPHI detector at LEP, at centre of mass energies between 88.2 and 94.2 GeV. Using the process e+e−→H 0+Z 0*,Z 0*→ff, it is possible to exclude the existence of the standard model Higgs particle with a mass between 0 and 210 MeV/c2 at the 99% confidence level. Extending this analysis to the minimal supersymmetric standard model restricts the lightest neutral Higgs boson to masses above 28 GeV/c2 irrespective of the value of the mixing angle.

Measurements of the cross section and forward-backward asymmetry for the reaction e+e− → μ+μ− using the DELPHI detector at LEP are presented. The data come from a scan around the Z0 peak at seven centre of mass energies, giving a sample of 3858 events in the polar angle region 22° < θ < 158°. From a fit to the cross section for 43° < θ < 137°, a polar angle region for which the absolute efficiency has been determined, the square root of the product of the Z0 → e+e− and Z0 → μ+μ− partial widths is determined to be (ΓeΓμ)12 = 85.0 ± 0.9(stat.) ± 0.8(syst.) MeV. From this measurement of the partial width, the value of the effective weak mixing angle is determined to be sin2(θw) = 0.2267 ± 0.0037. The ratio of the hadronic to muon pair partial widths is found to be Γh/Γμ = 19.89 ± 0.40(stat.) ± 0.19(syst.). The forward-backward asymmetry at the resonance peak energy ECMS = 91.22 GeV is found to be AFB = 0.028 ± 0.020(stat.) ± 0.005(syst.). From a combined fit to the cross section and forward-backward asymmetry data, the products of the electron and muon vector and axial-vector coupling constants are determined to be VeVμ = 0.0024 ± 0.0015(stat.) ± 0.0004(syst.) and AeAμ = 0.253 ± 0.003(stat.) ± 0.003 (syst.). The results are in good agreement with the expectations of the minimal standard model.