Ken W Bell

We report an experimental determination of the cross section for e+e− → hadrons from a scan around the Z0 pole. On the basis of 4350 hadronic events collected over seven energy points between 89.26 GeV and 93.26 GeV we obtain a mass of mz=91.01±0.05±0.05 GeV, and a total decay width of Γz=2.60±0.13 GeV. In the context of the standard model t these results imply 3.1 ± 0.4 neutrino generations.

A search for the minimal standard model Higgs boson (H0) has been performed with data from e+e− collisions in the OPAL detector at LEP. The analysis is based on approximately 8 pb−1 of data taken at centre-of-mass energies between 88.2 and 95.0 GeV. The search concentrated on the reaction e+e−→(e+e−, μ+μ−, vv or τ+τ−)H0, H0→(qq or τ+τ−) for Higgs boson masses above 25 GeV/c2. No Higgs boson candidates have been observed. The present study, combined with previous OPAL publications, excludes the existence of a standard model Higgs boson with mass in the range 3<mH0<44GeV/c2 at the 95% confidence level.

A study of b quark hadronisation is presented using inclusively reconstructed B hadrons in about four million hadronic Z decays recorded in 1992-2000 with the OPAL detector at LEP. The data are compared to different theoretical models, and fragmentation function parameters of these models are fitted. The average scaled energy of weakly decaying B hadrons is determined to be <xe>=0.7193+-0.0016(stat)+0.0036-0.0031(syst)

We have searched for Z0 decays into acoplanar pairs of leptons or jets. The data were recorded with the OPAL detector during an energy scan around the Z0 peak and correspond to about 17000 produced Z0,s. We have determined model independent limits on branching ratios for the Z0 to decay into pairs of heavy particles with subsequent decays leading to the above topologies. In the context of supersymmetric models, mass bounds close to the kinematic limit were obtained for the scalar leptons, ẽ, g̃m, g̃t, and for the chargino, X̃±.

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.

Quark and gluon jets with equal energies are identified in three-jet hadronicZ 0 events, using reconstructed secondary vertices from heavy quark decay in conjunction with energy ordering of the jets to anti-tag the gluon jets. Selection of jets from a symmetric event topology allows their properties to be compared in a simple and direct manner. The jets under study have an energy of about 24 GeV. It is observed that gluon jets have a larger angular width than quark jets and yield a softer particle energy spectrum. Correspondingly, the mean particle multiplicity is found to be larger for gluon than for quark jets. Correcting the distributions for residual misidentification of the quark and gluon jets, the ratio of mean particle multiplicty of gluon relative to quark jets is measured to be $$\frac{{\left\langle n \right\rangle _{gluon} }}{{\left\langle n \right\rangle _{quark} }} = 1.27 \pm 0.04(stat.) \pm 0.06(syst.),$$ where the jets are defined using thek ⊥ jet finder. The numerical value of this ratio is found to be sensitive to the choice of the jet algorithm. The experimental results are compared to Monte Carlo calculations which incorporate perturbative QCD along with different assumptions about the hadronization process.

We report on a measurement of the mass of the Z0 boson, its total width, and its partial decay widths into hadrons and leptons. On the basis of 25 801 hadronic decays and 1999 decays into electrons, muons or taus, selected over eleven energy points between 88.28 GeV and 95.04 GeV, we obtain from a combined fit to hadrons and leptons a mass of Mz = 91.154 ± 0.021 (exp) ± 0.030 (LEP) GeV, and a total width of Γz = 2.536 ± 0.045 GeV. The errors on Mz have been separated into the experimental error and the uncertainty due to the LEP beam energy. The measured leptonic partial widths are Γee = 81.2 ± 2.6 MeV, Γμμ = 82.6 ± 5.8 MeV, and Γττ = 85.7 ± 7.1 MeV, consistent with lepton universality. From a fit assuming lepton universality we obtain Γℓ+ ℓ− = 81.9 ± 2.0 MeV. The hadronic partial width is Γhad = 1838 ± 46 MeV. From the measured total and partial widths a model independent value for the invisible width is calculated to be Γinv = 453 ± 44 MeV. The errors quoted include both the statistical and the systematic uncertainties.

New data are presented on the reaction π−p→K0Λ0 at 13 incident momenta between 1395 and 2375 MeV/c. A partial-wave analysis has been performed using these data and previous data at lower momenta. The resulting resonance parameters are in broad agreement with recent πN analyses and with the predictions of SU(6)W × O(3) for the couplings.

