P. Achard

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.).

Single- and multi-photon events with missing energy in e+e - collisions at LEP

Single-and multi-photon events with missing energy are selected in 619 pb −1 of data collected by the L3 detector at LEP at centre-of-mass energies between 189 GeV and 209 GeV. The cross sections of the process e + e − → ν ¯ νγ(γ) are found to be in agreement with the Standard Model expectations, and the number of light neutrino species is determined, including lower energy data, to be N ν = 2.98 ± 0.05 ± 0.04. Selection results are given in the form of tables which can be used to test future models involving single-and multi-photon signatures at LEP. These final states are also predicted by models with large extra dimensions and by several supersymmetric models. No evidence for such models is found. Among others, lower limits between 1.5 TeV and 0.65 TeV are set, at 95% confidence level, on the new scale of gravity for the number of extra dimensions between 2 and 8.

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 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.

Study of the

The process e+e- ->W+ W- gamma is studied using the data collected by the L3 detector at LEP. New results, corresponding to an integrated luminosity of 427.4 pb-1 at centre-of-mass energies from 192 GeV to 207 GeV, are presented. The W+W- gamma cross sections are measured to be in agreement with Standard Model expectations. No hints of anomalous quartic gauge boson couplings are observed. Limits at 95% confidence level are derived using also the process e+e- -->nu nubar gamma gamma.

W^{+} W^{-} \gamma

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.

process and limits on anomalous quartic gauge boson couplings at LEP

A Higgs particle produced in association with a Z boson and decaying into two photons is searched for in the data collected by the L3 experiment at LEP. All possible decay modes of the Z boson are investigated. No signal is observed in 447.5 pb^-1 of data recorded at centre-of-mass energies up to 209 GeV. Limits on the branching fraction of the Higgs boson decay into two photons as a function of the Higgs mass are derived. A lower limit on the mass of a fermiophobic Higgs boson is set at 105.4 GeV at 95% confidence level.A Higgs particle produced in association with a Z boson and decaying into two photons is searched for in the data collected by the L3 experiment at LEP. All possible decay modes of the Z boson are investigated. No signal is observed in 447.5 pb^-1 of data recorded at centre-of-mass energies up to 209 GeV. Limits on the branching fraction of the Higgs boson decay into two photons as a function of the Higgs mass are derived. A lower limit on the mass of a fermiophobic Higgs boson is set at 105.4 GeV at 95% confidence level.

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

Double-tag events in two-photon collisions are studied using the L3 detector at LEP centre-of-mass energies from root(s)=189 GeV to 209 GeV. The cross sections of the e+e- ->e+e- hadrons and gamma*gamma* ->hadrons processes are measured as a function of the photon virtualities, Q1^2 and Q2^2, of the two-photon mass, W_gammagamma, and of the variable Y=ln(W_gammagamma^2/(Q1 Q2)), for an average photon virtuality<Q2>= 16 GeV2. The results are in agreement with next-to-leading order calculations for the process gamma*gamma* ->q qbar in the interval 2<= Y<= 5. An excess is observed in the interval 5<Y<= 7, corresponding to W_gammagamma greater than 40 GeV . This may be interpreted as a sign of resolved photon QCD processes or the onset of BFKL phenomena.Double-tag events in two-photon collisions are studied using the L3 detector at LEP centre-of-mass energies from root(s)=189 GeV to 209 GeV. The cross sections of the e+e- -> e+e- hadrons and gamma*gamma* -> hadrons processes are measured as a function of the photon virtualities, Q2_1 and Q2_2, of the two-photon mass, W_gammagamma, and of the variable Y=ln(W2_gammagamma/(Q_1Q_2)), for an average photon virtuality Q2 = 16 GeV2. The results are in agreement with next-to-leading order calculations for the process gamma*gamma* -> q qbar in the interval 2 = Y = 5. An excess is observed in the interval 5 Y = 7, corresponding to W_gammagamma greater than 40 GeV . This may be interpreted as a sign of resolved photon QCD processes or the onset of BFKL phenomena.

Search for a Higgs boson decaying into two photons at LEP

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.