C. Ciocca

.Approximately 438 pb-1 of e + e- data from the OPAL detector, taken with the LEP collider running at centre-of-mass energies of 192-209 GeV, are analyzed to search for evidence of chargino pair production, \({\mathrm{e}}^ + {\mathrm{e}}^-\to\tilde\chi^ + _1\tilde\chi^-_1\), or neutralino associated production, \({\mathrm{e}}^ + {\mathrm{e}}^-\to\tilde\chi^0_2\tilde\chi^0_1\). Limits are set at the 95% confidence level on the product of the cross-section for the process \({\mathrm{e}}^ + {\mathrm{e}}^-\to\tilde\chi^ + _1\tilde\chi^-_1\) and its branching ratios to topologies containing jets and missing energy, or jets with a lepton and missing energy, and on the product of the cross-section for \({\mathrm{e}}^ + {\mathrm{e}}^-\to\tilde\chi^0_2\tilde\chi^0_1\) and its branching ratio to jets. R-parity conservation is assumed throughout this paper. When these results are interpreted in the context of the Constrained Minimal Supersymmetric Standard Model, limits are also set on the masses of the \(\tilde\chi^{\pm}_1, \tilde\chi^0_1\) and \(\tilde\chi^0_2\), and regions of the parameter space of the model are ruled out. Nearly model-independent limits are also set at the 95% confidence level on \(\sigma({\mathrm{e}}^ + {\mathrm{e}}^-\to\tilde\chi^ + _1\tilde\chi^-_1)\) with the assumption that each chargino decays via a W boson, and on \(\sigma({\mathrm{e}}^ + {\mathrm{e}}^-\to\tilde\chi^0_2\tilde\chi^0_1)\) with the \(\tilde\chi^0_2\) assumed to decay via a Z0.

Photonic events with large missing energy have been observed in $\rm e^+e^-$ collisions at a centre-of-mass energy of 189 GeV using the OPAL detector at LEP. Results are presented for event topologies consistent with a single photon or with an acoplanar photon pair. Cross-section measurements are performed within the kinematic acceptance of each selection, and the number of light neutrino species is measured. Cross-section results are compared with the expectations from the Standard Model process $\mathrme^+\mathrme^-\to \nu\overline{\nu}$ + photon(s). No evidence is observed for new physics contributions to these final states. Upper limits on $\sigma(\mathrme^+\mathrme^-\to\mathrm{X}\mathrm{Y})\cdot\mathrm{BR}(\mathrm{X}\to\mathrm{Y}\gamma)$ and $\sigma(\mathrme^+\mathrme^-\to\mathrm{XX})\cdot\mathrm{BR}^2(\mathrm{X}\to\mathrm{Y}\gamma)$ are derived for the case of stable and invisible $\mathrm{Y}$ . These limits apply to single and pair production of excited neutrinos ( $\mathrm{X} = \nu^*, \mathrm{Y} = \nu$ ), to neutralino production ( $\mathrm{X}={{{\tilde{\chi}}^{0}}_{2}}, \mathrm{Y}={{{\tilde{\chi}}^{0}}_{1}}$ ) and to supersymmetric models in which $\mathrm{X} ={{{\tilde{\chi}}^{0}}_{1}}$ and $\mathrm{Y}={\tilde{\mathrm{G}}}$ is a light gravitino. The case of macroscopic decay lengths of particle X is considered for $\mathrme^+\mathrme^- \to \mathrm{XX}$ , $\rm X \to Y \gamma$ , when $M_{\mathrm Y}\approx 0$ . The single-photon results are also used to place upper limits on superlight gravitino pair production as well as graviton-photon production in the context of theories with additional space dimensions.

The mass and width of the W boson are measured using e+e– → W+W– events from the data sample collected by the OPAL experiment at LEP at centre-of-mass energies between 170 GeV and 209 GeV. The mass (m W) and width (Γ W) are determined using direct reconstruction of the kinematics of W+W– → ${\text{q\ifmmode\expandafter\bar\else\expandafter\=\fi{q}}}{\ell }\nu$ and W+W– → ${\text{q\ifmmode\expandafter\bar\else\expandafter\=\fi{q}q\ifmmode\expandafter\bar\else\expandafter\=\fi{q}}}$ events. When combined with previous OPAL measurements using W+W– → $ {\ell }\nu {\ell }\nu $ events and the dependence on of the WW production cross-section at threshold, the results are determined to be $ \begin{array}{*{20}c} {{m_{{\text{W}}} = 80.415 \pm 0.042 \pm 0.030 \pm 0.009\;{\text{GeV}}}} \\ {{\Gamma _{{\text{W}}} = 1.996 \pm 0.096 \pm 0.102 \pm 0.003\;{\text{GeV}}}} \\ \end{array} $ where the first error is statistical, the second systematic and the third due to uncertainties in the value of the LEP beam energy. By measuring in the channel using several different determinations of the direction of jets with differing sensitivities to soft particles, a limit is also obtained on possible final-state interactions due to colour reconnection effects inW+W– → ${\text{q\ifmmode\expandafter\bar\else\expandafter\=\fi{q}q\ifmmode\expandafter\bar\else\expandafter\=\fi{q}}}$ events. The consistency of the results for the W mass and width with those inferred from other electroweak parameters provides an important test of the Standard Model of electroweak interactions.

Hadronic event shape distributions from e+e- annihilation measured by the OPAL experiment at centre-of-mass energies between 91 GeV and 209 GeV are used to determine the strong coupling {\alpha}S. The results are based on QCD predictions complete to the next-to- next-to-leading order (NNLO), and on NNLO calculations matched to the resummed next-to-leading-log-approximation terms (NNLO+NLLA). The combined NNLO result from all variables and centre-of-mass energies is {\alpha}S(mZ0) = 0.1201 {\pm} 0.0008(stat.) {\pm} 0.0013(exp.) {\pm} 0.0010(had.) {\pm} 0.0024(theo.). while the combined NNLO+NLLA result is {\alpha}S(mZ0) = 0.1189 {\pm} 0.0008(stat.) {\pm} 0.0016(exp.) {\pm} 0.0010(had.) {\pm} 0.0036(theo.). The completeness of the NNLO and NNLO+NLLA results with respect to missing higher order contributions, studied by varying the renormalization scale, is improved compared to previous results based on NLO or NLO+NLLA predictions only. The observed energy dependence of {\alpha}S agrees with the QCD prediction of asymptotic freedom and excludes the absence of running.