A measurement of the single photon production cross-section is presented based on a data-sample of 40.5 pb−1 collected with the OPAL detector at centre-of-mass energies within 3 GeV of the Z0 mass. Single photon events arise from initial state radiation and the production of an "invisible" final state consisting of neutrinos or possibly particles such as sneutrinos or photinos. The single photon topology is also sensitive to new Z0 decays such as $$Z^0 \to \bar vv* \to \bar vv\gamma $$ orZ 0→γX, X→invisible particles. A total of 447 single photon candidates were observed with energy exceeding 1.75 GeV in the polar angle region |cosθ|<0.7. The estimated background from processes with visible reaction products, mainly e+e−→e+e−γ, is 37±6 events. Interpreting the cross-sections as being solely due to Z0 decay to invisible particles and the expected W-contributions, the Z0 invisible with is determined to be 539±26±17 MeV corresponding toN v=3.23±0.16±0.10 light neutrino generations. The differential cross-section with photon energy is presented. Upper limits are set on additional invisible contributions to the Z0 width, on possible non-resconant processes, and on Z0 decays to single photons. The energy spectra are used to constrain exotic sources of high energy single photons. In particular, the radiative twobody decay of the Z0 to a new particle X, with mass below 64 GeV and an invisible signature, has a Z0 branching ratio of less than 4.3×10−6 at 95% confidence level.

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/c2 at the 95% confidence level has been set for the doubly charged Higgs particle in left–right symmetric models. This is the first search for doubly charged Higgs bosons at centre-of-mass energies larger than 91 GeV.

There exists a topographically organized projection from the globus pallidus and ventral pallidum to the subthalamic nucleus and adjacent lateral hypothalamus. The participation of GABA as a neurotransmitter in this projection was evaluated by retrograde labeling of cells in the pallidal area from an iontophoretic deposit of Fluor-Gold in the subthalamus combined with in situ hybridization for mRNA of the GABA synthetic enzyme, glutamate decarboxylase (GAD). A rostrocaudal gradient in the contribution of GABA to the projection was demonstrated with a relatively small percentage of retrogradely labeled cells in the rostral ventral pallidum containing GAD mRNA (7%) compared to the caudal globus pallidus which had over 70% of the Fluoro-Gold containing cells double-labeled for GAD mRNA. Overall the ventral pallidum contribution to the subthalamic nucleus was less GABAergic than the portion arising from the globus pallidus (35% vs. 61%, respectively).

Using radiative Z0→ τ+τ−γevents collected with the OPAL detector at LEP at s=MZ during 1990–95, a direct study of the electromagnetic current at the τγ vertex has been performed in terms of the anomalous magnetic form factor F2 of the τ lepton. The analysis is based on a data sample of 1429 e+e−→τ+τ−γevents which are examined for a deviation from the expectation with F2=0. From the non-observation of anomalous τ+τ−γproduction a limit of−0.068<F2<0.065is obtained. This can also be interpreted as a limit on the electric dipole form factor F3 as|eF3|<3.7×10−16ecm.The above ranges are valid at the 95% confidence level.

Using a sample of 5558 Z0 → τ+τ− decays produced at LEP a direct test of CP-invariance in the neutral current reaction e+e− → τ+τ− is performed. Samples of events where eachy τ decays into a single particle have been isolated for the construction of CP-odd observables. Three different event classes are considered: lepton-lepton, lepton-hadron, and hadron-hadron. No evidence for a non-zero expectation value of the considered CP-observables and hence for CP-violation is observed. Quantitatively, we deduce from this null result an estimate on the weak dipole moment d̃τ(m2Z = (−4.5 ± 5.3 ± 1.4) × 10−17 e cm for the lepton-lepton signature and d̃τ(m2Z = (1.4 ± 3.7 ± 1.3) × 10−17 e cm for the hadron-hadron signature. Combining these results we place a limit with 95% confidence of |d̃τ|⩽7.0 × 10−17 e cm.

The amplitude of the signal collected from the PbWO4 crystals of the CMS electromagnetic calorimeter is reconstructed by a digital filtering technique. The amplitude reconstruction has been studied with test beam data recorded from a fully equipped barrel supermodule. Issues specific to data taken in the test beam are investigated, and the implementation of the method for CMS data taking is discussed.

