A. Barczyk

Muon-pair and tau-pair production in two-photon collisions at LEP

The QED processes e^+ e^- ->e^+ e^- \mu^+ \mu^- and e^+ e^- ->e^+ e^- \tau^+ \tau^- 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 e\nu\nu and the other into \pi\pi\nu . The cross sections are measured as a function of sqrt{s}. For muon pairs, the cross section of the \gamma\gamma ->\mu^+\mu^- process is also measured as a function of the two-photon centre-of-mass energy for 3 GeV<W_{\gamma\gamma}<40 GeV. Good agreement is found between these measurements and the O(\alpha^4) QED expectations. In addition, limits on the anomalous magnetic and electric dipole moments of the tau lepton are extracted.

Measurement of the Z-boson mass using e+e- →Zγ events at centre-of-mass energies above the Z pole

Events from the e + e − → Zγ process with hard initial-state radiation collected with the L3 detector at centre-of-mass energies between 183 GeV and 209 GeV are used to measure the mass of the Z boson. Decays of the Z boson into hadrons or muon pairs are considered and the Z mass is determined to be 91.272±0.032 (stat.)± 0.033 (syst.) GeV, in agreement with the value measured at the Z resonance. Alternatively , assuming this measured value of the Z mass, the method determines the LEP centre-of-mass energy, found to be 175 ± 68 (stat.) ± 68 (syst.) MeV lower than the nominal value.

Search for colour singlet and colour reconnection effects in hadronic Z decays at LEP

A search is performed in symmetric 3-jet hadronic Z-decay events for evidence of colour singlet production or colour reconnection effects. Asymmetries in the angular separation of particles are found to be sensitive indicators of such effects. Upper limits on the level of colour singlet production or of colour reconnection effects are established for a variety of models.  2003 Published by Elsevier B.V. Open access under CC BY license. 22 L3 Collaboration / Physics Letters B 581 (2004) 19–30

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 |\eta|<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 /<p_t>)

Search for R-parity violating decays of supersymmetric particles in e + e − collisions at √ s = 189 GeV

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