Anomalous quartic couplings between the electroweak gauge bosons may contribute to the $\ensuremath{\nu}\overline{\ensuremath{\nu}}\ensuremath{\gamma}\ensuremath{\gamma}$ and $q\overline{q}\ensuremath{\gamma}\ensuremath{\gamma}$ final states produced in ${e}^{+}{e}^{\ensuremath{-}}$ collisions. This analysis uses the LEP2 OPAL data sample at center-of-mass energies up to 209 GeV. Event selections identify $\ensuremath{\nu}\overline{\ensuremath{\nu}}\ensuremath{\gamma}\ensuremath{\gamma}$ and $q\overline{q}\ensuremath{\gamma}\ensuremath{\gamma}$ events in which the two photons are reconstructed within the detector acceptance. The cross section for the process ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}q\overline{q}\ensuremath{\gamma}\ensuremath{\gamma}$ is measured. Averaging over all energies, the ratio of the observed ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}q\overline{q}\ensuremath{\gamma}\ensuremath{\gamma}$ cross section to the standard model expectation is $\mathrm{R}(\mathrm{data}/\mathrm{SM})=0.92\ifmmode\pm\else\textpm\fi{}0.07\ifmmode\pm\else\textpm\fi{}0.04,$ where the errors represent the statistical and systematic uncertainties respectively. The $\ensuremath{\nu}\overline{\ensuremath{\nu}}\ensuremath{\gamma}\ensuremath{\gamma}$ and $q\overline{q}\ensuremath{\gamma}\ensuremath{\gamma}$ data are used to constrain possible anomalous ${W}^{+}{W}^{\ensuremath{-}}\ensuremath{\gamma}\ensuremath{\gamma}$ and $\mathrm{ZZ}\ensuremath{\gamma}\ensuremath{\gamma}$ couplings. Combining with previous OPAL results from the ${W}^{+}{W}^{\ensuremath{-}}\ensuremath{\gamma}$ final state, the 95% confidence level limits on the anomalous coupling parameters ${a}_{0}^{\mathrm{Z}},$ ${a}_{\mathrm{c}}^{\mathrm{Z}},$ ${a}_{0}^{\mathrm{W}}$ and ${a}_{\mathrm{c}}^{\mathrm{W}}$ are found to be $\ensuremath{-}0.007 {\mathrm{GeV}}^{\ensuremath{-}2}<{a}_{0}^{\mathrm{Z}}/{\ensuremath{\Lambda}}^{2}<0.023 {\mathrm{GeV}}^{\ensuremath{-}2},$ $\ensuremath{-}0.029 {\mathrm{GeV}}^{\ensuremath{-}2}<{a}_{\mathrm{c}}^{\mathrm{Z}}/{\ensuremath{\Lambda}}^{2}<0.029 {\mathrm{GeV}}^{\ensuremath{-}2},$ $\ensuremath{-}0.020 {\mathrm{GeV}}^{\ensuremath{-}2}<{a}_{0}^{\mathrm{W}}/{\ensuremath{\Lambda}}^{2}<0.020 {\mathrm{GeV}}^{\ensuremath{-}2},$ $\ensuremath{-}0.052 {\mathrm{GeV}}^{\ensuremath{-}2}<{a}_{\mathrm{c}}^{\mathrm{W}}/{\ensuremath{\Lambda}}^{2}<0.037 {\mathrm{GeV}}^{\ensuremath{-}2},$ where $\ensuremath{\Lambda}$ is the energy scale of the new physics. Limits found when allowing two or more parameters to vary are also presented.

.Using the OPAL detector at LEP, the running of the effective QED coupling α(t) is measured for space-like momentum transfer from the angular distribution of small-angle Bhabha scattering. In an almost ideal QED framework, with very favourable experimental conditions, we obtain: \( \Delta \alpha {\left( { - 6.07\;{\text{GeV}}^{2} } \right)} - \Delta \alpha {\left( { - 1.81\;{\text{GeV}}^{2} } \right)} = {\left( {440 \pm 58 \pm 43 \pm 30} \right)} \times 10^{{ - 5}} ,\) where the first error is statistical, the second is the experimental systematic and the third is the theoretical uncertainty. This agrees with current evaluations of α(t). The null hypothesis that α remains constant within the above interval of -t is excluded with a significance above 5σ. Similarly, our results are inconsistent at the level of 3σ with the hypothesis that only leptonic loops contribute to the running. This is currently the most significant direct measurement where the running α(t) is probed differentially within the measured t range.

Searches for a scalar top quark and a scalar bottom quark have been performed using a data sample of 182 pb−1 at a centre-of-mass energy of s=189 GeV collected with the OPAL detector at LEP. No evidence for a signal was found. The 95% confidence level (C.L.) lower limit on the scalar top quark mass is 90.3 GeV if the mixing angle between the supersymmetric partners of the left- and right-handed states of the top quark is zero. In the worst case, when the scalar top quark decouples from the Z0 boson, the lower limit is 87.2 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. From a search for the scalar bottom quark, a mass limit of 88.6 GeV was obtained if the mass difference between the scalar bottom quark and the lightest neutralino is larger than 7 GeV. These limits significantly improve the previous OPAL limits.

We measure the mass of the τ to be 1775.1±1.6(mcnstat.)±1.0(mcnsys.) MeV using τ from Z0 decays. To test CPT invariance we compare the masses of the positively and negatively charged τ. The relative mass difference is found to be smaller than 3.0×10−3 at the 90% confidence level.

The magnitude of the Cabibbo-Kobayashi-Maskawa matrix element Vcb has been measured using B̄0→D∗+ℓ−ν̄ decays recorded on the Z0 peak using the OPAL detector at LEP. The D∗+→D0π+ decays were reconstructed both in the particular decay modes D0→K−π+ and D0→K−π+π0 and via an inclusive technique. The product of |Vcb| and the decay form factor of the B̄0→D∗+ℓ−ν̄ transition at zero recoil F(1) was measured to be F(1)|Vcb|=(37.1±1.0±2.0)×10−3, where the uncertainties are statistical and systematic respectively. By using Heavy Quark Effective Theory calculations for F(1), a value of|Vcb|=(40.7±1.1±2.2±1.6)×10−3was obtained, where the third error is due to theoretical uncertainties in the value of F(1). The branching ratio Br(B̄0→D∗+ℓ−ν̄) was also measured to be (5.26±0.20±0.46)%.

Searches for the neutral Higgs bosons predicted by the Standard Model (SM) and the Minimal Supersymmetric extension of the Standard Model (MSSM) have been performed with the OPAL detector at LEP. Approximately 170 pb $^{-1}$ of $\mathrm{e}^+ \mathrm{e}^-$ collision data collected at $\sqrt{s} \approx 189$ GeV were used to search for Higgs boson production in the SM process ${\mathrm{e}}^+{\mathrm{e}}^- \rightarrow \mathrm{H}^{0}\mathrm{Z}^{0}$ and the MSSM processes ${\mathrm{e}}^+{\mathrm{e}}^- \rightarrow \mathrm{h}^{0} \mathrm{Z}^{0}$ and ${\mathrm{e}}^+ {\mathrm{e}}^- \rightarrow \mathrm{A}^{0} \mathrm{h}^{0}$ . The searches are sensitive to the $\mathrm{b}\bar{\mathrm{b}}$ and $\tau^+\tau^-$ decay modes of the Higgs bosons, and also to the MSSM decay mode $\mathrm{h}^{0} \rightarrow \mathrm{A}^{0} \mathrm{A}^{0}$ . OPAL search results at lower centre-of-mass energies have been incorporated in the limits, which are valid at the 95% confidence level. For the SM Higgs boson, a lower mass bound of 91.0 GeV is obtained. In the MSSM, the limits are $m_{\mathrm{H}} >74.8$ GeV and $m_{\mathrm{A}} >76.5$ GeV, assuming $\tan\beta > 1$ , that the mixing of the scalar top quarks is either zero or maximal, and that the soft SUSY-breaking masses are 1 TeV. For the case of zero scalar top mixing, the values of $\tan\beta$ between 0.72 and 2.19 are excluded.