The production of K0 mesons in e+e− interactions at center of mass energies in the region of the Z0 mass has been investigated with the OPAL detector at LEP. The rate is found to be 2.10±0.02±0.14 K0, Z0 per hadronic event. The predictions from the JETSET and HERWIG generators agree very well with both the rate and the scale invariant cross section (1/σhadβ) (dσ/dxE) for K0 production. Comparisons of the inclusive momentum spectrum with predictions of an analytical QCD formula and with data from lower center of mass energies are presented.

We present evidence for the existence of the strange b-flavoured meson Bs0 in a data sample of 470 628 hadronic Z0 decays recorded with the OPAL detector at LEP. A signal of 18.3±5.2 (stat.)±0.9(syst.) Ds−l+ pairs (or charge conjugate, l = e or l = μ) is found after background subtraction, where the Ds meson is detected in the øπ and K∗0 K final states. Interpreting this signal as coming from the semileptonic decay Bs0→Ds−l+vX and combining t he two decay modes of the Ds, we find f (b→Bs0)B(Bs0→Ds−l+vX)BDs−→φ π−) = [3.9±1.1 (stat.)±0.8(syst.)]×10−4, where f (b→Bs0) is the fraction of b quarks that results in a B0s meson in Z0 decays. This signal for the Bs0 is supported by our observation of an excess of 147 ± 48 inclusive Ds mesons in the φπ and K∗0K modes above the number expected from B0 and B+ decays and from the fragmentation of primary c quarks. In addition, a search is made for the exclusive decay Bs0→J/ψφ. Based on one candidate event the 90% confidence level upper limit is determined to be f (b → Bs0) B (Bs0 → J/ψφ) < 0.22%.

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 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 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.

We report on a measurement of the processes e+e−→e+e−, e+e−→μ+μ−, and e+e−→τ+τ− near the Z0 pole. On the basis of 163 e+e−, 101 μ+μ− and 87 τ+τ− events we obtain Γee=89±4±4 MeV, Γμμ=85±9±6 MeV and Γττ=87±10±8 MeV, compatible with the standard model. Combining these with our previous results on hadronic Z0 decays, we find a hadronic width Γhad=1787±81±90 MeV and an invisible width Γinv=552±85±71 MeV.

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.

Performance tests of some aspects of the CMS ECAL were carried out on modules of the "barrel" sub-system in 2002 and 2003. A brief test with high energy electron beams was made in late 2003 to validate prototypes of the new Very Front End electronics. The final versions of the monitoring and cooling systems, and of the high and low voltage regulation were used in these tests. The results are consistent with the performance targets including those for noise and overall energy resolution, required to fulfil the physics programme of CMS at the LHC.

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.

Calibration of the relative response of the individual channels of the barrel electromagnetic calorimeter of the CMS detector was accomplished, before installation, with cosmic ray muons and test beams. One fourth of the calorimeter was exposed to a beam of high energy electrons and the relative calibration of the channels, the intercalibration, was found to be reproducible to a precision of about 0.3%. Additionally, data were collected with cosmic rays for the entire ECAL barrel during the commissioning phase. By comparing the intercalibration constants obtained with the electron beam data with those from the cosmic ray data, it is demonstrated that the latter provide an intercalibration precision of 1.5% over most of the barrel ECAL. The best intercalibration precision is expected to come from the analysis of events collected in situ during the LHC operation. Using data collected with both electrons and pion beams, several aspects of the intercalibration procedures based on electrons or neutral pions were investigated.

Using the OPAL data accumulated in 1991–1993 amounting to 74 pb−1 of integrated luminosity, corresponding to 1.64×106 selected multi-hadronic events, a search has been performed for charged particles with unusual mass or unusual charge. The mass was determined from a combination of momentum and ionization energy loss measurements. No isolation criteria were applied to the tracks examined, so that both isolated particles and particles produced in jets were valid candidates. For particles with chargeQ/e=−1, one candidate with a mass of approximately 4.2 GeV/c2 was found, which is compatible with the background rate expected according to a Monte Carlo simulation. The implications of this search for the mass limits of a conjectured stable or quasi-stable charged gluino composite $$(\tilde gq\bar q')^ \pm $$ are discussed. Limits are also presented for the production of fractionally-charged particles withQ/e=±2/3 and ±4/3 as well as for particles withQ/e=±2.

