[show abstract][hide abstract] ABSTRACT: From measurements of the cross sections for e+e---> hadrons and the cross sections and forward-backward charge-asymmetries for ee--->e+e-, mu+mu- and pi+pi- at several centre-of-mass energies around the Z0 pole with the DELPHI apparatus, using approximately 150 000 hadronic and leptonic events from 1989 and 1990, one determines the following Z0 parameters: the mass and total width MZ = 91.177 +/- 0.022 GeV, GammaZ = 2.465 +/- 0.020 GeV, the hadronic and leptonic partial widths Gammah = 1.726 +/- 0.019 GeV, Gammal = 83.4 +/- 0.8 MeV, the invisible width Gammainv = 488 +/- 17 MeV, the ratio of hadronic over leptonic partial widths RZ = 20.70 +/- 0.29 and the Born level hadronic peak cross section sigma0 = 41.84+/-0.45 nb. A flavour-independent measurement of the leptonic cross section gives very consistent results to those presented above (Gammal = 83.7 +/- 0.8rmMeV). From these results the number of light neutrino species is determined to be Nv = 2.94 +/-0.10. The individual leptonic widths obtained are: Gammae = 82.4+/-_1.2 MeV, Gammau = 86.9+/-2.1 MeV and Gammatau = 82.7 +/- 2.4 MeV. Assuming universality, the squared vector and axial-vector couplings of the Z0 to charged leptons are: V¯l2 = 0.0003+/-0.0010 andA¯l2 = 0.2508+/-0.0027. These values correspond to the electroweak parameters: varrhoeff = 1.003 +/- 0.011 and sin2thetaWeff = 0.241 +/- 0.009. Within the Minimal Standard Model (MSM), the results can be expressed in terms of a single parameter: sin2thetaWM¯S = 0.2338 +/- 0.0027. All these values are in good agreement with the predictions of the MSM. Fits yield 43
[show abstract][hide abstract] ABSTRACT: 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-?H0+Z0*,Z0*?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.
[show abstract][hide abstract] ABSTRACT: 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.
[show abstract][hide abstract] ABSTRACT: Measurements of the cross section and forward-backward asymmetry for the reaction e+e- --> mu+mu- 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° < theta < 158°. From a fit to the cross section for 43° < theta < 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 --> mu+mu- partial widths is determined to be (GammaeGammamu)1/2 = 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(thetaw) = 0.2267 +/- 0.0037. The ratio of the hadronic to muon pair partial widths is found to be Gammah/Gammamu = 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 VeVmu = 0.0024 +/- 0.0015(stat.) +/- 0.0004(syst.) and AeAmu = 0.253 +/- 0.003(stat.) +/- 0.003 (syst.). The results are in good agreement with the expectations of the minimal standard model.
[show abstract][hide abstract] ABSTRACT: The production rates for 2-, 3-, 4- and 5-jet hadronic final states have been measured with the DELPHI detector at the e+e− storage ring LEP at centre of mass energies around 91.5 GeV. Fully corrected data are compared to O(α2s) QCD matrix element calculations and the QCD scale parameter is determined for different parametrizations of the renormalization scale ω2. Including all uncertainties our result is αs(M2Z)=0.114±0.003[stat.]±0.004[syst.]±0.012[theor.].
[show abstract][hide abstract] ABSTRACT: We present a search for the third generation up type quark t and a possible fourth down type quark b' in hadronic Z0 decays observed in DELPHI at the LEP collider. For any scenario with a decay through the charged current or into a charged Higgs with a mass at least 6 GeV/c2 below the t and 3 GeVc2 below the b' mass, we set a lower limit for the t quark mass at 44.0 GeV/c2 and for the b' mass at 44.5 GeV/c2. For specific scenarios the mass limits are slightly higher, e.g. for charged current decays the limits are 44.5 and 45.0 GeV/c2 respectively, where all limits are given at a 95% confidence level.
[show abstract][hide abstract] ABSTRACT: Using a data sample corresponding to 10 000 hadronic Z0 decays, we have searched for the production of sleptons and gauginos in the two-prong decays of Z0. No candidate remains after straightforward selections. For neutralinos, we use selection methods developed in our previous search for neutral Higgs particles. The negative results are translated into improved mass limits and parameter constraints on the minimal supersymmetric extension of the standard model.
[show abstract][hide abstract] ABSTRACT: The correlations in rapidity in hadron production from e+e− annihilation near the Z0 resonance were studied by means of the method of factorial moments, using data taken with the DELPHI detector at LEP. The parton shower hadronization model was found to be in quantitative agreement with the data, in contrast with previous results at lower energies.
[show abstract][hide abstract] ABSTRACT: A determination of the partial width Gammacc¯ of the Z0 boson into charm quark pairs is presented, based on a total sample of 36 900 Z0 hadronic decays measured with the DELPHI detector at the LEP collider. The production rate of cc¯ events is derived from the inclusive analysis of charged pions coming from the decay of charmed meson D*+-->D0pi+ and D*--->D¯0 pi- where the pi+/- is constrained by kinematics to have a low pT with respect to the axis. The probability to produce these pi+/- from D*+/- decay in cc¯ events is taken to be 0.31 +/-_0.05 as measured at &surd;s=10.55 GeV. The measured relative partial width Gammacc¯/Gammah = 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 Gammah this implies Gammacc¯ = 282+/-53 (stat.)+/-88(syst.) MeV.