This Letter describes a direct search for pair produced magnetic monopoles in e+e− collisions. The analysis is based on 62.7 pb−1 of data collected with the OPAL detector at an average centre-of-mass energy of s=206.3GeV. The monopole signal was assumed to be characterized by two back-to-back particles with an anomalously high ionization energy loss dE/dx in the tracking chambers. No evidence for production of monopoles was observed. Upper limits were obtained on the magnetic monopole pair-production cross-section (σ) in the mass range 45GeV/c2<mM<102GeV/c2. The average limit is σ<0.05pb and is essentially independent of the magnetic monopole mass. The cross-section limit is derived at the 95% confidence level and is valid for spin-1/2 magnetic monopoles.

Abstract A search is made for charged Higgs bosons predicted by Two-Higgs-Doublet extensions of the Standard Model (2HDM) using electron-positron collision data collected by the OPAL experiment at $\sqrt{s}=189\mbox{--}209\ \mbox{GeV}$ , corresponding to an integrated luminosity of approximately 600 pb −1 . Charged Higgs bosons are assumed to be pair-produced and to decay into $\mathrm{q} \bar{\mathrm{q}}$ , τν τ or AW ± . No signal is observed. Model-independent limits on the charged Higgs-boson production cross section are derived by combining these results with previous searches at lower energies. Under the assumption $\mathrm{BR} (\mathrm{H}^{\pm} \to \tau\nu_{\tau}) + \mathrm{BR} (\mathrm{H}^{\pm} \to \mathrm{q} \bar{\mathrm{q}}) = 1$ , motivated by general 2HDM type II models, excluded areas on the $[m_{\mathrm{H}^{\pm}} , \mathrm{BR} (\mathrm {H}^{\pm} \to \tau\nu_{\tau})]$ plane are presented and charged Higgs bosons are excluded up to a mass of 76.3 GeV at 95 % confidence level, independent of the branching ratio BR(H ± → τν τ ). A scan of the 2HDM type I model parameter space is performed and limits on the Higgs-boson masses $m_{\mathrm{H}^{\pm}}$ and m A are presented for different choices of tan β .

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 $_{t_{side}}$ , and longitudinal, r $_l$ , dimensions is observed, indicating that the emitting source of identical pions, as observed in the Longitudinally CoMoving System, has an elongated shape. This is observed with a variety of selection techniques. Specifically, the values of the parameters obtained by fitting the extended Goldhaber parametrisation to the correlation function ${\mathrm C'} ={\mathrm C^{DATA}}/{\mathrm C^{MC}}$ for two-jet events, selected with the Durham algorithm and resolution parameter y $_{cut}$ = 0.04, are r $_{t_{side}}$ = (0.809 $\pm$ 0.009 (stat) $^{+0.019}_{-0.032}$ (syst)) fm, r $_l$ = (0.989 $\pm$ 0.011 (stat) $^{+0.030}_{-0.015}$ ({\it syst})) fm and r $_l$ /r $_{t_{side}}$ = 1.222 $\pm$ 0.027 (stat) $^{+0.075}_{-0.012}$ (syst). The results are discussed in the context of a recent model of Bose-Einstein correlations based on string fragmentation. The results of a unidimensional analysis are also presented.

Flavour inclusive, udsc and b fragmentation functions in unbiased jets, and flavour inclusive, udsc, b and gluon fragmentation functions in biased jets are measured in e+e- annihilations from data collected at centre-of-mass energies of 91.2, and 183-209 GeV with the OPAL detector at LEP. The unbiased jets are defined by hemispheres of inclusive hadronic events, while the biased jet measurements are based on three-jet events selected with jet algorithms. Several methods are employed to extract the fragmentation functions over a wide range of scales. Possible biases are studied in the results are obtained. The fragmentation functions are compared to results from lower energy e+e- experiments and with earlier LEP measurements and are found to be consistent. Scaling violations are observed and are found to be stronger for the fragmentation functions of gluon jets than for those of quarks. The measured fragmentation functions are compared to three recent theoretical next-to-leading order calculations and to the predictions of three Monte Carlo event generators. While the Monte Carlo models are in good agreement with the data, the theoretical predictions fail to describe the full set of results, in particular the b and gluon jet measurements.

A search is performed for Higgs bosons decaying into invisible final states, produced in association with a Z0 boson in e+e− collisions at energies between 183 and 209 GeV. The search is based on data samples collected by the OPAL detector at LEP corresponding to an integrated luminosity of about 660 pb−1. The analysis aims to select events containing the hadronic decay products of the Z0 boson and large missing momentum, as expected from Higgs boson decay into a pair of stable weakly interacting neutral particles, such as the lightest neutralino in the Minimal Supersymmetric Standard Model. The same analysis is applied to a search for nearly invisible Higgs boson cascade decays into stable weakly interacting neutral particles. No excess over the expected background from Standard Model processes is observed. Limits on the production of invisibly decaying Higgs bosons produced in association with a Z0 boson are derived. Assuming a branching ratio BR(h0→invisible)=1, a lower limit of 108.2 GeV is placed on the Higgs boson mass at the 95% confidence level. Limits on the production of nearly invisibly decaying Higgs bosons are also obtained.

A study of W+W− events accompanied by hard photon radiation produced in e+e− collisions at LEP is presented. Events consistent with two on-shell W-bosons and an isolated photon are selected from 183 pb−1 of data recorded at s=189 GeV. From these data, 17 W+W−γ candidates are selected with photon energy greater than 10 GeV, consistent with the Standard Model expectation. These events are used to measure the e+e−→W+W−γ cross-section within a set of geometric and kinematic cuts, σ̂WWγ=136±37±8 fb, where the first error is statistical and the second systematic. The photon energy spectrum is used to set the first direct, albeit weak, limits on possible anomalous contributions to the W+W−γγ and W+W−γZ0 vertices:−0.070GeV−2<a0/Λ2<0.070GeV−2,−0.13GeV−2<ac/Λ2<0.19GeV−2,−0.61GeV−2<an/Λ2<0.57GeV−2,where Λ represents the energy scale for new physics.

In the Standard Model, b quarks produced in e+e− annihilation at the Z0 peak have a large average longitudinal polarization of −0.94. Some fraction of this polarization is expected to be transferred to b-flavored baryons during hadronization. The average longitudinal polarization of weakly decaying b baryons, 〈PLΛb〉, is measured in approximately 4.3 million hadronic Z0 decays collected with the OPAL detector between 1990 and 1995 at LEP. Those b baryons that decay semileptonically and produce a Λ baryon are identified through the correlation of the baryon number of the Λ and the electric charge of the lepton. In this semileptonic decay, the ratio of the neutrino energy to the lepton energy is a sensitive polarization observable. The neutrino energy is estimated using missing energy measurements. From a fit to the distribution of this ratio, the value 〈PLΛb〉=−0.56+0.20−0.13±0.09 is obtained, where the first error is statistical and the second systematic.

We compared the multiplicities of $\pi^0, \eta, \rm K^0 $ and of charged particles in quark and gluon jets in 3-jet events, as measured by the OPAL experiment at LEP. The comparisons were performed for distributions unfolded to 100% pure quark and gluon jets, at an effective scale $\rm Q_{jet} $ which took into account topological dependences of the 3-jet environment. The ratio of particle multiplicity in gluon jets to that in quark jets as a function of $\rm Q_{jet}$ for $ \pi^0, \eta$ or $\rm K^0 $ was found to be independent of the particle species. This is consistent with the QCD prediction that the observed enhancement in the mean particle rate in gluon jets with respect to quark jets should be independent of particle species. In contrast to some theoretical predictions and previous observations, we observed no evidence for an enhancement of $ \eta $ meson production in gluon jets with respect to quark jets, beyond that observed for charged particles. We measured the ratio of the slope of the average charged particle multiplicity in gluon jets to that in quark jets, C, and we compared it to a next-to-next-to-next-to leading order calculation. Our result, $\rm C=2.27\pm 0.20(stat.+syst.),$ is about one standard deviation higher than the perturbative prediction.

