F rom the analysis of a data sample corresponding to an integrated luminosity of 4.63 pb -1 taken during the 1990 run of LEP at centre of mass energies between 88.2GeV an 94.2GeV, the tau decays r ~ e g e v T , r -* ,u-~ .v~ , r ~ z c ( K ) v T , r r p v ~ and their charge conjugates have been studied. The following branching ratios have been measured; B R ( r ~ e aTeVT) = 18.6 + 0.8 (stat.) _ 0.6 (sys.)%, BR (z---+ /~17u vT) = 17.4 • 0.7 + 0.6%, B R ( r ~ z c ( K ) v T ) = 11.9_+0.7_ 0.7%, BR ( r > p vT) = 22.4 + 0.8 + 1.3%, in good agreement with world averages. The measured electronic and muonic branching ratios lead to a measurement of the strong coupling constant, es (mT) = 0.26 + 0.09 Extrapolating the c L value from -0 .12" m T to m z yields cL(mz)=0 .109 +0.012 0.028" The average polarization P~ of taus produced in Z---, r + r decays has also been measured using the above decay modes. The weighted mean of the polarizations obtained from the four decay modes is PT = 0.24 _+ 0.07. This value of PT gives, in the improved Born approximation, a ratio between the axial and vector coupling constants of the tau of vT/aT=0.12+__O.04, and hence a value of the effective electroweak mixing parameter sin 2 0 w(m~) = 0.220 _ 0.009.

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.

A study of inclusive production of the meson resonances r!°, [{•o(892), f 0 (975) and f 2 (1270) in hadronic decays of the zo is presented. The measured mean meson multiplicity per hadronic event is 0.83 ± 0.14 for the (Jo, 0.64 ± 0.24 for the [{• (892), 0.10 ± 0.04 for the fo(975) in the momentum range p > 0.05Pbeam (xp > 0.05) and 0.11 ±0.05 for the fz(l270) for Xp > 0.1. These values and the corresponding differential cross sections 1/ O'hadr · dO' / dxp for the vector mesons are in good agreement with the predictions of the JETSET 7.3 PS and HERWIG 5.4 models. The fz(1270) production is overestimated by HER\VIG but its Xpshape is correctly reproduced. The measured ratios of the production cross sections 0'(!2(1270))/0'((Jo) = 0.22 ± 0.08 and 0'(!2(1270))/0'(!0 (975)) = 3~i for Xp > 0.1 are consistent with the results obtained in hadronic reactions. (Submitted to Physics Letters B)

sion 7.2, giving: e (b) = (8 + ~ ___ 2)10 -3. The corresponding value of the mean fraction of the beam energy taken by a B hadron in the fragmentation of a b quark is: + 0 02 X~=0.69 0"03-t-0.01. If the values of F~ and F n are taken from the Standard Model, the following value is obtained for the mean semi-leptonic braching fraction of B hadrons" BR~I=(10.1 +0.7)%. Taking the value of Fb~/F n from an independent analysis of DELPHI data based on the use of the boosted sphericity product, a value: BRs~ =(10.1 -+ 1.3)% is obtained. 1 I n t r o d u c t i o n In the Standard Model the Z ~ boson couples with different strengths to up and down type quarks. Experimentally jets produced by heavy quarks are the easiest to isolate because of the use of characteristic properties of heavy hadron production and decay. In the present paper, semi-leptonic decays of B hadrons are used to isolate the Z ~ decays into bb pairs. Studying the distributions of the lepton energy and transverse momentum relative to the jet axis allows one to select this channel. This measurement provides a value for the coupling of the Z ~ to b quarks weighted by the mean semi-leptonic branching fraction of B hadrons. DELPHI has measured previously the fraction of b quarks produced in hadronic events using the distribution of an event shape variable, the boosted sphericity product [1], and also by studying the impact parameter distribution of charged tracks at the level of the beam interaction point [2]. Combining these measurements allows one to give a value for the mean semi-leptonic branching fraction of B hadrons produced in Z ~ decays. The lepton energy distribution is sensitive to the energy distribution of heavy hadrons and a comparison between data and Monte Carlo simulations allows the fragmentation distributions of the b quark to be studied. Leptons coming from the decays of charm particles do not have such distinctive features as leptons from direct B decays and with the present statistics only very crude measurements could be extracted on c g production. For this reason, in the following analysis, it was assumed that the production of c quarks is given by the Standard Model. After a description of the event selection and of the aspects of the apparatus that are relevant for this analysis, measurements obtained with selected data samples enriched in muons and in electrons are presented separately and then combined to get the final results. 2 D a t a a n d d e t e c t o r 2.1 Event selection and apparatus For this analysis, the sample of 120 K hadronic events recorded in DELPHI in 1990, were required to fulfill the following selection criteria: at least 7 reconstructed charged particles with momentum greater than 100 MeV/c; a total charged energy greater than 14% of the centre of mass energy; the thrust axis of the event at more than 32 ~ from the beam axis; the muon chambers had to be operational for the muon analysis; the barrel electromagnetic calorimeter (the HPC) had to be operational for the electron analysis. The resulting samples of about 100 K events were analyzed for the presence of electron and muon candidates. To define the solid angle covered by the detectors the following conventions were used. The z axis was taken along the electron beam direction and the y axis was vertical. Polar coordinates of a point in the transverse (x, y) plane were labeled R and ~. The 0 angle was used to define a direction relative to the z axis. The muon identification relied mainly on the muon chambers, a set of drift chambers providing three dimensional information. In the barrel part of the detector (52~ 0 < 128 ~ there are 3 sets of chambers (see Fig. 1). One set of chambers is located just inside the hadron calorimeter and two sets are just beyond it, with 2 layers I X ' ~ DELPHI InteraCtive Analysis i