[show abstract][hide abstract] ABSTRACT: Measurements are presented of the cross section ratios Rl=sigmal(e+e--->l+l-)/sigmah(e+e--->hadrons) for l=e, mu and tau using data taken from a scan around the Z0. The results are Re=(5.09+/-o.32+/-0.18)%, Rmu=(0.46+/-0.35+/-0.17)% and Rtau=(4.72+/-0.38+/-0.29)% where, for the ratio Re, the t-channel contribution has been subtracted. These results are consistent with the hypothesis of lepton universality and test this hypothesis at the energy scale s~8300 GeV2. The absolute cross sections sigmal(e+e--->l+l-) have also been measured. From the cross sections the leptonic partial widths Gammae=(83.2+/-3.0+/-2.4) MeV, (GammaeGammamu)1/2=(84.6+/-3.0+/-2.4) MeV and (GammaeGammatau)1/2=(82.6+/-3.3+/-3.2) MeV have been extracted. Assuming lepton universality the ratio Gammal/Gammah=(4.89+/-0.20+/-0.12) × 10-2 w was obtained, together with Gammal=(83.6+/-1.8+/-2.2) MeV. The number of light neutrino species is determined to be Nv=3.12+/-0.24+/-0.25. Al the data are consistent with the predictions of the standard model.
[show abstract][hide abstract] ABSTRACT: 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 Mz, the total width Gammaz, the product of the electronic and hadronic partial widths GammaeGammah, and the unfolded pole cross section sigma0: MZ=91.171+/-0.030(stat)+/-0.030 (beam) GeV, GammaZ=2.511+/-0.065 GeV, GammaeGammah=0.148+/-0.006 (stat.)+/-0.004 (syst.) GeV2, sigma0=41.6+/-0.7(stat.)+/-1.1 (syst.) nb, Good agreement with the predictions of the standard model is observed. From a two-parameter fit the number of massless neutrino generations is found to be Nnu = 2.97+/-0.26. Thus the hypothesis of a fourth neutrino with mass less than 40 GeV is excluded with 95% confidence level. Combining the cross section measurements with the ratio Gammal/Gammah reported in another DELPHI paper [Phys. Lett. B 241 (1990) 425], the hadronic, leptonic and invisible widths are found to be Gammah=1741+/-61 MeV, Gammal=85.1+/-2.9 MeV, Gammah/Gammal=20.45+/-0.98, Gammainv=515+/-54 MeV, in good agreement with the standard model.
[show abstract][hide abstract] ABSTRACT: A search has been made for pairs oade for pairs of scalar quarks (squarks) produced in e+e- annihilations at LEP (sqrt(s)≅Mz0), and decaying into a standard quark and a neutral, non-interacting, stable, massive particle (the lightest supersymmetric particle, LSP). The search has been conducted for differences in the mass of the squark and LSP of 2 GeV/c2 and above. Up squarks with masses below 42 GeV/c2 and down squarks below 43 GeV/c2 were excluded. Six squark flavours degenerate in mass were excluded below 45 GeV/c2.
[show abstract][hide abstract] ABSTRACT: We have studied the energy-energy angular correlations in hadronic final states from Z0 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 Lambda(5)/MS=104+25-20(stat.)+25-20(syst.)+3000)theor.). MeV, which corresponds to alphas(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 alphas. In the Monte Carlo simulation the scale of alphas as well as the fragmentation parameters have been optimized to described reasonably well all aspects of multihadron production.
[show abstract][hide abstract] ABSTRACT: Hadronic decays of Z0 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, pinT and poutT are compared with values found at lower energy e+e− colliders.
[show abstract][hide abstract] ABSTRACT: Using a sample of Z0's corresponding to about 12 000 events, we have searched for the production of charged scalars, primarily charged Higgs particles, decaying into cscs, taunu + jets, and taunutaunu. The average detection efficiency is 20%. No candidate was found in the leptonic modes. Masses in the range up to 30-36 GeV/c2 are excluded, extending the mass domain covered by previous e+e- machines.
[show abstract][hide abstract] ABSTRACT: 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 beta1 and tan beta
[show abstract][hide abstract] ABSTRACT: First measurements of the mass and width of the Z0 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 Z0 mass. The values found for the mass and width are M(Z0)=91.06+/-0.09 (stat) +/-0.045 (syst.) GeV and Gamma(Z0)=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 Nnu=2.4+/-0.4 (stat.) +/-0.5 (syst.).
[show abstract][hide abstract] ABSTRACT: Single diffraction dissociation was measured in the reaction at the centre-of-mass energy . The mass M of the system X was deduced from the pseudorapidity distribution of the observed charged tracks. The cross section of single diffraction dissociation for M2/s⩽0.05 isσsd=9.4±0.7 mb. Comparison to the ISR data shows that σsd increases with energy less fast than the total and the elastic cross sections.
[show abstract][hide abstract] ABSTRACT: Proton-antiproton elastic scattering was measured at the CERN SPS Collider at the centr-of-mass energy GeV in the Coulomb interference region. The data provide information on the phase of the hadronic amplitude in the forward direction. The conventional analysis gives for the ratio ϱ of the real to the imaginary part of the hadronic amplitude the result ϱ=0.24±0.04.