Elements of the spin density matrix for W bosons in e+e−→W+W−→qq̄′ℓνℓ events are measured from data recorded by the OPAL detector at LEP. This information is used to calculate polarised differential cross-sections and to search for CP-violating effects. Results are presented for W bosons produced in e+e− Collisions with centre-of-mass energies between 183 GeV and 209 GeV. The average fraction of W bosons that are longitudinally polarised is found to be (23.9±2.1±1.1)% compared to a Standard Model prediction of (23.9±0.1)%. All results are consistent with CP conservation.

The helicity density matrix elements $\rho_{00}$ of $\rho(770)^{\pm}$ and $\omega(782)$ mesons produced in Z $^0$ decays have been measured using the OPAL detector at LEP. Over the measured meson energy range, the values are compatible with 1/3, corresponding to a statistical mix of helicity $-1$ , 0 and +1 states. For the highest accessible scaled energy range 0.3 $<$ $x_E$ $<$ 0.6, the measured $\rho_{00}$ values of the $\rho^{\pm}$ and the $\omega$ are 0.373 $\pm$ 0.052 and 0.142 $\pm$ 0.114, respectively. These results are compared to measurements of other vector mesons.

We present the first experimental results based on the jet boost algorithm, a technique to select unbiased samples of gluon jets in ${e}^{+}{e}^{\ensuremath{-}}$ 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 ${e}^{+}{e}^{\ensuremath{-}}$ 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.

The effects of the final state interaction phenomenon known as colour reconnection are investigated at centre-of-mass energies in the range $\sqrt s$ ≃ 189–209 using the OPAL detector at LEP. Colour reconnection is expected to affect observables based on charged particles in hadronic decays of . Measurements of inclusive charged particle multiplicities, and of their angular distribution with respect to the four jet axes of the events, are used to test models of colour reconnection. The data are found to exclude extreme scenarios of the Sjöstrand-Khoze Type I () model and are compatible with other models, both with and without colour reconnection effects. In the context of the model, the best agreement with data is obtained for a reconnection probability of 37%. Assuming no colour reconnection, the charged particle multiplicity in hadronically decaying W bosons is measured to be ${\left\langle {n^{{{\text{qq}}}}_{{{\text{ch}}}} } \right\rangle }$ =19.38±0.05(stat.)±0.08(syst.).

From a total data sample of 701.1 pb-1 recorded with e+e- centre-of-mass energies of $\sqrt{s} =$ 161–209 GeV with the OPAL detector at LEP, 11693 W-pair candidate events are selected. These data are used to obtain measurements of the W-pair production cross sections at 10 different centre-of-mass energies. The ratio of the measured cross sections to the standard model expectation is found to be: ${\text{data}}/{{\text{SM}}} = 1.002\pm0.011 ({\text{stat.}}) \pm0.007 ({\text{syst.}}) \pm0.005 ({\text{theory}})$ , where the uncertainties are statistical, experimental systematics and theory systematics respectively. The data are used to determine the W boson branching fractions, which are found to be consistent with lepton universality of the charged current interaction. Assuming lepton universality, the branching ratio to hadrons is determined to be 67.41±0.37(stat.)±0.23(syst.)%, from which the CKM matrix element |Vcs| is determined to be 0.969±0.017(stat.)±0.012(syst.). The differential cross section as a function of the W- production angle is measured for the qqeν and qqμν final states. The results described in this paper are consistent with the expectations from the standard model.

Hadronic final states with a hard isolated photon are studied using data taken at centre-of-mass energies around the mass of the Z boson with the OPAL detector at LEP. The strong coupling αs is extracted by comparing data and QCD predictions for event shape observables at average reduced centre-of-mass energies ranging from 24 GeV to 78 GeV, and the energy dependence of αs is studied. Our results are consistent with the running of αs as predicted by QCD and show that within the uncertainties of our analysis event shapes in hadronic Z decays with hard and isolated photon radiation can be described by QCD at reduced centre-of-mass energies. Combining all values from different event shape observables and energies gives αs(MZ)=0.1182±0.0015(stat.)±0.0101(syst.).

A study of Bose–Einstein correlations in pairs of identically charged pions produced in e+e- annihilations at the Z0 peak has been performed for the first time assuming a non-static emitting source. The results are based on the high statistics data obtained with the OPAL detector at LEP. The correlation functions have been analyzed in intervals of the average pair transverse momentum and of the pair rapidity, in order to study possible correlations between the pion production points and their momenta (position–momentum correlations). The Yano–Koonin and the Bertsch–Pratt parameterizations have been fitted to the measured correlation functions to estimate the geometrical parameters of the source as well as the velocity of the source elements with respect to the overall centre-of-mass frame. The source rapidity is found to scale approximately with the pair rapidity, and both the longitudinal and transverse source dimensions are found to decrease for increasing average pair transverse momenta.

Using a data sample of 57 pb−1 recorded at a centre-of-mass energy of 183 GeV with the OPAL detector at LEP, 282 W+W−→qqqq and 300 W+W−→qqℓνℓ candidate events are used to obtain a measurement of the mass of the W boson, MW=80.39±0.13(stat.)±0.05(syst.) GeV, assuming the Standard Model relation between MW and ΓW. A second fit provides a direct measure of the width of the W boson and gives ΓW=1.96±0.34(stat.)±0.20(syst.) GeV. These results are combined with previous OPAL results to obtain MW=80.38±0.12(stat.)±0.05(syst.) GeV and ΓW=1.84±0.32(stat.)±0.20(syst.) GeV.

We present an observation at LEP of the production of χc2 mesons in the collisions of two quasi-real photons using the OPAL detector. The χc2 mesons are reconstructed in the decay channel χc2→J/ψγ→ℓ+ℓ−γ (with ℓ = e,μ) using all data taken at e+e− centre-of-mass energies of 91 and 183 GeV, corresponding to integrated luminosities of 167 and 55 pb−1 respectively. The two-photon width of the χc2 is determined to be Γ(χc2→γγ)=1.76±0.47±0.37±0.15 keV, where the first error is statistical, the second is systematic and the third comes from branching ratio uncertainties.

Proceedings of ISMD08

The branching ratio for the decay τ−→e−ν̄eντ has been measured using Z0 decay data collected by the OPAL experiment at LEP. In total 33073 τ−→e−ν̄eντ candidates were identified from a sample of 186197 selected τ decays, giving a branching ratio of B(τ−→e−ν̄eντ)=(17.81±0.09(stat)±0.06(syst))%. This result is combined with other measurements to test e - μ and μ - τ universality in charged-current weak interactions. Additionally, the strong coupling constant αs(mτ2) has been extracted from B(τ−→e−ν̄eντ) and evolved to the Z0 mass scale, giving αs(mZ2)=0.1204±0.0011 (exp)±0.0019 (theory).