A search for decays of the Bc meson was performed using data collected from 1990–1995 with the OPAL detector on or near the Z0 peak at LEP. The decay channels Bc+→J/ψπ+, Bc+→J/ψa1+ and Bc+→J/ψℓ+ν were investigated, where ℓ denotes an electron or a muon. Two candidates are observed in the mode Bc+→J/ψπ+, with an estimated background of (0.63±0.20) events. The weighted mean of the masses of the two candidates is (6.32±0.06) GeV/c2, which is consistent with the predicted mass of the Bc meson. One candidate event is observed in the mode Bc+→J/ψℓ+ν, with an estimated background of (0.82±0.19) events. No candidate events are observed in the Bc+→J/ψa1+ decay mode, with an estimated background of (1.10±0.22) events. Upper bounds at the 90% confidence level are set on the production rates for these processes.

A search was performed for charginos with masses close to the mass of the lightest neutralino in e+e- collisions at centre-of-mass energies of 189-209 GeV recorded by the OPAL detector at LEP. Events were selected if they had an observed high-energy photon from initial state radiation, reducing the dominant background from two-photon scattering to a negligible level. No significant excess over Standard Model expectations has been observed in the analysed data set corresponding to an integrated luminosity of 570pb-1. Upper limits were derived on the chargino pair-productin cross-section, and lower limits on the chargino mass were derived in the context of the Minimal Supersymmetric Extension of the Standard Model for the gravity and anomaly mediated Supersymmetry breaking scenarios.

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.

Within a Two-Higgs-Doublet Model (2HDM) a search for a light Higgs boson in the mass range of 4–12 GeV has been performed in the Yukawa process $\mathrme^+ \mathrme^-\rightarrow \mathrm{b}\bar{\mathrm{b}}\mathrm{A}/\mathrm{h}\rightarrow\mathrm{b} \bar{\mathrm{b}}\tau^+\tau^-$ , using the data collected by the OPAL detector at LEP between 1992 and 1995 in $\mathrme^+ \mathrme^-$ collisions at about 91 GeV centre-of-mass energy. A likelihood selection is applied to separate background and signal. The number of observed events is in good agreement with the expected background. Within a CP-conserving 2HDM type II model the cross-section for Yukawa production depends on $\xi^{\mathrm{A}}_d=|\tan\beta|$ and $\xi^{\mathrm{h}}_d=|\sin\alpha/\cos\beta|$ for the production of the CP-odd A and the CP-even h, respectively, where $\tan\beta$ is the ratio of the vacuum expectation values of the Higgs doublets and $\alpha$ is the mixing angle between the neutral CP-even Higgs bosons. From our data 95% C.L. upper limits are derived for $\xi^{\mathrm{A}}_d$ within the range of 8.5 to 13.6 and for $\xi^{\mathrm{h}}_d$ between 8.2 to 13.7, depending on the mass of the Higgs boson, assuming a branching fraction into $\tau^+\tau^-$ of 100%. An interpretation of the limits within a 2HDM type II model with Standard Model particle content is given. These results impose constraints on several models that have been proposed to explain the recent BNL measurement of the muon anomalous magnetic moment.

Measurements of helicity density matrix elements have been made for the φ(1020), D*± and B* vector mesons in multihadronic Z0 decays in the OPAL experiment at LEP. Results for inclusive φ produced with high energy show evidence for production preferentially in the helicity zero state, with ρ00 = 0.54 ± 0.08, compared to the value of 1/3 expected for no spin alignment. The corresponding element for the D*± has a value of 0.40 ± 0.02, also suggesting a deviation from 1/3. The B* result, with ρ00 = 0.36 ± 0.09, is consistent with no spin alignment. Off-diagonal elements have been measured for the f and D* mesons; for the D* the element Re ρ1−1 is non-zero, indicating non-independent fragmentation of the primary quarks.

We report on an improved measurement of the value of the strong coupling constant σs at the Z0 peak, using the asymmetry of the energy-energy correlation function. The analysis, based on second-order perturbation theory and a data sample of about 145000 multihadronic Z0 decays, yields αs(Mz0 = 0.118±0.001(stat.)±0.003(exp.syst.)−0.004+0.0009 (theor. syst.), where the theoretical systematic error accounts for uncertainties due to hadronization, the choice of the renormalization scale and unknown higher-order terms. We adjust the parameters of a second-order matrix element Monte Carlo followed by string hadronization to best describe the energy correlation and other hadronic Z0 decay data. The αs result obtained from this second-order Monte Carlo is found to be unreliable if values of the renormalization scale smaller than about 0.15 Ecm are used in the generator.

