Since the end of 1995 the LEP electron-positron collider at CERN is operated at center-of-mass energies well above the Z resonance. Results on hadronic and leptonic total cross sections and forward-backward asymmetries obtained with the L3 detector are reported. These measurements which are sensitive to the interference of photon and Z exchange are in agreement with the Standard Model.
7/9,20 Submitted 23 May, pa07-040
From 1989 to 1995, LEP had been running at the Z resonance with high luminosity. About 4 millions Z events have been accumulated by the L3 experiment. Together with careful systematic studies, these data allow a high precision determination of the properties of the Z boson such as the mass and the widths. Asymmetries are used to determine the effective neutral current coupling constants of charged leptons. Our results are compared with Standard Model predictions.
7/9,20, 23 May, pa07-044 Contributd paper will appear as part of the EWWG contribution in July
Using the data recorded with the L3 detector at LEP, we study the process e+e- --> mu+mu- for events with hard initial-state photon radiation. 293 muonpairs at effetive center-of-mass energies between 50 GeV and 86 GeV are selected in a total luminosity of 103.5/pb. These events are used to determine the cross section and forward-backward asymmetry at center-of-mass energies below the Z resonance.
7/9,20, 23 May, pa07-041
Results on total cross sections and asymmetries at the Z resonance obtained with the L3 detector at LEP are used to obtain limits on the mass of a heavy gauge boson Z' and the Z-Z' mixing angle. Recent measurements performed above the Z resonance are included which improves the mass limit. In addition, limits on the couplings of fermions to the Z' are derived.
7,12,13/9,12,15, 23 May, pa07-042
The results of the searches for neutral Higgs boson using L3 Detector are reported, focusing on Higgs boson masses below 70 GeV. The data sample consists of three million hadronic Z decays collected by the L3 experiment from 1991 through 1994 near the Z peak. No signal was found in all the mass range covered leading to a lower limit on the mass of the Standard Model Higgs boson of 60.2 GeV at 95% CL. These results along with the results of the search for the pair produced hA Higgs bosons are interpreted in the framework of the general two doublet model and limits on the Higgs boson production crossection and branching fractions are set.
11,13/9,12,20, 23 May, pa11-017
A search for supersymmetric particles (charginos, neutralinos, sleptons and stop quarks) has been performed with data collected by the L3 detector during the November 1995 run of the LEP collider at center of mass energies between 130 and 140 GeV with a total integrated luminosity of 5.1 pb-1. We observe no signal for supersymmetric particles and we set improved exclusion limits on their production cross sections and masses. The results of the search for supersymmetric particles are also interpreted in the framework of the Minimal Supersymmetric Standard Model. We also report on the latest preliminary results on the search for new particles in data collected in 1996, at the highest center of mass energy reached at LEP.
A search for unstable sequential neutral and charged heavy leptons has been made at center-of-mass energies 130 and 136 GeV with the L3 detector at LEP. The neutral leptons are assumed to decay via mixing to electrons and muons. No evidence for their existence was found. We exclude unstable Dirac neutrinos for masses below 59.3 (57.9) GeV and unstable Majorana neutrinos below 48.6 (47.2) GeV if the neutrino couples to the electron(muon) family. We exclude unstable charged heavy leptons for masses below 61 GeV for a wide range of the associated neutral lepton mass.
13/12, 23 May, pa13-016
We report on a search for the excited electrons, muons, taus and neutrinos in e+e- collisions at center of mass energies between 130 and 140 GeV using the L3 detector at LEP. No evidence has been found for their existence. From an analysis of the expected pair produced scalar leptons we determine the lower mass limits at 95 CL. From an analysis of the expected single produced sleptons we determine upper limits on their couplings.
13/12, 23 May, pa13-017
We report on the results of the search for lepton flavour violating Z decays which was carried out with the full data sample collected at LEP 1 from 1990 to 1995. We searched for Z decays into a pair of different flavour leptons. Taking advantage of the precision measurements of electrons and muons we set stringent limits on the production rates for these decays.
7/9,10, 23 May, pa07-045
A search for four fermion events with two leptons, with all the four fermions in the final state seen in the detector, has been done in the data collected by the L3 detector at LEP at center of mass energies between 130 and 140 GeV. The data sample correspond to an integrated luminosity of 5 pb-1. The number of observed events in the two channels is consistent with the predictions from known physics processes. The production of four-jet final states has also been investigated, to search for purely hadronic decays of new heavy particles. Preliminary results show no significant evidence of a signal from new particles in the data analysed.
13/12, 23 May, pa13-018
We have studied the structure of hadronic events with a hard radiated final state photon (e+e- -> Z -> hadrons + gamma) recorded by the L3 detector at center-of-mass energy around 91 GeV. Event shape distributions and jet rates are measured in the CM frame of the hadronic system. The corresponding reduced CM energy varies between 30 GeV and 86 GeV. The shapes of the distributions and the energy dependence of their mean values are compared with the predictions of several QCD models. From a fit to the data with a resummed O(alphaS^2) QCD calculation, we determine the strong coupling constant alphaS over a wide range of energy.