K0SK0S correlations have been studied in a sample of 717 511 hadronic events collected by the DELPHI detector at LEP during 1992. An enhancement is found in the production of pairs of K0S of similar momenta, as compared with a Monte Carlo simulated reference sample. The measured values for the strength of the correlation and the radius of the emitting source of kaons are λ = 1.13 ± 0.54 (stat) ± 0.23 (syst) and r = 0.90 ± 0.19 (stat) ± 0.10 (syst) fm. This enhancement is consistent with the hypothesis that K0SK0S pairs display an enhancement, regardless of whether they come from a K0K̄0 or from a K0K0 (K̄0K̄0) system.

The study of the properties of inclusive production ofDs mesons and of events in which a ϕ and a muon are present in the same jet provides two independent measurements of the probability,fsw, for a heavy quark to hadronize into a strangeB orD meson. The data sample analysed corresponds to 243,000 hadronicZ0 decays. The combined value of these measurements isfsw=0.19±0.06±0.08. From the flight distance distributions ofDs and of (ϕ-lepton) secondary vertices, with the lepton emitted at high transverse momentum relative to the jet axis, two values are obtained for theBs0 meson lifetime. Combining these measurements with a previous result based on the study ofDs-μ events, theBs0 meson lifetime is measured to be: 0.96±0.37 ps.

The study of the properties of inclusive production ofD s mesons and of events in which a ϕ and a muon are present in the same jet provides two independent measurements of the probability,f s w , for a heavy quark to hadronize into a strangeB orD meson. The data sample analysed corresponds to 243,000 hadronicZ0 decays. The combined value of these measurements isf s w =0.19±0.06±0.08. From the flight distance distributions ofDs and of (ϕ-lepton) secondary vertices, with the lepton emitted at high transverse momentum relative to the jet axis, two values are obtained for theB s 0 meson lifetime. Combining these measurements with a previous result based on the study ofDs-μ events, theB s 0 meson lifetime is measured to be: 0.96±0.37 ps.

A study ofB meson decays intoDlX final states is presented. In these events, neutral and chargedD mesons originate predominantly fromB+ andB0 decays, respectively. The dilution of this correlation due toD** production has been taken into account. From 263 700 hadronicZ0 decays collected in 1991 with the DELPHI detector at the LEP collider, 92D0?K-p+, 35D+?K-p+p+ and 61D*0?D0p+ followed byD0?K-p+ orD0?K-p+p+p-, are found with an associated lepton of the same charge as the kaon. From theD0l- andD*+l-, the probabilityfd that ab quark hadronizes into aB- (or <img src="/fulltext-image.asp?format=htmlnonpaginated&src=G841512G76668453_html\10052_2005_Article_BF01565048_TeX2GIFIE1.gif" border="0" alt=" $$\bar B^0 $$ " />) meson is found to be 0.44 ±0.08±0.09, corresponding to a total (Bs+?b) hadronization fraction of 0.12-0.12+0.24. By reconstructing the energy of eachB meson, theb quark fragmentation is directly measured for the first time. The mean value of theB meson energy fraction is: $$\left\langle {X_E (B)} \right\rangle = 0.695 \pm 0.015(stat.) \pm 0.029(syst.)$$

The tau lepton lifetime is measured using four different methods with the DELPHI detector. Three measurements using one prong decays are combined, accounting for correlations, resulting in ττ=298 ±7 (stat.)±4 (syst.) fs while the decay length distribution of three prong decays gives ππ=298±13 (stat)±(syst.) fs. The combined result is ττ=298±7 fs. The ratio of the Fermi coupling constant from tau decay relative to that from muon decay is found to be 0.985±0.013, compatible with lepton universality.

A new measurement of alpha(s), is obtained from the distributions in thrust, heavy jet mass, energy-energy correlation and two recently introduced jet broadening variables following a method proposed by Catani, Trentadue, Turnock and Webber. This method includes the full calculation of O(alpha(s)2) terms and leading and next-to-leading logarithms resummed to all orders of alpha(s). The analysis is based on data taken with the DELPHI detector at LEP during 1991. I its found that the inclusion of the resummed leading and next-to-leading logarithms reduces the scale dependence of alpha(s) and allows an extension of the fit range towards the infrared limit of the kinematical range. The combined value for alpha(s) obtained at the scale mu2 = M(Z)2 is: alpha(s)(M(Z)2)= 0.123 +/- 0.006.