A search for the resonant production of high mass photon pairs associated with a leptonic or hadronic system has been performed using a data sample of 57.7 pb−1 collected at an average center-of-mass energy of 182.6 GeV with the OPAL detector at LEP. No evidence for contributions from non-Standard Model physics processes was observed. The observed candidates are used to place limits on B(H0→γγ) assuming a Standard Model production rate for Higgs boson masses up to 92 GeV, and on the production cross section for a scalar resonance decaying into di-photons up to a mass of 170 GeV. Upper limits on the product of cross section and branching ratios, σ(e+e−→XY)×B(X→γγ)×B(Y→ff̄), as low as 70 fb are obtained over the range 10<MX<170 GeV for the case where 10<MY<160 GeV and MX+MY>90 GeV, independent of the nature of Y provided it decays to a fermion pair and has negligible width. Higgs scalars which couple only to gauge bosons at Standard Model strength are ruled out up to a mass of 90.0 GeV at the 95% confidence level. Limits are also placed on non-minimal Higgs sectors having triplet representations.

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.

The rates are measured per hadronic $\mathrm{Z}^0$ decay for gluon splitting to $\mathrm{b\overline{b}}$ quark pairs, $g_{\mathrm{b\overline{b}}}$ , and of events containing two $\mathrm{b\overline{b}}$ quark pairs, $g_{\mathrm{4b}}$ , using a sample of four-jet events selected from data collected with the OPAL detector. Events with an enhanced signal of gluon splitting to $\mathrm{b\overline{b}}$ quarks are selected if two of the jets are close in phase-space and contain detached secondary vertices. For the event sample containing two $\mathrm{b\overline{b}}$ quark pairs, three of the four jets are required to have a significantly detached secondary vertex. Information from the event topology is combined in a likelihood fit to extract the values of $g_{\mathrm{b\overline{b}}}$ and $g_{\mathrm{4b}}$ , namely \begin{eqnarray*} g_{\mathrm{b\overline{b}}} & = & (3.07 \pm 0.53 \mathrm{(stat)} \pm 0.97\mathrm{(syst)})\times 10^{-3},\\ g_{\mathrm{4b}} & = & (0.36\pm 0.17 \mathrm{(stat)} \pm 0.27\mathrm{(syst)})\times 10^{-3}. \end{eqnarray*}

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 0±34(stat.)±27(syst.)MeV.

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/c2 is set on the mass at the 95% confidence level.

Data from e+e- annihilation into hadrons at centre-of-mass energies between 91 GeV and 209 GeV collected with the OPAL detector at LEP, are used to study the four-jet rate as a function of the Durham algorithm resolution parameter ycut. The four-jet rate is compared to next-to-leading order calculations that include the resummation of large logarithms. The strong coupling measured from the four-jet rate is alphas(Mz0)= 0.1182+-0.0003(stat.)+-0.0015(exp.)+-0.0011(had.)+-0.0012(scale)+-0.0013(mass) in agreement with the world average. Next-to-leading order fits to the D-parameter and thrust minor event-shape observables are also performed for the first time. We find consistent results, but with significantly larger theoretical uncertainties.

The process e+e−→γγ(γ) is studied using data recorded with the OPAL detector at LEP. The data sample taken at a centre-of-mass energy of 189 GeV corresponds to a total integrated luminosity of 178 pb−1. The measured cross-section agrees well with the expectation from QED. A fit to the angular distribution is used to obtain improved limits at 95% CL on the QED cut-off parameters: Λ+> 304 GeV and Λ−> 295 GeV as well as a mass limit for an excited electron, Me∗> 306 GeV assuming equal e∗eγ and eeγ couplings. Graviton exchange in the context of theories with higher dimensions is excluded for scales G+< 660 GeV and G−< 634 GeV. No evidence for resonance production is found in the invariant mass spectrum of photon pairs. Limits are obtained for the cross-section times branching ratio for a resonance decaying into two photons and produced in association with another photon.

A search is described for the generic process e+e−→XY, where X is a neutral heavy scalar boson decaying into a pair of photons, and Y is a neutral heavy boson (scalar or vector) decaying into a fermion pair. The search is motivated mainly by the cases where either X, or both X and Y, are Higgs bosons. In particular, we investigate the case where X is the Standard Model Higgs boson and Y the Z0 boson. Other models with enhanced Higgs boson decay couplings to photon pairs are also considered. The present search combines the data set collected by the OPAL collaboration at 189 GeV collider energy, having an integrated luminosity of 182.6 pb−1, with data samples collected at lower energies. The search results have been used to put 95% confidence level bounds, as functions of the mass MX, on the product of the cross-section and the relevant branching ratios, both in a model independent manner and for the particular models considered.

The predicted effects of final state interactions such as colour reconnection are investigated by measuring properties of hadronic decays of W bosons, recorded at a centre-of-mass energy of s≃182.7 GeV in the OPAL detector at LEP. Dependence on the modelling of hadronic W decays is avoided by comparing W+W−→qq′qq′ events with the non-leptonic component of W+W−→qq′ℓνℓ events. The scaled momentum distribution, its mean value, 〈xp〉, and that of the charged particle multiplicity, 〈nch〉, are measured and found to be consistent in the two channels. The measured differences are:Δ〈nch〉=〈nch4q〉−2〈nchqqℓν〉=+0.7±0.8±0.6Δ〈xp〉=〈xp4q〉−〈xpqqℓν〉=(−0.09±0.09±0.05)×10−2.In addition, measurements of rapidity and thrust are performed for W+W−→qq′qq′ events. The data are described well by standard QCD models and disfavour one model of colour reconnection within the ARIADNE program. The current implementation of the Ellis-Geiger model of colour reconnection is excluded. At the current level of statistical precision no evidence for colour reconnection effects was found in the observables studied. The predicted effect of colour reconnection on OPAL measurements of MW is also quantified in the context of models studied.

A study of Z boson pair production in e+e− annihilation at center-of-mass energies near 183 GeV and 189 GeV is reported. Final states containing only leptons, (ℓ+ℓ−ℓ+ℓ− and ℓ+ℓ−νν), quark and lepton pairs, (qqℓ+ℓ−, qqνν) and the all-hadronic final state (qqqq) are considered. In all states with at least one Z boson decaying hadronically, qq and bb final states are considered separately using lifetime and event-shape tags, thereby improving the cross-section measurement. At s=189 GeV the Z-pair cross section was measured to be 0.80+0.14−0.13(stat.)+0.06−0.05(syst.)pb, consistent with the Standard Model prediction. At s=183 GeV the 95% C.L. upper limit is 0.55pb. Limits on anomalous ZZγ and ZZZ couplings are derived.

The inclusive production of charged hadrons in the collisions of quasi-real photons (e+e−→e+e−+X) has been measured using the OPAL detector at LEP. The data were taken at e+e− centre-of-mass energies from 183 to 209GeV. 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 αs. The data are also compared to a measurement by the L3 Collaboration, in which a large deviation from the NLO predictions is observed.

Inclusive jet production (e+e- -> e+e- +jet+X) is studied in collisions of quasi-real photons radiated by the LEP beams at e+e- centre-of-mass energies sqrt see from 189 to 209 GeV. Jets are reconstructed using the kp jet algorithm. The inclusive differential cross-section is measured as a function of the jet transverse momentum, ptjet, in the range 5 <ptjet < 40 GeV for pseudo-rapidities, etaj, in the range -1.5 < etaj < 1.5. The results are compared to predictions of perturbative QCD in next-to-leading order in the strong coupling constant.

