Determination of the LEP beam energy using radiative fermion-pair events

We present a determination of the LEP beam energy using “radiative return” fermion-pair events recorded at centre-of-mass energies from 183 to 209 GeV. We find no evidence of a disagreement between the OPAL data and the LEP Energy Working Group's standard calibration. Including the energy-averaged 11 MeV uncertainty in the standard determination, the beam energy we obtain from the OPAL data is higher than that obtained from the LEP calibration by View the MathML source0±34(stat.)±27(syst.)MeV

Flavour independent search for Higgs bosons decaying into hadronic final states in e^(+)e^(−)e^(+)e^(−) collisions at LEP

A search for the Higgsstrahlung process e^(+)e^(−)→hZ is described, where the neutral Higgs boson h is assumed to decay into hadronic final states. In order to be sensitive to a broad range of models, the search is performed independent of the flavour content of the Higgs boson decay. The analysis is based on e^(+)e^(−) collision data collected by the OPAL detector at energies between 192 and 209 GeV. The search does not reveal any significant excess over the Standard Model background prediction. Results are combined with previous searches at energies around 91 and at 189 GeV. A limit is set on the product of the cross-section and the hadronic branching ratio of the Higgs boson, as a function of the Higgs boson mass. Assuming the hZ coupling predicted by the Standard Model, and a Higgs boson decaying only into hadronic final states, a lower bound of 104 GeV/c2104 GeV/c^(2) is set on the mass at the 95% confidence level

W boson polarisation at LEP2

This is the publisher's version, also available electronically from http://www.sciencedirect.com/science/article/pii/S0370269304002576

Consistency of data on soft photon production in hadronic interactions

The glob model of Lichard and Van Hove and the modified soft annihilation model (MSAM) of Lichard and Thompson are used as a phenomenological tool for relating results from various experiments on soft photon production in high energy collisions. The total phenomenological expectation is composed of contributions from classical bremsstrahlung, the soft annihilation model and the glob model. The empirical excess above the background from hadronic decays at very small longitudinal momenta of photons is well reproduced, as well as that for transverse momenta pT >~ 10 MeV/c. Some data do not require the glob model and MSAM components in the phenomenological mixture, but do not exclude them. On the basis of consistency of all data with the total theoretical expectation we argue that the results of all experiments are mutually consistent. The models are unable to describe the excess of ultrasoft photons (pT <~ 10 MeV/c), seen by some, but not all, experiments. This may indicate an as yet unknown projectile-mass-dependent production mechanism. Possible relations of soft photon production to other phenomena are discussed. A simple-to-use, but physically equivalent version of the glob model is developed, which enables an easy check of presented results.Comment: 25 pages, RevTeX, epsf.sty, 12 embedded figure

Aspects of four-jet production in polarized proton-proton collisions

We examine the intrinsic spin-dependence of the dominant $gg \rightarrow gggg$ subprocess contribution to four-jet production in polarized proton-proton collisions using helicity amplitude techniques. We find that the partonic level, longitudinal spin-spin asymmetry, $\hat{a}_{LL}$, is intrinsically large in the kinematic regions probed in experiments detecting four isolated jets. Such events may provide another qualitative or semi-quantitative test of the spin-structure of QCD in planned polarized $pp$ collisions at RHIC.Comment: 9 pages, LaTeX, 2 uuencoded postscript files attache

Search for scalar top and scalar bottom quarks at LEP

Searches for a scalar top quark and a scalar bottom quark have been performed using a data sample of 438 pb^(−1) at centre-of-mass energies of sqrt(s) = –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 partners of the left- and right-handed states of the top quark is zero. When the scalar top quark decouples from the Z0 boson, the lower limit is 95.7 GeV. These limits were obtained assuming that the scalar top quark decays into a charm quark and the lightest neutralino, and that the mass difference between the scalar top quark and the lightest neutralino is larger than 10 GeV. The complementary decay mode of the scalar top quark decaying into a bottom quark, a charged lepton and a scalar neutrino has also been studied. The lower limit on the scalar top quark mass is 96.0 GeV for this decay mode, if the mass difference between the scalar top quark and the scalar neutrino is greater than 10 GeV and if the mixing angle of the scalar top quark is zero. From a search for the scalar bottom quark, a mass limit of 96.9 GeV was obtained if the mass difference between the scalar bottom quark and the lightest neutralino is larger than 10 GeV

Bose–Einstein correlations of π^0 pairs from hadronic Z^0 decays

We observe Bose–Einstein correlations in π^0 pairs using back-to-back two jet hadronic events from Z^0 decays in the data sample collected by the OPAL detector at LEP 1 from 1991 to 1995. Using a static Gaussian picture for the pion emitter source, we obtain the chaoticity parameter λ=0.55±0.10±0.10 and the source radius R=(0.59±0.08±0.05) fm. According to the JETSET and HERWIG Monte Carlo models, the Bose–Einstein correlations in our data sample largely connect π^0s originating from the decays of different hadrons. Prompt pions formed at string break-ups or cluster decays only form a small fraction of the sample

Search for new physics in rare B decays

A search for the decay B^±→K^±K^±π^∓ was performed using data collected by the OPAL detector at LEP. These decays are strongly suppressed in the Standard Model but could occur with a higher branching ratio in supersymmetric models, especially in those with R-parity violating couplings. No evidence for a signal was observed and a 90% confidence level upper limit of 1.29×10^(−4) was set for the branching ratio

Experimental studies of unbiased gluon jets from e^+e^− annihilations using the jet boost algorithm

This is the publisher's version, also available electronically from http://journals.aps.org/prd/abstract/10.1103/PhysRevD.69.032002.We present the first experimental 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 Z^0 decays observed 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 calculations 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 theoretical results are found to be in global agreement with the data, although the factorial moments are not well described for jet energies below about 14 GeV

Inclusive production of charged hadrons in photon–photon collisions

The inclusive production of charged hadrons in the collisions of quasi-real photons (e+e−→e+e−+Xe+e−→e+e−+X) has been measured using the OPAL detector at LEP. The data were taken at e+e−e+e− centre-of-mass energies from 183 to 209 GeV209 GeV. The differential cross-sections as a function of the transverse momentum and the pseudorapidity of the hadrons are compared to theoretical calculations of up to next-to-leading order (NLO) in the strong coupling constant αsαs. The data are also compared to a measurement by the L3 Collaboration, in which a large deviation from the NLO predictions is observed