arXiv subjects

L3 Collaboration

Publications and source records attributed to L3 Collaboration.

At least 37 records · Page 2Linked to original sources

arXiv2005

Compton Scattering of Quasi-Real Virtual Photons at LEP

Compton scattering of quasi-real virtual photons, gamma e+- -> gamma e+-, is studied with 0.6fb-1 of data collected by the L3 detector at the LEP e+e- collider at centre-of-mass energies root(s')=189-209GeV. About 4500 events produced by the interaction of virtual photons emitted by e+- of one beam with e-+ of the opposite beam are collected for effective centre-of-mass energies of the photon-electron and photon-positron systems in the range from root(s')= 35GeV up to root(s')=175GeV, the highest energy at which Compton scattering was ever probed. The cross sections of the gamma e+- -> gamma e+- process as a function of root(s') and of the rest-frame scattering angle are measured, combined with previous L3 measurements down to root(s')~20GeV, and found to agree with the QED expectations.

hep-ex↗

arXiv2005

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.

astro-ph↗

arXiv2005

Study of Spin and Decay-Plane Correlations of W Bosons in the e+e- -> W+W- Process at LEP

Data collected at LEP at centre-of-mass energies \sqrt(s) = 189 - 209 GeV are used to study correlations of the spin of W bosons using e+e- -> W+W- -> lnqq~ events. Spin correlations are favoured by data, and found to agree with the Standard Model predictions. In addition, correlations between the W-boson decay planes are studied in e+e- -> W+W- -> lnqq~ and e+e- -> W+W- -> qq~qq~ events. Decay-plane correlations, consistent with zero and with the Standard Model predictions, are measured.

hep-ex↗

arXiv2005

Search for an Invisibly-Decaying Higgs Boson at LEP

A search for a Higgs boson produced in e^+e^- collisions in association with a Z boson and decaying into invisible particles is performed. Data collected at LEP with the L3 detector at centre-of-mass energies from 189 GeV to 209 GeV are used, corresponding to an integrated luminosity of 0.63/fb. Events with hadrons, electrons or muons with visible masses compatible with a Z boson and missing energy and momentum are selected. They are consistent with the Standard Model expectations. A lower limit of 112.3 GeV is set at 95% confidence level on the mass of the invisibly-decaying Higgs boson in the hypothesis that its production cross section equals that of the Standard Model Higgs boson. Relaxing this hypothesis, upper limits on the production cross section are derived.

hep-ex↗

arXiv2005

A Combination of Preliminary Electroweak Measurements and Constraints on the Standard Model

This note presents a combination of published and preliminary electroweak results from the four LEP collaborations and the SLD collaboration which were prepared for the 2004 summer conferences. [...] The main changes with respect to the experimental results presented in summer 2003 are updates to the W branching fractions and four-fermion cross sections measured at LEP-2, and the SLD/LEP heavy-flavour results measured at the Z pole. The results are compared with precise electroweak measurements from other experiments, notably the final result on the electroweak mixing angle determined in neutrino-nucleon scattering by the NuTeV collaboration, the latest result in atomic parity violation in Caesium, and the measurement of the electroweak mixing angle in Moller scattering. The parameters of the Standard Model are evaluated, first using the combined LEP electroweak measurements, and then using the full set of high-$Q^2$ electroweak results.

hep-ex↗

arXiv2004

Measurement of Exclusive rho^0 rho^0 Production in Mid-Virtuality Two-Photon Interactions at LEP

Exclusive rho^0 rho^0 production in two-photon collisions between a quasi-real and a mid-virtuality photon is studied with data collected at LEP at centre-of-mass energies 183GeV < sqrt{s} < 209GeV with a total integrated luminosity of 684.8/pb. The cross section of the process gamma gamma* -> rho^0 rho^0 is determined as a function of the photon virtuality, q^2, and the two-photon centre-of-mass energy, Wgg, in the kinematic region: 0.2GeV^2 < q^2 < 0.85GeV^2 and 1.1GeV < Wgg < 3GeV.