4/4,5, 23 May, pa04-021
We present a study of the structure of hadronic events recorded by the L3 detector at center-of-mass energies of 130 and 136 GeV. The data sample corresponds to an integrated luminosity of 5 pb^-1 collected during the high energy run of 1995. The shapes of the event shape distributions and the energy dependence of their mean values are well reproduced by QCD models. From a comparison of the data with resummed O(alphaS^2) QCD calculations, we determine the strong coupling constant to be alphaS(133 GeV) = 0.107 +- 0.005 (exp) +- 0.006 (theor).
4/4,5, 23 May, pa04-022
A search for the neutral Higgs boson in the processes e+e- -> Z -> Higgs + gamma -> qqbar gamma and e+e- -> Z -> Z^* Higgs -> qqbar gammagamma has been performed using 2.8 million hadronic Z decays collected with the L3 detector at LEP from 1991 through 1994. No evidence for these processes has been observed. Upper limits at 95\Šonfidence level for the corresponding cross sections have been set and the results have been compared with theoretical predictions beyond the Standard Model.
13/12, 23 May, pa13-019
The average lifetime of the tau lepton has been measured using data collected in 1994 by the L3 detector at LEP, running at sqrt{s}= M(Z) . The precise track position information of the new Silicon Microvertex Detector has been exploited. The tau lepton lifetime is determined from the impact parameter distribution for 30322 tau decays in one charged particle and from the decay length distribution for 3891 tau decays in three charged particles. The preliminary results are: tau_(tau) = 287.5 +- 3.8 (stat) +- 3.9(syst) fs and \\ tau_(tau) = 293.0 +- 5.3(stat) +- 2.5(syst) fs, respectively. Including the measurement from the data sample collected in 1991-1993 the combined result is: tau_(tau) = 291.6 +- 3.4 fs. The branching ratios of the decays tau -> e nu_e nu_tau and tau -> mu nu_mu nu_tau are measured from the data sample collected in 1991-1994 to be (17.67 +- 0.14)% and (17.34 +- 0.14)%, respectively. The errors are statistical only. The systematic errors are on the same level but not yet finalized.
The Michel parameters rho, eta, xi and xi delta, the chirality parameter xi_h and the tau polarization P_tau are measured using 32000 tau pair decays. Their values are extracted from the energy spectra of leptons and hadrons in tau -> l nu nu and tau -> pi nu decays, the energy and decay angular distributions in tau -> rho nu decays, and the correlations in the energy spectra and angular distributions of the decay products. Assuming universality in leptonic and semileptonic tau decays, the results are rho=0.794 +- 0.039 +- 0.031, eta=0.25 +- 0.17 +- 0.11, xi =0.94 +- 0.21 +- 0.07 and xi delta = 0.81 +- 0.14 +- 0.06, the chirality parameter is xi_h = -0.970 +- 0.053 +- 0.011, and the tau polarization P_tau = -0.154 +- 0.018 +- 0.012. The values for all Michel parameters are in agreement with a V-A structure of the weak charged current interaction in tau lepton decays. The measurement of the chirality parameter xi_h agrees with only left handed tau neutrinos in semileptonic tau decays. The value for the tau polarization P_tau obtained in this analysis is in agreement with the result of our previous tau polarization measurement.
7,9,10/7,9, 23 May, pa07-051
With a data sample corresponding to the integrated luminosity of 84 pb-1 collected during the 1991-1994 LEP runs at the Z resonance we have analyzed radiative decays Z -> tau tau gamma. The energy spectrum of the photons is compared to the prediction of QED including an additional magnetic dipole term in the matrix element. Neglecting terms of O(m_tau^2/M_Z^2), the contribution of the latter, expressed as anomalous magnetic moment of the tau lepton, a_tau=(g-2)/2, results to a_tau^2 =-0.00279 +- 0.00197(stat)^{+0.00101}_ {-0.00072}(syst). >From this an upper limit for a_tau^2 of 0.00235 at 90\% confidence level is derived using Bayesian approach. This leads to |a_tau|< 0.049. All results are preliminary.
7,10/7,9, 23 May, pa07-053
The tau flight direction is reconstructed for 899 Z ->tau^+ tau^- where both tau's decay into rho nu_tau. >From azimuthal asymmetries we perform measurements of the electric weak dipole moment d^w_tau and the anomalous weak magnetic moment a^w_tau. The preliminary results are d_tau^w = (3.11 +- 6.61) * 10^-17 e cm and a_tau^w = 0.007 +- 0.012. Corresponding upper limits at the 95Šonfidence level are d_tau^w < 1.39 *10^-16 e cm and a_tau^w < 0.026.
7,10/7,9, 23 May, pa07-055
With a data sample of 58000 events Z -> tau^+ tau^- collected in 1994 we have performed a preliminary measurement of A_e and A_tau from the polarization of tau leptons as a function of the production polar angle using the following decay modes: tau -> e nu_e nu_tau, tau -> mu nu_mu nu_tau, tau -> pi(K) nu_tau, tau -> rho nu_tau and tau -> a_1 nu_tau.