A search for minimal standard model (MSM) and minimal supersymmetric model (MSSM), Higgs bosons with masses larger than 3 GeV/c2 has been performed by the OPAL collaboration one + e − data from LEP corresponding to an integrated luminosity of 1.24 pb−1. The limits for MSM Higgs bosons have been obtained using the channelsZ 0→Z 0* H 0,Z 0*→(v $$\bar v$$ ore + e − or µ+, µ−. The search for MSSM Higgs bosons has been performed using the channelsZ 0→Z 0* H 0,v $$\bar v$$ ore + e − or µ+µ−),h 0 →q $$\bar q$$ andZ 0→h 0 A 0,h 0 A 0→(4 jet orτ + τ −→ or 4τ), whereh 0 andA 0 are the two lightest neutral MSSM Higgs bosons. No Higgs boson signal has been observed. The MSM Higgs boson is excluded in the mass range 3 GeV < $$m_{H^0 }$$ <25.3 GeV/c2 at the 95% confidence level; limits on the masses of the two lightest neutral MSSM Higgs bosons are obtained forh 0 mases up to 40.5 GeV/c2.

Excited leptons have been searched for using data recorded by the OPAL detector at LEP. No evidence for such particles has been found. From the study of e+e− → ℓ+ℓ−γγ events, lower limits on the masses of spin-12 excited leptons are found to be 44.9 GeV at 95% confidence level. From the study of e+e− →ℓ+ℓ−γ events, upper limits on their couplings are set up to ℓ∗ masses close to the mass of the Z0 boson.

Ensuring the radiation hardness of PbWO4 crystals was one of the main priorities during the construction of the electromagnetic calorimeter of the CMS experiment at CERN. The production on an industrial scale of radiation hard crystals and their certification over a period of several years represented a difficult challenge both for CMS and for the crystal suppliers. The present article reviews the related scientific and technological problems encountered.

A search for charged and neutral excited leptons is performed in 217 pb^-1 of e+e- collision data collected with the L3 detector at LEP at centre-of-mass energies from 202 GeV up to 209 GeV. The pair- and single-production mechanisms of excited electrons, muons and taus, as well as of excited electron-, muon- and tau-neutrinos,are investigated and no signals are detected. Combining with L3 results from searches at lower centre-of-mass energies, gives improved limits on the masses and couplings of excited leptons.

Measurements have been made in the OPAL experiment at LEP of the inclusive production of strange vector φ(1020) and K*(892)0 mesons, and the tensor meson K 2 * (1430)0. The overall production rates per hadronic Z0 decay have been determined to be 0.100±0.004stat.±0.007syst. φ(1020) mesons, 0.74±0.03stat.±0.03syst. K*(892)0 mesons and (forx E <0.3) 0.19±0.04stat.±0.06syst. K 2 * (1430)0 mesons. The measurements for the vector states update previously published results based on lower statistics, while the K 2 * (1430)0 rate represents the first direct measurement of a strange tensor state in Z0 decay. For the vector states, both the overall production rates and normalised differential cross sections, with respect to the scaled energy variablex E , have been compared to JETSET and HERWIG predictions. The peak positions in the ζ=ln(1/x p ) distributions have been measured and compared to measurements of other hadron states.

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.

A search for the Minimal Standard Model Higgs boson (H0) has been performed with data from e+e− collisions collected by the OPAL detector at LEP. The search was made for events of the types e+e−→(e+e−,μ+μ− or νν)H0, H0→qq̄ and was based on approximately 78 pb−1 of data taken at center-of-mass energies between 88 and 95 GeV in the years 1990–1993. The present study, combined with previous OPAL publications, excludes the existence of a Minimal Standard Model Higgs boson with mass below 56.9 GeV at the 95% confidence level.

