G. Alemanni

Measurement of the atmospheric muon spectrum from 20 to 3000 GeV

The absolute muon flux between 20 GeV and 300 GeV is measured with the L3 magnetic muon spectrometer for zenith angles ranging from 0 degree to 58 degrees. Due to the large exposure of about 150 m2 sr d, and the excellent momentum resolution of the L3 muon chambers, a precision of 2.3% at 150 GeV in the vertical direction is achieved. The ratio of positive to negative muons is studied between 20 GeV and 500 GeV, and the average vertical muon charge ratio is found to be 1.285 +- 0.003 (stat.)+- 0.019 (syst.).

Search for single top production at LEP

Single top production in e^+e^- annihilations is searched for in data collected by the L3 detector at centre-of-mass energies from 189 to 209 GeV, corresponding to a total integrated luminosity of 634 pb-1. Investigating hadronic and semileptonic top decays, no evidence of single top production at LEP is obtained and upper limits on the single top cross section as a function of the centre-of-mass energy are derived. Limits on possible anomalous couplings, as well as on the scale of contact interactions responsible for single top production are determined.Single top production in e+e- annihilations is searched for in data collected by the L3 detector at centre-of-mass energies from 189 to 209 GeV, corresponding to a total integrated luminosity of 634 pb-1. Investigating hadronic and semileptonic top decays, no evidence of single top production at LEP is obtained and upper limits on the single top cross section as a function of the centre-of-mass energy are derived. Limits on possible anomalous couplings, as well as on the scale of contact interactions responsible for single top production are determined.

Measurement of triple gauge boson couplings of the W boson at LEP

The CP-conserving triple-gauge-boson couplings, gZ 1 , κγ , λγ , g Z 5 , κZ andλZ are measured using hadronic and semi-lepto W-pair events selected in 629 pb −1 of data collected at LEP with the L3 detector at centre-of-mass energies between 1 209 GeV. The results are combined with previous L3 measurements based on data collected at lower centre-of-mass e with the results from single-W production and from events with a single-photon and missing energy. Imposing the co 154 L3 Collaboration / Physics Letters B 586 (2004) 151–166 d Model κZ = gZ 1 − tan2 θW(κγ − 1) andλZ = λγ , we obtain for the C and P conserving couplings the results: gZ 1 = 0.966± 0.033(stat) ± 0.015(syst), κγ = 1.013± 0.066(stat) ± 0.026(syst), λγ = −0.021± 0.035(stat) ± 0.017(syst). Results from the analysis of fully leptonic W-pair decays are also given. All results are in agreement with the Standar expectations and confirm the existence of self-couplings among electroweak gauge bosons.  2004 Published by Elsevier B.V.

Measurement of the anomalous magnetic and electric dipole moments of the tau lepton

Abstract We analyse e+e−→ττγ events using 100 pb −1 of data collected by the L3 experiment during the 1991-1995 LEP runs at the Z pole. From the energy of the photons and their isolation from the tau decay products, we determine the anomalous magnetic and electric dipole moments of the tau to be, respectively: a τ =0.004±0.027±0.023; d τ =(0.0±1.5±1.3)×10 −16 e· cm . This is a direct measurement of these τ form factors at q2=0.

Measurement of η′(958) formation in two-photon collisions at LEP1

Abstract The formation of the η′ in the reaction e+e−→e+e−η′→e+e−π+π−γ has been measured by the L3 detector at a centre-of-mass energy of 91 GeV . The radiative width of the η′ has been found to be Γ γγ =4.17±0.10 (stat.) ±0.27 (sys.) keV . The Q2 dependence of the η′ formation cross section has been measured for Q 2 ≤10 GeV 2 and the η′ electromagnetic transition form factor has been determined. The form factor can be parametrised by a pole form with Λ=0.900±0.046 (stat.) ±0.022 (sys.) GeV . It is also consistent with recent non-perturbative QCD calculations.

Study of the hadronic photon structure function F(2)**gamma at LEP

Abstract The hadronic photon structure function Fγ2 is studied in the reaction e+e−→e+e−hadrons at LEP with the L3 detector. The data, collected from 1991 to 1995 at a centre-of-mass energy s ≃91 GeV, correspond to an integrated luminosity of 140 pb−1. The photon structure function Fγ2 is measured in the Q2 interval 1.2 GeV2 ≤Q2≤9.0 GeV2 and the x interval 0.002

Measurement of the cross section of W-boson pair production at LEP

The cross section of W-boson pair-production is measured with the L3 detector at LEP. In a data sample corresponding to a total luminosity of 629.4 pb−1, collected at centre-of-mass energies ranging from 189 to 209 GeV, 9834 four-fermion events with W bosons decaying into hadrons or leptons are selected. The total cross section is measured with a precision of 1.4% and agrees with the Standard Model expectation. Assuming charged-lepton universality, the branching fraction for hadronic W-boson decays is measured to be: Br(W → hadrons) = 67.50 ± 0.42(stat.) ± 0.30(syst.)%, in agreement with the Standard Model. Differential cross sections as a function of the W− production angle are also measured for the semi-leptonic channels qqeν and qqμν.  2004 Published by Elsevier B.V. Open access under CC BY license.

Measurement of exclusive ρ0ρ0 production in two-photon collisions at high Q2 at LEP

Exclusive ρ 0 ρ 0 production in two-photon collisions involving a single highly virtual photon is studied with data collected at LEP at centre-of-mass energies 89 GeV < √ s < 209 GeV with a total integrated luminosity of 854.7 pb −1. The cross section of the process γγ * → ρ 0 ρ 0 is determined as a function of the photon virtuality, Q 2 , and the two-photon centre-of-mass energy, W γγ , in the kinematic region: 1.2 GeV 2 < Q 2 < 30 GeV 2 and 1.1 GeV < W γγ < 3 GeV.

Measurement of the Shadowing of High-Energy Cosmic Rays by the Moon: A Search for TeV-Energy Antiprotons

The shadowing of high-energy cosmic rays by the Moon has been observed with a significance of 9.4 standard deviations with the L3 + C muon spectrometer at CERN. A significant effect of the Earth magnetic field is observed. Since no event deficit on the east side of the Moon has been observed, an upper limit at 90% confidence level on the antiproton to proton ratio of 0.11 is obtained for primary energies around 1 TeV.

Study of the e+e-→Zγ process at LEP and limits on triple neutral-gauge-boson couplings

The process e^+e^- -->Z gamma, where the Z boson decays into hadrons or neutrinos, is studied with data collected with the L3 detector at LEP at centre-of-mass energies from 189 GeV up to 209 GeV. The cross sections are measured and found to be in agreement with the Standard Model predictions. Limits on triple neutral-gauge-boson couplings, forbidden in the Standard Model at tree level, are derived. Limits on the energy scales at which the anomalous couplings could be manifest are set. They range from 0.3 TeV to 2.3 TeV depending on the new physics effect under consideration.