7/9, 23 May, pa07-056
Resonance formation in gamma - gamma collisions has been studied in the L3 detector at LEP with data from 1991 to 1994. Results are presented for the decay channels pi+ pi- gamma and pi+ pi- pi0. The Q2 dependence and the form factor of the eta '(958) have been measured. In the pi+ pi- pi0 final state we observe the a2(1320) meson and evidence for a radially excited state, a2(1765), of JP=2+ produced predominantly in the helicity l=2 channel. The intermediate states considered for a2(1765) are rho-pi and f2-pi.
The reaction e+ e- --> e+ e- l+ l- has been studied in the L3 detector for l=e, mu ,tau with the 1991-94 data. Untagged data as well as data tagged in two ranges of Q2 : 1.4< Q2 < 7.6 (GeV/c)2 and 60.< Q2 < 140. (GeV/c)2 have been analysed. Good agreement is found with QED predictions. For the Q2 = 1.7 (GeV/c)2 data the polar and azimuthal angular distributions of the leptons in the gamma-gamma center of mass frame have been measured . The leptonic F2 structure function has been extracted.
Parallel session 3 Plenary 2 or 4, 24 May, ps03-022 ps03-022b
The charmonium state Chi_c has been observed in photon-photon collisions with the L3 detector at LEP in the decay channel Chi_c --> J/psi gamma (J/psi --> mu+ mu- , e+ e- ). The gamma - gamma width of the resonance has been measured. The event sample also contains events with non zero Q2.
Parallel session 1 Plenary 1, 24 May, pa01-065
The data taken with the L3 detector during the high energy LEP 1995 run e+e- --> e+e- gamma* gamma* --> e+e- hadrons have been analysed . Several caracteristics of the events are compared with theoretical models which include non perturbative vector dominance effects as well as perturbative QED and QCD diagrams.
Parallel session 2 Plenary 2 or 4, 24 May, pa02-036.ps
We have measured the time integrated B^0 Bbar^0 mixing parameter and the forward-backward asymmetry in the process e^+ e^- -->bb using hadronic events containing muons or electrons. The analysis presented is based on a fit to the momentum and transverse momentum distributions for single lepton and dilepton events. The determination of the effective electroweak mixing angle will also be presented.
7,8,10/7,8,9,10, 24 May, pa07-060 No contributed paper planned
We present measurements of J/psi and chi_c production using 3 million hadronic Z decays collected by the L3 experiment from 1991-1994. J/psi are identified via their decays into e^+e^- or mu^+mu^-. chi_c are reconstructed through their decays into a J/psi and a photon. The inclusive Z --> J/psi and Z --> chi_c branching fractions are measured. The extension of this study up to the Upsilon mass region is also reported, which includes a search for the rare decays B --> e^+e^- and B --> mu^+mu^-. In addition we search for exclusive B decay modes containing J/psi. Upper limits on the decay channels B --> J/psi eta and B --> J/psi pi^0 are given. The decay channel B_c --> J/psi l nu is used to set an upper limit on the B_c production rate.
5,8,10,13/7,10,12, 24 May, pa05-046
We report on the measurements of the average B-hadron lifetime and of the inclusive semileptonic branching ratios b --> e nu X, b --> mu nu X, b --> l nu X and b --> nu X at LEP. The lifetime analysis is based on the data collected by the L3 detector during the 1994 running period and it uses the information from the precise L3 Silicon Microvertex Detector. The semileptonic decay measurement is based on hadronic Z decays collected between 1991 and 1992. A new approach that treats the information from events with large missing-energy and from events with identified leptons has been used. Both experimental analyses can be combined to provide a precise determination of the V_cb component of the Kobayashi-Maskawa matrix.
5,7,8,10/7,8,9, 24 May, pa05-047
Time-dependent B^0_d Bbar^0_d mixing has been studied using 1.5 million hadronic Z decays registered by L3 in 1994 with a fully operational silicon microvertex detector. b tagging is performed by identifying leptons from semileptonic B decays and using the charge correlation between leptons in opposite hemispheres as a signature for mixing. The proper time of the B meson decay is measured by reconstructing the primary and decay vertices. A result for Delta(m_d) will be given.
5,8,10/7,8,9,10, 24 May, pa05-048
We present a new measurement of R_b = Gamma(Z-->bb) / Gamma(Z-->hadrons) using different double tag methods. The first analysis uses 1.5 million hadronic Z events collected by L3 in 1994 and exploits the capabilities of the silicon microvertex detector. The lifetime tagging of b-events can be performed either by identifying secondary vertices displaced significantly from the primary vertex or by using tracks with large impact parameter with respect to the e+e- collision point. In the second analysis, a high P_t lepton tag is used to enrich the fraction of b-events in a high statistics hadronic sample.
5,7,10/6,7,9, 24 May, pa05-049 pa05-049.ps