A search for stable and long-lived massive particles of electric charge |Q/e|=1 or 2/3, pair-produced in e+e− collisions at centre-of-mass energies from 130 to 183 GeV, is reported by the OPAL collaboration at LEP. No evidence for production of these particles was observed in a mass range between 45 and 89.5 GeV. Model-independent upper limits on the production cross-section between 0.05 and 0.19 pb have been derived for scalar and spin-1/2 particles with charge ±1. Within the framework of the minimal supersymmetric model (MSSM), this implies a lower limit of 82.5 (83.5) GeV on the mass of long-lived right- (left-)handed scalar muons and scalar taus. Long-lived charged leptons and charginos are excluded for masses below 89.5 GeV. For particles with charge ±2/3 the upper limits on the production cross-section vary between 0.05 and 0.2 pb. All limits, on masses and on cross-sections, are valid at the 95% confidence level for particles with lifetimes longer than 10−6 s.

The performance of electromagnetic calorimeter modules made of proton-irradiated PbWO4 crystals has been studied in beam tests. The modules, similar to those used in the Endcaps of the CMS electromagnetic calorimeter (ECAL), were formed from 5×5 matrices of PbWO4 crystals, which had previously been exposed to 24 GeV protons up to integrated fluences between 2.1× 1013 and 1.3× 1014 cm−2. These correspond to the predicted charged-hadron fluences in the ECAL Endcaps at pseudorapidity η = 2.6 after about 500 fb−1 and 3000 fb−1 respectively, corresponding to the end of the LHC and High Luminosity LHC operation periods. The irradiated crystals have a lower light transmission for wavelengths corresponding to the scintillation light, and a correspondingly reduced light output. A comparison with four crystals irradiated in situ in CMS showed no significant rate dependence of hadron-induced damage. A degradation of the energy resolution and a non-linear response to electron showers are observed in damaged crystals. Direct measurements of the light output from the crystals show the amplitude decreasing and pulse becoming faster as the fluence increases. The latter is interpreted, through comparison with simulation, as a side-effect of the degradation in light transmission. The experimental results obtained can be used to estimate the long term performance of the CMS ECAL.

We report a measurement of the Cabibbo-Kobayashi-Maskawa matrix element |Vcb|. From approximately 4.2 million hadronic Z0 decays recorded with the OPAL deteector, a sample is selected containing 1251 ± 125B0 → D∗+ℓ−νℓ candidates, where ℓ is either an electron or a muon. Using Heavy Quark Effective Theory calculations for the decay form factor at zero recoil of the D∗+ meson in the B0 rest frame, we derive |Vcb| = [36.0 ± 2.1 (stat) ± 2.4 (syst) ± 1.2 (theory)] × 10−3.

The tau lepton lifetime has been measured using two independent techniques; an impact parameter analysis of the 1-prong decays and a decay length analysis of the 3-prong decays. Approximately 5 000 Z0 decays to τ+τ− have been selected from the data collected with the OPAL detector at LEP during 1990. The results of the two statistically independent measurements are, respectively, τ1=0.293 ± 0.013 (stat.)±0.013 (syst.) ps and τ3=0.327±0.017 (stat.)±0.011 (syst.) ps. After combining the statistical and systematic errors for each analysis in quadrature, the weighted average lifetime is calculated to be ττ=0.308 ± 0.013 ps.

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.

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.

Data collected with the OPAL detector during 1990–1994 are used to measure the time dependence of $$B_d^0 \leftrightarrow \bar B_d^0 $$ mixing. A sample of 348 D*± candidates with a lepton in the opposite hemisphere are reconstructed, of which 167±25 are expected to be from B d 0 decays. The B d 0 oscillation frequency is measured to be $$\Delta m_d = 0.567 \pm 0.089(stat)_{ - 0.023}^{ + 0.029} (syst) ps^{ - 1} .$$ . A previously published analysis ofΔm d using D*± and lepton candidates in the same hemisphere and jet charge is also updated with a larger data sample. From 1200 D*±ℓ∓ candidates, of which 778±84 are expected to be from B d 0 decays, we find a value of: $$\Delta m_d = 0.539 \pm 0.060(stat) \pm 0.024(syst) ps^{ - 1} .$$ . The combined result of these two analyses is $$\Delta m_d = 0.548 \pm 0.050(stat)_{ - 0.019}^{ + 0.023} (syst) ps^{ - 1} .$$ .