QCD coherence effects are studied based on measurements of correlations of particles with either restricted transverse momenta, pT<pTcut, where pT is defined with respect to the thrust axis, or restricted absolute momenta, p≡|p|<pcut, using about four million hadronic Z decays recorded at LEP with the OPAL detector. The correlations are analyzed in terms of normalized factorial and cumulant moments. The analysis is inspired by analytical QCD calculations which, in conjunction with Local Parton–Hadron Duality (LPHD), predict that, due to colour coherence, the multiplicity distribution of particles with restricted transverse momenta should become Poissonian as pTcut decreases. The expected correlation pattern is indeed observed down to pTcut≈1GeV but not at lower transverse momenta. Furthermore, for pcut→0GeV a strong rise is observed in the data, in disagreement with theoretical expectation. The Monte Carlo models reproduce well the measurements at large pTcut and pcut but underestimate their magnitudes at the lowest momenta. The e+e− data are also compared to the measurements in deep-inelastic e+p collisions at HERA. It is shown that for soft particles, the often assumed equivalence of a single hemisphere in e+e− annihilation with the current region in the Breit frame of a deep-inelastic collision may be misleading. Our study indicates difficulties with the LPHD hypothesis when applied to many-particle inclusive observables of soft hadrons.

This paper describes a topological search for an invisibly decaying Higgs boson, H, produced via the Bjorken process (e+e-→HZ). The analysis is based on data recorded using the OPAL detector at LEP at centre-of-mass energies from 183 to 209 GeV corresponding to a total integrated luminosity of 629 pb-1. In the analysis only hadronic decays of the Z boson are considered. A scan over Higgs boson masses from 1 to 120 GeV and decay widths from 1 to 3000 GeV revealed no indication for a signal in the data. From a likelihood ratio of expected signal and standard model background we determine upper limits on cross-section times branching ratio to an invisible final state. For moderate Higgs boson decay widths, these range from about 0.07 pb (MH=60 GeV) to 0.57 pb (MH=114 GeV). For decay widths above 200 GeV the upper limits are of the order of 0.15 pb. The results can be interpreted in general scenarios predicting a large invisible decay width of the Higgs boson. As an example we interpret the results in the so-called stealthy Higgs scenario. The limits from this analysis exclude a large part of the parameter range of this scenario experimentally accessible at LEP 2.

We report the first observation of Z/γ∗ production in Compton scattering of quasi-real photons. This is a subprocess of the reaction e+e−→e+e−Z/γ∗, where one of the final state electrons is undetected. Approximately 55pb−1 of data collected in the year 1997 at an e+e− centre-of-mass energy of 183 GeV with the OPAL detector at LEP have been analysed. The Z/γ∗ from Compton scattering has been detected in the hadronic decay channel. Within well defined kinematic bounds, we measure the product of cross-section and Z/γ∗ branching ratio to hadrons to be (0.9±0.3±0.1) pb for events with a hadronic mass larger than 60 GeV, dominated by (e)eZ production. In the hadronic mass region between 5 GeV and 60 GeV, dominated by (e)eγ∗ production, this product is found to be (4.1±1.6±0.6) pb. Our results agree with the predictions of two Monte Carlo event generators, grc4f and PYTHIA.

Data collected around $\sqrt{s}=91$ GeV by the OPAL experiment at the LEP e+e− collider are used to study the mechanism of baryon formation. As the signature, the fraction of Σ− hyperons whose baryon number is compensated by the production of a $\overline{\Sigma^{-}},\overline{\Lambda}$ or $\overline{\Xi^{-}}$ antihyperon is determined. The method relies entirely on quantum number correlations of the baryons, and not rapidity correlations, making it more model independent than previous studies. Within the context of the JETSET implementation of the string hadronization model, the diquark baryon production model without the popcorn mechanism is strongly disfavored with a significance of 3.8 standard deviations including systematic uncertainties. It is shown that previous studies of the popcorn mechanism with $\Lambda \overline{\Lambda}$ and $\mathrm{p}\uppi \overline{\mathrm{p}}$ correlations are not conclusive, if parameter uncertainties are considered.

In the process e+e−→hadrons, one of the effects of gluon emission is to modify the (1+cos2θ) form of the angular distribution of the thrust axis, an effect which may be quantified by the longitudinal cross-section. Using the OPAL detector at LEP, we have determined the longitudinal to total cross-section ratio to be σL/σtot=0.0127±0.0016±0.0013 at the parton level, in good agreement with the expectation of QCD computed to O(αs2). Comparisons at the hadron level with Monte Carlo models are presented. The dependence of the longitudinal cross-section on the value of thrust has also been studied, and provides a new test of QCD.

The production of charged hadrons and ${\rm K}^0_{\rm S}$ mesons in the collisions of quasi-real photons has been measured using the OPAL detector at LEP. The data were taken at $\mbox{e}^+\mbox{e}^-$ centre-of-mass energies of 161 and 172 GeV. The differential cross-sections as a function of the transverse momentum and the pseudorapidity of the charged hadrons and ${\rm K}^0_{\rm S}$ mesons have been compared to the leading order Monte Carlo simulations of PHOJET and PYTHIA and to perturbative next-to-leading order (NLO) QCD calculations. The distributions have been measured in the range $10 < W < 125$ GeV of the hadronic invariant mass W. By comparing the transverse momentum distribution of charged hadrons measured in $\gamma\gamma$ interactions with $\gamma$ -proton and meson-proton data we find evidence for hard photon interactions in addition to the purely hadronic photon interactions.

The branching ratios of the decay of the $\tau$ lepton to five charged hadrons have been measured with the OPAL detector at LEP using data collected between 1991 and 1995 at ${\rm e^+e^-}$ centre-of-mass energies close to the ${\rm Z}^0$ resonance. The branching ratios are measured to be \[ B(\tau^- \to 3h^- 2h^+ \nu_\tau) = (0.091 \pm 0.014 \pm 0.006)\% \] \[ B(\tau^- \to 3h^- 2h^+ \pi^0 \nu_\tau) = (0.027 \pm 0.018 \pm 0.009)\% \] where the first error is statistical and the second systematic.

An optimized use of the Grid computing resources in the ATLAS experiment requires the enforcement of a mechanism of job priorities and of resource sharing among the different activities inside the ATLAS VO. This mechanism has been implemented through the VOViews publication in the information system and the fair share implementation per UNIX group in the batch system. The VOView concept consists of publishing resource information, such as running and waiting jobs, as a function of VO groups and roles. The ATLAS Italian Cloud is composed of the CNAF Tier1 and Roma Tier2, with farms based on the LSF batch system, and the Tier2s of Frascati, Milano and Napoli based on PBS/Torque. In this paper we describe how test and deployment of the job priorities has been performed in the cloud, where the VOMS-based regional group /atlas/it has been created. We show that the VOViews are published and correctly managed by the WMS and that the resources allocated to generic VO users, users with production role and users of the /atlas/it group correspond to the defined share.