The B0−B0 mixing was studied by using about 250 000 hadronic decays of the Z0, collected with the DELPHI detector at LEP. With 1665 dilepton events, the probability for a b quark to become a b before decaying was found to be χ = 0.121−0.040+0.044 ± 0.017. The semileptonic branching ratio of the b was measured from the dilepton and single lepton events and found to be Br(b → l) = (10.0 ± 0.7 ± 0.7)%.

A search has been made for pairs and for single production of scalar leptoquarks of the first and second generations using a data sample of 392 000 Z0 decays from the DELPHI detector at LEP 1. No signal was found and limits on the leptoquark mass, production cross section and branching ratio were set. A mass limit at 95% confidence level of 45.5 GeV/c2 was obtained for leptoquark pair production. The search for the production of a single leptoquark probed the mass region above this limit abd its results exclude first and second generation leptoquarks D0 with masses below 65 GeV/c2 and 73 GeV/c2 respectively, at 95% confidence level, assuming that the D0lq Yukawa coupling αλ is equal to the electromagnetic one. An upper limit is also given on the coupling αλ as a function of the leptoquark mass mD

The DELPHI Microvertex detector, which has been in operation since the start of the 1990 LEP run, consists of three layers of silicon microstrip detectors at average radii of 6.3, 9.0 and 11.0 cm. The 73728 readout strips, oriented along the beam, have a total active area of 0.42 m2. The strip pitch is 25 μm and every other strip is read out by low power charge amplifiers, giving a signal to noise ratio of 15:1 for minimum ionizing particles. On-line zero suppression results in an average data size of 4 kbyte for Z0 events. After a mechanical survey and an alignment with tracks, the impact parameter uncertainty as determined from hadronic Z0 decays is well described by (69pt)2 + 242 μm, with pt in GeV/c. For the 45 GeV/c tracks from Z0 → μ− decays we find an uncertainty of 21 μm for the impact parameter, which corresponds to a precision of 8 μm per point. The stability during the run is monitored using light spots and capacitive probes. An analysis of tracks through sector overlaps provides an additional check of the stability. The same analysis also results in a value of 6 μm for the intrinsic precision of the detector.

The strong coupling constant for b quarks has been determined, and its flavour independence, as predicted by QCD, investigated. The analysis involved events with lepton candidates selected from approximately 356 000 hadronic decays of the Z0, collected by the DELPHI detector at LEP in 1990 and 1991. A method based on a direct comparison of the three-jet fraction in a b enriched sample, selected by requiring leptons with large momenta, to that of the entire hadronic sample, illustrated the significant effect of the b quark mass on the multi-jet cross section, and verified the flavour independence of the strong coupling constant to an accuracy of ±6%. A second procedure based on a fit to the momentum and transverse momentum spectra of the lepton candidates in both two-jet and three (or more)-jet event samples simultaneously determined the b content in each, and, using second order QCD calculations, gave an absolute measurement of αs for b quarks of 0.118±0.004 (stat.)±0.003 (syst.)±0.008 (scale). A comparison with αs for all quark flavours, as measured from the three-jet fraction in all hadronic events, further allowed the coupling strength for b quarks to be expressed in terms relative to that for udsc quarks, thereby cancelling certain common systematic uncertainties, and yielded αsbαsudsc = 1.00 ± 0.04 (stat.) ± 0.03 (syst.).

A determination of the hadronic fragmentation functions of the Z-boson is presented from a study of the inclusive hadron production wit the DELPHI detector at LEP. These fragmentation functions were compared with the ones at lower energies, thus covering data in a large kinematic range: 196 0.08 A large scaling violation was observed, which was used to extract the strong coupling constant in second order QCD: alpha_s(Mz) =0.118 +\- 0.005. The corresponding QCD scale for five quark flavours is: Lambda_MS_bar = 230 +/- 60 MeV.

An analysis of the production of the LAMBDA baryon in the hadronic decays of the Z0 is presented, based on about 993K multihadronic events collected by the DELPHI detector at LEP during 1991 and 1992. The differential cross section of the LAMBDA and the correlations between LAMBDA and LAMBDABAR produced in the same event are compared to current models, based both on string fragmentation and on cluster decay. The predictions of the string fragmentation model are found to give satisfactory agreements with the data, clearly better than those of the cluster model.

This constitutes a convincing model-independent proof of the existence of the triple-gluon vertex, since its contribution is directly proportional toCA/CF. The results are in agreement with the values expected from QCD:CA/CF=2.25, andNC/NA=3/8.

Based on a sample equivalent to 365000 hadronic Z0 decays, the search in DELPHI data for pairs of leptons accompanied by a pair of charged particles is described. A total of 11 events were found in the electron channel, 9 in the muon channel and 7 in the tau channel. Results on lepton pairs with a radiated photon are also presented. The data from all channels are compatible with the expectations from standard processes. However, one event was found in the tau channel with an unusually high mass of the charged particle pair.