hep-ex↗

arXiv2004

Inclusive Jet Production in Two-Photon Collisions at LEP

Inclusive jet production, e+e- -> e+e- \ee$ jet X, is studied using 560/pb of data collected at LEP with the L3 detector at centre-of-mass energies between 189 and 209 GeV. The inclusive differential cross section is measured using a k_t jet algorithm as a function of the jet transverse momentum, pt, in the range 3<pt<50 GeV for a pseudorapidity, eta, in the range -1<eta<1. This cross section is well represented by a power law. For high pt, the measured cross section is significantly higher than the NLO QCD predictions, as already observed for inclusive charged and neutral pion production.

hep-ex↗

arXiv2004

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, 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 qqev and qqmv.

hep-ex↗

arXiv2004

Measurement of Exclusive ρ^+ρ^- Production in High-Q^2 Two-Photon Collisions at LEP

Exclusive rho^+ rho^- 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 < \sqrt{s} < 209 GeV with a total integrated luminosity of 854.7 pb^-1. The cross section of the process gamma gamma^* -> rho^+ rho^- is determined as a function of the photon virtuality, Q^2, and the two-photon centre-of-mass energy, W_gg, in the kinematic region: 1.2 GeV^2 < Q^2 < 30 GeV^2 and 1.1 GeV < W_gg < 3 GeV. The ρ^+ρ^- production cross section is found to be of the same magnitude as the cross section of the process gamma gamma^* -> rho^0 rho^0, measured in the same kinematic region by L3, and to have similar W_gg and Q^2 dependences.

hep-ex↗

arXiv2004

Search for Branons at LEP

We search, in the context of extra-dimension scenarios, for the possible existence of brane fluctuations, called branons. Events with a single photon or a single Z-boson and missing energy and momentum collected with the L3 detector in e^+ e^- collisions at centre-of-mass energies sqrt{s}=189-209$ GeV are analysed. No excess over the Standard Model expectations is found and a lower limit at 95% confidence level of 103 GeV is derived for the mass of branons, for a scenario with small brane tensions. Alternatively, under the assumption of a light branon, brane tensions below 180 GeV are excluded.

hep-ex↗

arXiv2004

Study of the e^+e^- --> Z gamma 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.

hep-ex↗

arXiv2004

Studies of Hadronic Event Structure in e+e- Annihilation from 30 GeV to 209 GeV with the L3 Detector

In this Report, QCD results obtained from a study of hadronic event structure in high energy e^+e^- interactions with the L3 detector are presented. The operation of the LEP collider at many different collision energies from 91 GeV to 209 GeV offers a unique opportunity to test QCD by measuring the energy dependence of different observables. The main results concern the measurement of the strong coupling constant, α_s, from hadronic event shapes and the study of effects of soft gluon coherence through charged particle multiplicity and momentum distributions.

hep-ex↗

arXiv2004

Search for Anomalous Couplings in the Higgs Sector at LEP

Anomalous couplings of the Higgs boson are searched for through the processes e^+ e^- -> H gamma, e^+ e^- -> e^+ e^- H and e^+ e^- -> HZ. The mass range 70 GeV < m_H < 190 GeV is explored using 602 pb^-1 of integrated luminosity collected with the L3 detector at LEP at centre-of-mass energies sqrt(s)=189-209 GeV. The Higgs decay channels H -> ffbar, H -> gamma gamma, H -> Zγand H -> WW^(*) are considered and no evidence is found for anomalous Higgs production or decay. Limits on the anomalous couplings d, db, Delta(g1z), Delta(kappa_gamma) and xi^2 are derived as well as limits on the H -> gamma gamma and H -> Z gamma decay rates.

hep-ex↗

arXiv2004

A Combination of Preliminary Electroweak Measurements and Constraints on the Standard Model