Bose-Einstein correlations between like-charge pions are studied in hadronic final states produced by $\mathrme^+\mathrme^-$ annihilations at center-of-mass energies of 172 and 183 GeV. Three event samples are studied, each dominated by one of the processes $\mathrm{W}^+\mathrm{W}^-\rightarrow\mathrm{q\overline{q}}\ell\overline{\nu}_{\ell}$ , $\mathrm{W}^+\mathrm{W}^-\rightarrow\mathrm{q\overline{q}}\mathrm{q\overline{q}}$ , or $(\mathrm{Z}^0/\gamma)^{*}\rightarrow\mathrm{q\overline{q}}$ . After demonstrating the existence of Bose-Einstein correlations in W decays, an attempt is made to determine Bose-Einstein correlations for pions originating from the same W boson and from different W bosons, as well as for pions from $(\mathrm{Z}^0/\gamma)^{*}\rightarrow\mathrm{q\overline{q}}$ events. The following results are obtained for the individual chaoticity parameters $\lambda$ , assuming a common source radius R: \begin{eqnarray} \lambda^{\mathrm{ same}} & = & 0.63 \pm 0.19 \pm 0.14 \nonumber , \lambda^{\mathrm{ diff}} & = & 0.22 \pm 0.53 \pm 0.14 \nonumber , \noalign{} \lambda^{\mathrm Z^{*}} & = & 0.47 \pm 0.11 \pm 0.08 \nonumber , R\, & = & 0.92\pm0.09\pm 0.09 \mathrm{fm} \nonumber. \end{eqnarray} In each case, the first error is statistical and the second is systematic. At the current level of statistical precision it is not established whether Bose-Einstein correlations, between pions from different W bosons exist or not.

A new scalar resonance, called the radion, with couplings to fermions and bosons similar to those of the Higgs boson, is predicted in the framework of Randall–Sundrum models, proposed solutions to the hierarchy problem with one extra dimension. An important distinction between the radion and the Higgs boson is that the radion would couple directly to gluon pairs, and in particular its decay products would include a significant fraction of gluon jets. The radion has the same quantum numbers as the Standard Model (SM) Higgs boson, and therefore they can mix, with the resulting mass eigenstates having properties different from those of the SM Higgs boson. Existing searches for the Higgs bosons are sensitive to the possible production and decay of radions and Higgs bosons in these models. For the first time, searches for the SM Higgs boson and flavour-independent and decay-mode independent searches for a neutral Higgs boson are used in combination to explore the parameter space of the Randall–Sundrum model. In the dataset recorded by the OPAL experiment at LEP, no evidence for radion or Higgs particle production was observed in any of those searches at centre-of-mass energies up to 209 GeV. The results are used to set limits on the radion and Higgs boson masses. For all parameters of the Randall–Sundrum model, the data exclude masses below 58 GeV for the mass eigenstate which becomes the Higgs boson in the no-mixing limit.

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.

Events with a final state consisting of two or more photons and large missing transverse energy have been observed in e+e− collisions at centre-of-mass energies in the range 192–209 GeV using the OPAL detector at LEP. Cross-section measurements are performed within the kinematic acceptance of the selection and compared with the expectations from the Standard Model process e+e−→νν¯γγ(γ). No evidence for new physics contributions to this final state is observed. Upper limits on σ(e+e−→XX)⋅BR2(X→Yγ) are derived for the case of stable and invisible Y. In the case of massive Y the combined limits obtained from all the data range from 10 to 60 fb, while for the special case of massless Y the range is 20 to 40 fb. The limits apply to pair production of excited neutrinos (X=ν*,Y=ν), to neutralino production (X=χ˜20,Y=χ˜10) and to supersymmetric models in which X=χ˜10 and Y=G˜ is a light gravitino.

Di-jet production is studied in collisions of quasi-real photons radiated by the LEP beams at e $^+$ e $^-$ centre-of-mass energies $\sqrt{s}_{\rm ee}=161$ and 172 GeV. The jets are reconstructed using a cone jet finding algorithm. The angular distributions of direct and double-resolved processes are measured and compared to the predictions of leading order and next-to-leading order perturbative QCD. The jet energy profiles are also studied. The inclusive di-jet cross-section is measured as a function of $E_{\rm T}^{\rm jet}$ and $|\eta^{\rm jet}|$ and compared to next-to-leading order perturbative QCD calculations. The inclusive di-jet cross-section as a function of $|\eta^{\rm jet}|$ is compared to the prediction of the leading order Monte Carlo generators PYTHIA and PHOJET. The Monte Carlo predictions are calculated with different parametrisations of the parton distributions of the photon. The influence of the ‘underlying event’ has been studied to reduce the model dependence of the predicted jet cross-sections from the Monte Carlo generators.

The process e+e−→γγ(γ) is studied using data recorded with the OPAL detector at LEP. The data sample corresponds to a total integrated luminosity of 56.2 pb−1 taken at a centre-of-mass energy of 183 GeV. The measured cross-section agrees well with the expectation from QED. A fit to the angular distribution is used to obtain improved limits at 95% CL on the QED cut-off parameters: Λ+> 233 GeV and Λ−> 265 GeV as well as a mass limit for an excited electron, Me∗> 227 GeV assuming equal e∗eγ and eeγ couplings. No evidence for resonance production is found in the invariant mass spectrum of photon pairs. Limits are obtained for the cross-section times branching ratio for a resonance decaying into two photons.

ATLAS data are distributed centrally to Tier-1 and Tier-2 sites. The first stages of data selection and analysis take place mainly at Tier-2 centres, with the final, iterative and interactive, stages taking place mostly at Tier-3 clusters. The Italian ATLAS cloud consists of a Tier-1, four Tier-2s, and Tier-3 sites at each institute. Tier-3s that are grid-enabled are used to test code that will then be run on a larger scale at Tier-2s. All Tier-3s offer interactive data access to their users and the possibility to run PROOF. This paper describes the hardware and software infrastructure choices taken, the operational experience after 10 months of LHC data, and discusses site performances.

One of the most demanding tasks which Computing in High Energy Physics has to deal with is reliable and high throughput transfer of large data volumes. Maximization and optimization of the data throughput are therefore key issues which have to be addressed by detailed investigations of the involved infrastructures and services.

In the framework of WLCG, Tier-1s need to manage large volumes of data ranging in the PB scale. Moreover they need to be able to transfer data, from CERN and with the other centres (both Tier-1s and Tier-2s) with a sustained throughput of the order of hundreds of MB/s over the WAN offering at the same time a fast and reliable access also to the computing farm. In order to cope with these challenging requirements, at INFN Tier-1 we have adopted a storage model based on StoRM/GPFS/TSM for the D1T0 and D1T1 Storage Classes and on CASTOR for the D0T1. In this paper we present the results of the commissioning tests of this system for the ATLAS experiment reproducing the real production case with a full matrix transfer from the Tier-0 and with all the other involved centres. Noticeably also the new approach of direct file access from farm to data is covered showing positive results. GPFS/StoRM has also been sucessfully deployed, configured and commissioned as storage solution for an ATLAS INFN Tier-2, specifically the one of Milano. The results are shown and discussed in this paper together with the ones obtained for the Tier-1.

Searches for final states expected in models with light gravitinos have been performed, including experimental topologies with multi-leptons with missing energy, leptons and photons with missing energy, and jets and photons with missing energy. No excess over the expectations from the Standard Model has been observed. Limits are placed on production cross-sections in the different experimental topologies. Additionally, combining with searches for the anomalous production of lepton and photon pairs with missing energy, results are interpreted in the context of minimal models of gauge mediated SUSY breaking. Exclusion limits at the 95% confidence level on the supersymmetric particle masses of mℓ̃> 83 GeV and mχ̃10> 85 GeV for tanβ=2, and mτ̃> 69 GeV, mẽ,μ̃> 88 GeV and mχ̃10> 76 GeV for tanβ=20, are established.

