Giovanni Abbiendi

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Cross-sections and angular distributions for hadronic and lepton pair final states in e+e− collisions at a centre-of-mass energy near 189 GeV, measured with the OPAL detector at LEP, are presented and compared with the predictions of the Standard Model. The results are used to measure the energy dependence of the electromagnetic coupling constant αem, and(More)
QCD coherence effects are studied based on measurements of correlations of particles with either restricted transverse momenta, p T < p cut T , where p T is defined with respect to the thrust axis, or restricted absolute momenta, p ≡ |p| < p cut , using about four million hadronic Z decays recorded at LEP with the OPAL detector. The correlations are(More)
The measurement of small–angle Bhabha scattering is used to determine the luminosity at the OPAL interaction point for the LEP I data recorded between 1993 and 1995. The measurement is based on the OPAL Silicon-Tungsten Luminometer which is composed of two calorimeters encircling the LEP beam pipe, on opposite sides of the interaction point. The luminometer(More)
Searches for a scalar top quark and a scalar bottom quark have been performed using a data sample of 438 pb at centre-of-mass energies of √ s =192 – 209 GeV collected with the OPAL detector at LEP. No evidence for a signal was found. The 95% confidence level lower limit on the scalar top quark mass is 97.6 GeV if the mixing angle between the supersymmetric(More)
A search for pair-produced doubly charged Higgs bosons has been performed using data samples corresponding to an integrated luminosity of about 614 pb −1 collected with the OPAL detector at LEP at centre-of-mass energies between 189 GeV and 209 GeV. No evidence for a signal has been observed. A mass limit of 98.5 GeV/c 2 at the 95% confidence level has been(More)
Photonic events with large missing energy have been observed in e + e − collisions at centre-of-mass energies of 130, 136 and 183 GeV collected in 1997 using the OPAL detector at LEP. Results are presented for event topologies with a single photon and missing transverse energy or with an acoplanar photon pair. Cross-section measurements are performed within(More)
A search for pair produced scalar fermions with couplings that violate R-parity has been performed using a data sample corresponding to an integrated luminosity of 56 pb at a centre-of-mass energy of √ s = 183 GeV collected with the OPAL detector at LEP. An important consequence of R-parity breaking interactions is that the lightest supersymmetric particle(More)
A selection of di-lepton events with significant missing transverse momentum has been performed using a total data sample of 237.4 pb −1 at e + e − centre-of-mass energies of 183 GeV and 189 GeV. The observed numbers of events — 78 at 183 GeV and 301 at 189 GeV — are consistent with the numbers expected from Standard Model processes, which arise(More)
Bose-Einstein correlations in pairs of identical charged pions produced in a sample of 4.3 million Z 0 hadronic decays are studied as a function of the three components of the momentum difference, transverse (" out " and " side ") and longitudinal with respect to the thrust direction of the event. A significant difference between the transverse, r t side ,(More)