This note presents a combination of published and preliminary electroweak results from the four LEP collaborations and the SLD collaboration which were prepared for the 2003 summer conferences. Averages from $\Zzero$ resonance results are derived for hadronic and leptonic cross sections, the leptonic forward-backward asymmetries, the $τ$ polarisation asymmetries, the $\bb$ and $\cc$ partial widths and forward-backward asymmetries and the $\qq$ charge asymmetry. Above the $\Zzero$ resonance, averages are derived for di-fermion cross sections and forward-backward asymmetries, photon-pair, W-pair, Z-pair, single-W and single-Z cross sections, electroweak gauge boson couplings, W mass and width and W decay branching ratios. Also, an investigation of the interference of photon and Z-boson exchange is presented, and colour reconnection and Bose-Einstein correlation analyses in W-pair production are combined. The main changes with respect to the experimental results presented in summer 2002 are updates to the mass of the W boson, four-fermion cross sections and gauge couplings, all measured at LEP-2, and the LEP heavy-flavour results measured at the Z pole. The results are compared with precise electroweak measurements from other experiments, notably the recent final result on the electroweak mixing angle determined in neutrino-nucleon scattering by the NuTeV collaboration and the new result in atomic parity violation in Caesium. The parameters of the Standard Model are evaluated, first using the combined LEP electroweak measurements, and then using the full set of electroweak results.

hep-ex↗

arXiv2004

Muon-Pair and Tau-Pair Production in Two-Photon Collisions at LEP

The QED processes e^+ e^- -> e^+ e^- μ^+ μ^- and e^+ e^- -> e^+ e^- τ^+ τ^- are studied with the L3 detector at LEP using an untagged data sample collected at centre-of-mass energies 161 GeV < sqrt{s} < 209 GeV. The tau-pairs are observed through the associated decay of one tau into eννand the other into ππν. The cross sections are measured as a function of sqrt{s}. For muon pairs, the cross section of the γγ-> μ^+μ^- process is also measured as a function of the two-photon centre-of-mass energy for 3 GeV < W_{γγ} < 40 GeV. Good agreement is found between these measurements and the O(α^4) QED expectations. In addition, limits on the anomalous magnetic and electric dipole moments of the tau lepton are extracted.

hep-ex↗

arXiv2004

Inclusive Lambda Production in Two-Photon Collisions at LEP

The reactions e^+e^- -> e^+e^- Lambda X and e^+e^- -> e^+e^- Lambda X are studied using data collected at LEP with the L3 detector at centre-of-mass energies between 189 and 209 GeV. Inclusive differential cross sections are measured as a function of the lambda transverse momentum, p_t, and pseudo-rapidity, eta, in the ranges 0.4 GeV < p_t < 2.5 GeV and |η| < 1.2. The data are compared to Monte Carlo predictions. The differential cross section as a function of p_t is well described by an exponential of the form A exp (- p_t / )$.

hep-ex↗

arXiv2004

Measurement of Triple-Gauge-Boson Couplings of the W Boson at LEP

The CP-conserving triple-gauge-boson couplings, g1Z, kappa_gamma, lambda_gamma, g5Z, kappa_Z and lambda_Z are measured using hadronic and semi-leptonic W-pair events selected in 629 pb-1 of data collected at LEP with the L3 detector at centre-of-mass energies between 189 and 209 GeV. The results are combined with previous L3 measurements based on data collected at lower centre-of-mass energies and with the results from single-W production and from events with a single-photon and missing energy. Imposing the constraints kappa_Z = g1Z - tan^2(theta_W) (kappa_gamma-1) and lambda_Z = lambda_gamma, we obtain for the C and P conserving couplings the results: g1Z = 0.966 +/- 0.033 (stat.) +/- 0.015 (syst.) kappa_gamma = 1.013 +/-0.066 (stat.) +/- 0.026 (syst.) lambda_gamma = -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 Standard Model expectations and confirm the existence of self-couplings among electroweak gauge bosons.

hep-ex↗

arXiv2004

Flavour Independent Search for Neutral Higgs Bosons at LEP

A flavour independent search for the CP-even and CP-odd neutral Higgs bosons h and A is performed in 624/pb of data collected with the L3 detector at LEP at centre-of-mass energies between 189 and 209GeV. Higgs boson production through the e^+e^- -> Z h and the e^+e^- ->h A processes is considered and decays of the Higgs bosons into hadrons are studied. No significant signal is observed and 95% confidence level limits on the hZZ and hAZ couplings are derived as a function of the Higgs boson masses. Assuming the Standard Model cross section for the Higgs-strahlung process and a 100% branching fraction into hadrons, a 95% confidence level lower limit on the mass of the Higgs boson is set at 110.3GeV.

hep-ex↗