INTEGRAL is the forthcoming European Space Agency's (ESA) satellite mission for gamma-ray astronomy, which will be launched in 2002. IBIS is the imaging telescope onboard INTEGRAL and will produce images of the gamma-ray sky in the region between 15 keV and 10 MeV by means of a two-layer position sensitive detection plane coupled with a coded aperture mask. The detection plane of IBIS comprises two detectors: ISGRI, operative in the 15 keV - 1 MeV range, and PICsIT, 150 keV - 10 MeV. The PICsIT instrument, which is the high energy plane of the IBIS imager, comprises 8 individual modules of 512 detection elements. The modules are arranged in a 4 x 2 pattern, while the pixels are in a 16 x 32 array within each module. Detailed simulation programs of PICsIT qualification and flight model have been set up in order to provide a complete scientific characterization of the detector in terms of spectral and imaging performances. These simulation programs have also been used to reproduce the on-ground calibration results, and will be the basis for the production of the response matrix.

Using the entire OPAL LEP1 on-peak Z hadronic decay sample, Z→qq̄γ decays were selected by tagging hadronic final states with isolated photon candidates in the electromagnetic calorimeter. Combining the measured rates of Z→qq̄γ decays with the total rate of hadronic Z decays permits the simultaneous determination of the widths of the Z into up- and down-type quarks. The values obtained, with total errors, were Γu=300+19−18 MeVandΓd=381+12−12 MeV. The results are in good agreement with the Standard Model expectation.

Using data collected with the OPAL detector at LEP, we have searched for the processes e+e−→Z0→pe−,pμ− and the charge conjugate final-states. These would violate the conservation of the baryon-number B, lepton-number L and the fermion-number n = (B+L). No evidence for such decays has been found, and the 95% confidence level upper limits on the partial widths Γ(Z0→pe) and Γ(Z0→pμ) are found to be 4.6 and 4.4 keV respectively.

The inclusive charm hadron semileptonic branching fractions ${\rm B}({\rm c}\to{\rm e})$ and ${\rm B}({\rm c}\to\mu)$ in $\mathrm{Z}^0\to\mathrm c\bar{\mathrm c}$ events have been determined using 4.4 million hadronic $\mathrm{Z}^0$ decays collected with the OPAL detector at LEP. A charm-enriched sample is obtained by selecting events with reconstructed $\mathrm{D}^{*\pm}$ mesons. Using leptons found in the hemisphere opposite that of the $\mathrm{D}^{*\pm}$ mesons, the semileptonic branching fractions of charm hadrons are measured to be \[ {\rm B}({\rm c}\to{\rm e})= 0.103 \pm 0.009^{+0.009}_{-0.008} {\rm and} {\rm B}({\rm c}\to\mu) = 0.090 \pm 0.007^{+0.007}_{-0.006}\,, \] where the first errors are in each case statistical and the second systematic. Combining these measurements, an inclusive semileptonic branching fraction of charm hadrons of \[ {\rm B}({\rm c}\to\ell) = 0.095 \pm{0.006}^{+0.007}_{-0.006} \] is obtained.

INFN-CNAF, the Italian Tier-1, houses more than 10 PB of tape space and 6 PB of disk space used by the global physics user communities including the LHC experiments. In this context, an innovative solution to provide the highest level of reliability, scalability and performances on the installed mass storage system has been developed and used on the production system. The solution, called Grid Enabled Mass Storage System (GEMSS), is based on two components: a layer between the IBM GPFS and TSM HSM, and StoRM, designed to allow direct access to the storage resources by file protocol as well as standard Grid protocols. During this year, both LHC and non-LHC experiments have agreed to use GEMSS with SRM endpoint at INFN Tier-1 after the excellent results observed during their GEMSS validation phase. In this paper we describe the storage system setup, the data workflow and computing activities of the LHC experiments, and main results obtained at INFN-CNAF Tier-1 in the first year of LHC data taking. In particular, we show the system response to a sustained huge load on the mass storage system and the computing infrastructure.

The large amount of data produced by the ATLAS experiment needs new computing paradigms for data processing and analysis, which involve many computing centres spread around the world. The computing workload is managed by regional federations, called "clouds". The Italian cloud consists of a main (Tier-1) center, located in Bologna, four secondary (Tier-2) centers, and a few smaller (Tier-3) sites. In this contribution we describe the Italian cloud facilities and the activities of data processing, analysis, simulation and software development performed within the cloud, and we discuss the tests of the new computing technologies contributing to evolution of the ATLAS Computing Model.

The inclusive branching fraction of φ mesons from the decay of b hadrons produced in Z decays was measured to be Br(b→φX)=0.0282±0.0013 (stat.)±0.0019 (syst.), using data collected by the OPAL detector at LEP.

With this work we present the activity and performance optimization of the Italian computing centers supporting the ATLAS experiment forming the so-called Italian Cloud. We describe the activities of the ATLAS Italian Tier-2s Federation inside the ATLAS computing model and present some Italian original contributions. We describe StoRM, a new Storage Resource Manager developed by INFN, as a replacement of Castor at CNAF - the Italian Tier-1- and under test at the Tier-2 centers. We also show the failover solution for the ATLAS LFC, based on Oracle DataGuard, load-balancing DNS and LFC daemon reconfiguration, realized between CNAF and the Tier-2 in Roma. Finally we describe the sharing of resources between Analysis and Production, recently implemented in the ATLAS Italian Cloud with the Job Priority mechanism.

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.

The selection of top pair events in the fully hadronic final state with the CMS detector at LHC and the expected performance, the resulting cross section measurement and the top mass reconstruction accuracy are discussed.

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.

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.

IBIS, the imager on board the INTEGRAL satellite, is a coded mask telescope for X and gamma-ray astronomy with imaging and spectroscopy capabilities from 15 keV to 10 MeV. In order to cover this energy range, IBIS uses two position sensitive detectors: the low energy detector layer ISGRI and the high energy detector layer PICsIT. The sensitivity of a coded mask instrument depends on the detectors efficiencies, imaging efficiency, and on the diffuse photons and particles background count-rate. In the IBIS energy range, also the opacity at low energies of the open mask elements and the transparency of the closed elements at high energies give a significant effect on the sensitivities curves. In this work we present a Monte Carlo evaluation of the IBIS detectors efficiencies. The mask transparency data from the Flight Model (FM) are also presented.

The imager IBIS on board INTEGRAL is composed of two detector planes: ISGRI (15 keV - 1 MeV) and PICsIT (170 keV - 10 MeV). Here we describe the data structure of PICsIT and the instrument specific software, including the standard scientific analysis. Some examples of shadowgrams and spectra are shown, by using data collected during the End-to-End Tests of January 2002.

The imager IBIS on board INTEGRAL is composed of two detector planes: ISGRI (15 keV - 1 MeV) and PICsIT (170 keV - 10 MeV). Here we describe the data structure of PICsIT and the instrument specific software, including the standard scientific analysis. Some examples of shadowgrams and spectra are shown, by using data collected during the End-to-End Tests of January 2002.