The properties of final state photons in multihadronic decays of theZ 0 and those of the recoiling hadronic system are discussed and compared with theoretical expectations. The yield of two and three jet events with final state photons is found to be in good agreement with the expectation from a matrix element calculation ofO(αα s. Uncertainties in the interpretation of the theoretical calculation do not yet permit a final assessment of events with just one reconstructed jet. Comparing the rates of two jet events with a photon to those of three jet events in the inclusive multihadronic sample, the strong coupling constant in second order is determined asα s $$(M_{Z^0 } )$$ =0.122±0.010, taking into account only the statistical and experimental systematic errors. It is found that an abelian model of the strong interaction does not describe the data. The comparison of the total yield and the jet rates with QCD shower programs shows better agreement with the ARIADNE model than with the JETSET model. Both programs are found to describe well the photon properties and the properties of the residual hadronic event.

The magnitude of the CKM matrix element Vub is determined by measuring the inclusive charmless semileptonic branching fraction of beauty hadrons at OPAL based on b -> Xu l nu event topology and kinematics. This analysis uses OPAL data collected between 1991 and 1995, which correspond to about four million hadronic Z decays. We measure Br(b -> Xu l) to be (1.63 +/- 0.53 +0.55/-0.62) x 10^(-3). The first uncertainty is the statistical error and the second is the systematic error. From this analysis, Vub is determined to be: |Vub| = (4.00 +/- 0.65(stat) +0.67/-0.76(sys) +/- 0.19(HQE)) x 10^(-3). The last error represents the theoretical uncertainties related to the extraction of |Vub| from Br(b -> Xu l) using the Heavy Quark Expansion.

New measurements of the hadronic and leptonic cross sections and of the leptonic forward-backward asymmetries ine + e − collisions are presented. The analysis includes data recorded up to the end of 1991 by the OPAL experiment at LEP, with centre-of-mass energies within ±3 GeV of the Z0 mass. The results are based on a recorded total of 454 000 hadronic and 58 000 leptonic events. A model independent analysis of Z0 parameters based on an extension of the improved Born approximation is presented leading to test of lepton universality and an interpretation of the results within the Standard Model framework. The determination of the mass and width of the Z0 benefit from an improved understanding of the LEP energy calibration.

The τ lifetime has been measured with the OPAL detector at LEP, from analyses using the impact parameters in decays to single charged tracks, and the decay lengths from τ decays to three charged tracks. The 1991 sample of approximately 12300 τ-pair events, of which 70% contain silicon microvertex detector information, has been combined with a re-analysis of the 5100 events recorded during 1990. The two statistically-independent determinations give: $$\begin{gathered} \tau (one - prong) = 296.4 \pm 7.1 (stat) \pm 3.8 (sys) fs, \hfill \\ \tau (three - prong) = 286.3 \pm 7.4 (stat) \pm 5.2 (sys) fs. \hfill \\ \end{gathered} $$ The weighted average of these results after combination of the uncorrelated systematic errors is: $$\tau _\tau = 291.9 \pm 5.1 (stat) \pm 3.1 (sys) fs.$$ .

We have measured the cross sections for e+e− → e+e−, e+e− → μ+μ−, e+e− → γγ and e+e− → hadrons in an energy scan at center of mass energies between 39.79 and 46.72 GeV in 30 MeV steps. New spinless bosons, whose existence has been postulated as a possible means to explain the anomalously large radiative width of the Z0 found at the CERN SPS pp collider, are ruled out in the scan region. The data are used to set limits on the couplings to lepton, photon and quark pairs of bosons with masses above 46.72 GeV.

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.

The differential cross-sections for e+e− → e+e−, e+e− → μ+μ− and e+e− → τ+τ−, and the total cross-section for e+e− → qq̄ at centre-of-mass energies of 130–140 GeV were studied using about 5 pb−1 of data collected with the OPAL detector at LEP in October and November 1995. The results are in agreement with the Standard Model predictions. Four-fermion contact interaction models were fitted to the data and lower limits were obtained on the energy scale Λ at the 95% confidence level.

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.

A sample of 689 muon candidates and 665 electron candidates identified in multihadronic Z0 decays has been used to measure the average B hadron lifetime. These data were recorded with the OPAL detector during 1990. Maximum likelihood fits to the distributions of the lepton impact parameters yield an average B hadron lifetime of τb = 1.37 ± 0.07 ± 0.06 ps, where the first error is statistical and the second systematic. This result is a weighted average over the semileptonic branching fractions and production rates of the B hadrons produced in Z0 decays.

