Bose-Einstein Correlations in Multihadron Events at LEP

Bose-Einstein correlations in pairs of identical particles were analyzed in e+ e- multihadron annihilations at ~91.2 GeV at LEP. The first studies involved identical charged pions and the emitting source size was determined. Then the study of charged kaons suggested that the radius depends on the mass of the emitted particles. Subsequenty the dependence of the source radius on the event multiplicity was analyzed. The study of the correlations in neutral pions and neutral kaons extended these concepts to neutral particles. The shape of the source was analyzed in 3 dimensions and was found not to be spherically symmetric. In recent studies at LEP the correlations were analyzed in intervals of the average pair transverse momentum and of the pair rapidity to study the correlations between the pion production points and their momenta (position-momentum correlations). The latest e+ e- data are consistent with an expanding source.Comment: 8 pages, 10 eps figures. Invited paper at the ``Ninth Workshop on Non Perturbative QCD'', Institut d'Astrophysique de Paris, Paris, France, 4-8 June 200

Measurement of Rb in e+e- Collisions at 182 - 209 GeV

Measurements of Rb, the ratio of the bbbar cross-section to the qqbar cross- section in e+e- collisions, are presented. The data were collected by the OPAL experiment at LEP at centre-of-mass energies between 182 GeV and 209 GeV. Lepton, lifetime and event shape information is used to tag events containing b quarks with high efficiency. The data are compatible with the Standard Model expectation. The mean ratio of the eight measurements reported here to the Standard Model prediction is 1.055+-0.031+-0.037, where the first error is statistical and the second systematic.Comment: 21 pages, 5 figures, Submitted to Phys. Letts

Searches for Gauge-Mediated Supersymmetry Breaking Topologies in e+e- collisions at LEP2

In gauge-mediated supersymmetry (SUSY) breaking (GMSB) models the lightest supersymmetric particle (LSP) is the gravitino and the phenomenology is driven by the nature of the next-to-lightest SUSY particle (NLSP) which is either the lightest neutralino, the stau or mass degenerate sleptons. Since the NLSP decay length is effectively unconstrained, searches for all possible lifetime and NLSP topologies predicted by GMSB models in e+e- collisions are performed on the data sample collected by OPAL at centre-of-mass energies up to 209 GeV at LEP. Results independent of the NLSP lifetime are presented for all relevant final states including direct NLSP pair-production and, for the first time, also NLSP production via cascade decays of heavier SUSY particles. None of the searches shows evidence for SUSY particle production. Cross-section limits are presented at the 95% confidence level both for direct NLSP production and for cascade decays, providing the most general, almost model independent results. These results are then interpreted in the framework of the minimal GMSB (mGMSB) model, where large areas of the accessible parameter space are excluded. In the mGMSB model, the NLSP masses are constrained to be larger than 53.5 GeV/c^2, 87.4 GeV/c^2 and 91.9 GeV/c^2 in the neutralino, stau and slepton co-NLSP scenarios, respectively. A complete scan on the parameters of the mGMSB model is performed, constraining the universal SUSY mass scale Lambda from the direct SUSY particle searches: Lambda > 40, 27, 21, 17, 15 TeV/c^2 for messenger indices N=1, 2, 3, 4, 5 respectively, for all NLSP lifetimes.Comment: 4 pages, 2 figures. To appear in Proceedings of SUSY06, the 14th International Conference on Supersymmetry and the Unification of Fundamental Interactions, UC Irvine, California, 12-17 June 200

Bose Einstein correlations of neutral pion pairs at LEP

With the OPAL detector at LEP we measured at energies around the Z0 peak the Bose-Einstein Correlations (BECs) of neutral pion pairs. We compare the results of this measurement with former results obtained at LEP for hadrons including those obtained from Fermi-Dirac Correlations (FDCs).Comment: 4 pages, PDF format. Talk given by G. Giacomelli at DPF 2003, Philadelphia, USA, 5-8 April 200

Scaling violations of quark and gluon jet fragmentation functions in e+e- annihilations

Flavour inclusive, udsc and b fragmentation functions in biased and unbiased jets and gluon fragmentation functions in biased jets are measured in e+e- annihilations using OPAL data collected at energies Ecms = 91.2 and 183-209 GeV.Comment: 5 pages, 4 figures. To appear in Proceedings of Workshop on Deep Inelastic Scattering 2004 (DIS04), Strbske Pleso, Slovakia, 14-18.04.200

OPAL and More

We shall summarize some of the research activities performed in collaboration with Ben Shen in the OPAL experiment at LEP and in the CMS experiment at the LHC. And we shall recall the LEP legacy to particle physics in general and to the Standard Model in particular. Short recollections are made in other fields in which Ben was interested, in particular in Astroparticle Physics.Comment: Talk given at the Benjamin C. Shen Memorial Symposium, UCR, March 8, 2008. Changed reference

Review of Final LEP Results or A Tribute to LEP

After a comment on the performance of LEP some highlights of the LEP1 and LEP2 physics programmes are reviewed. The talk concentrates on the precision measurements at the Z resonance, two fermion production above the Z, W+W- production, ZZ production, indirect limits on the Higgs mass, LEP contributions to the exploration of the CKM matrix, and on the LEP measurements of alpha_s.Comment: Proceedings of the XX International Symposium on Lepton and Photon Interactions at High Energies Rome, Italy, July 200

Running of the QED coupling in small-angle Bhabha scattering at LEP

Using the OPAL detector at LEP, the running of the effective QED coupling alpha(t) is measured for space-like momentum transfer, 2 \leq -t \leq 6 GeV^2, from the angular distribution of small-angle Bhabha scattering. This is currently the most significant direct observation of the running of the QED coupling in a single experiment and the first clear evidence of the hadronic contribution to the running in the space-like region. Our result is in good agreement with standard evaluations of alpha(t), based on data in the time-like region.Comment: Talks given at the 19th Rencontres De Physique De La Vallee D'Aoste (27.2-5.3.05), La Thuile, Italy and at the 40th Rencontres De Moriond On Electroweak Interactions And Unified Theories (5.3-12.3.05), La Thuile, Italy. Added 1 figure and related text wrt vers.