G. Anagnostou

Experimental studies of unbiased gluon jets from e ¿ e ¿ annihilations using the jet boost algorithm

We present the first exptl. results based on the jet boost algorithm, a technique to select unbiased samples of gluon jets in e+e- annihilations, i.e. gluon jets free of biases introduced by event selection or jet finding criteria. Our results are derived from hadronic Z0 decays obsd. with the OPAL detector at the LEP e+e- collider at CERN. First, we test the boost algorithm through studies with HERWIG Monte Carlo events and find that it provides accurate measurements of the charged particle multiplicity distributions of unbiased gluon jets for jet energies larger than about 5 GeV, and of the jet particle energy spectra (fragmentation functions) for jet energies larger than about 14 GeV. Second, we apply the boost algorithm to our data to derive unbiased measurements of the gluon jet multiplicity distribution for energies between about 5 and 18 GeV, and of the gluon jet fragmentation function at 14 and 18 GeV. In conjunction with our earlier results at 40 GeV, we then test QCD calcns. for the energy evolution of the distributions, specifically the mean and first two nontrivial normalized factorial moments of the multiplicity distribution, and the fragmentation function. The theor. results are in global agreement with the data, although the factorial moments are not well described for jet energies below about 14 GeV. [on SciFinder (R)]

Constraints on anomalous quartic gauge boson couplings from ννbar γγ and qqbar γγ events at CERN LEP2

Anomalous quartic couplings between the electroweak gauge bosons may contribute to the nu(nu) over bar gammagamma and q (q) over bar gammagamma final states produced in e(+)e(-) collisions. This analysis uses the LEP2 OPAL data sample at center-of-mass energies up to 209 GeV. Event selections identify nu(nu) over bar gammagamma and q (q) over bar gammagamma events in which the two photons are reconstructed within the detector acceptance. The cross section for the process e(+)e(-)-->q (q) over bar gammagamma is measured. Averaging over all energies, the ratio of the observed e(+)e(-)-->q (q) over bar gammagamma cross section to the standard model expectation is R(data/SM)=0.92+/-0.07+/-0.04, where the errors represent the statistical and systematic uncertainties respectively. The nu(nu) over bar gammagamma and q (q) over bar gammagamma data are used to constrain possible anomalous W(+)W(-)gammagamma and ZZgammagamma couplings. Combining with previous OPAL results from the W(+)W(-)gamma final state, the 95% confidence level limits on the anomalous coupling parameters a(0)(Z), a(c)(Z), a(0)(W) and a(c)(W) are found to be -0.007 GeV-2<a(0)(Z)/Lambda(2)<0.023 GeV-2, -0.029 GeV-2<a(c)(Z)/Lambda(2)<0.029 GeV-2, -0.020 GeV-2<a(0)(W)/Lambda(2)<0.020 GeV-2, -0.052 GeV-2<a(c)(W)/Lambda(2)<0.037 GeV-2, where Lambda is the energy scale of the new physics. Limits found when allowing two or more parameters to vary are also presented.Anomalous quartic couplings between the electroweak gauge bosons may contribute to the νν γγ and qq γγ final states produced in e+e- collisions. This analysis uses the LEP2 OPAL data sample at center-of-mass energies up to 209 GeV. Event selections identify νν γγ and qq γγ events in which the two photons are reconstructed within the detector acceptance. The cross section for the process e+e-→qq γγ is measured. Averaging over all energies, the ratio of the observed e+e-→qq γγ cross section to the standard model expectation is R(data/SM)=0.92±0.07±0.04, where the errors represent the statistical and systematic uncertainties respectively. The νν γγ and qq γγ data are used to constrain possible anomalous W+W-γγ and ZZγγ couplings. Combining with previous OPAL results from the W+W-γ final state, the 95% confidence level limits on the anomalous coupling parameters a0Z, acZ, a0W and acW are found to be -0.007 GeV-2<a0Z/Λ2<0.023 GeV-2, -0.029 GeV-2<acZ/Λ2<0.029 GeV-2, -0.020 GeV-2<a0W/Λ2<0.020 GeV-2, -0.052 GeV-2<acW/Λ2<0.037 GeV-2, where Λ is the energy scale of the new physics. Limits found when allowing two or more parameters to vary are also presented.

Inclusive jet production in photon–photon collisions at View the sqrt(s(ee)) from 189 to 209 GeV

This is the publishers version, also available electronically from http://www.sciencedirect.com/science/article/pii/S0370269307010179. This item is published under a Creative Commons Attribution License.This is the publishers version, also available electronically from http://www.sciencedirect.com/science/article/pii/S0370269307010179. This item is published under a Creative Commons Attribution License.