The production of charm quarks is studied in deep-inelastic electron–photon scattering using data recorded by the OPAL detector at LEP at nominal e+e− centre-of-mass energies from 183 to 209 GeV. The charm quarks have been identified by full reconstruction of charged D★ mesons using their decays into D0π with the D0 observed in two decay modes with charged particle final states, Kπ and Kπππ. The cross-section σD★ for production of charged D★ in the reaction e+e−→e+e−D★X is measured in a restricted kinematical region using two bins in Bjorken x, 0.0014<x<0.1 and 0.1<x<0.87. From σD★ the charm production cross-section σ(e+e−→e+e−cc̄X) and the charm structure function of the photon F2,cγ are determined in the region 0.0014<x<0.87 and 5<Q2<100 GeV2 . For x>0.1 the perturbative QCD calculation at next-to-leading order agrees perfectly with the measured cross-section. For x<0.1 the measured cross-section is 43.8±14.3±6.3±2.8 pb with a next-to-leading order prediction of 17.0+2.9−2.3 pb.

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.

The production of Δ++ baryons has been measured using 3.5 million hadronic Z0 decays collected with the OPAL detector at LEP. The production rate and fragmentation function are presented. A total of 0.22 ± 0.04 ± 0.04 Δ++ + (Δ)−− per hadronic Z0 decay is observed. The fragmentation function is found to be softer than that predicted by the JETSET and HERWIG Monte Carlo event generators. With this measurement of Δ++ production, at least one baryon of each strangeness level in the lightest baryon decuplet has now been measured at LEP.

We report on a measurement of the tau lepton polarization and its forward-backward asymmetry at the Z0 resonance using the OPAL detector. This measurement is based on analyses of the $$\tau \to e\bar v_e v_\tau $$ , $$\tau \to \mu \bar v_\mu v_\tau $$ , τ→π(K)v τ and τ→ρ v τ from a sample of 30663 e+e+→τ+τ- events collected during the years 1990 to 1992. Assuming that the tau lepton decays according to V-A theory, we measure the average τ polarization to be <P τ>=(−14.9±1.9±1.3)% and the τ polarization forward-backward asymmetry to be A =(−8.9±2.2±0.9)%, where the first error is statistical and the second systematic. These results are consistent with the hypothesis of lepton universality. When combined under the assumption of universality, our results can be interpreted as a measurement of sin2 θ eff lept =0.2321±0.0023 within the context of the Standard Model.

The average lifetime of baryons containing a b quark has been measured with the OPAL detector using data collected between 1990 and 1992. Semileptonically decaying b-flavoured baryons which produce a Λ are identified through the charge correlation of the λ and lepton. The decay point of the baryon is estimated by the λ-lepton vertex, and the observed distribution of decay lengths is fitted to determine the average lifetime. In the data sample consisting of 261 right-sign and 104 wrong-sign λ-lepton charge combinations, the average b baryon lifetime is found to be τ = 1.05+0.23-0.20±0.08 ps, where the first error is statistical and the second systematic.

The b quark forward–backward asymmetry has been measured using hadronic Z0 decays collected by the OPAL experiment at LEP. Z0→bb̄ decays were selected using a combination of secondary vertex and lepton tags, and the sign of the b quark charge was determined using an inclusive tag based on jet, vertex and kaon charges. The results, corrected to the quark level, are: AFBb=0.0582±0.0153±0.0012ats=89.50GeV,AFBb=0.0977±0.0036±0.0018ats=91.26GeV,AFBb=0.1221±0.0123±0.0025ats=92.91 GeV, where the first error is statistical and the second systematic in each case. Within the framework of the Standard Model, the result is interpreted as a measurement of the effective weak mixing angle for electrons of sin2θeff,eW=0.23205±0.00068.

We have studied the reactionsγγ→μ + μ − andγγ→τ + τ − by measuring the properties of events of the typese + e −→e + e − μ + μ − ande + e −→e + e − τ + τ − ate + e − centre-of-mass energies between 88 and 94 GeV. The data sample corresponds to an integratede + e − luminosity of 40.4 pb−1 collected by the OPAL detector at LEP. The QED structure functionF 2 is extracted from a sample of 1462γγ→μ + μ − events in which one photon is off the mass shell (single-tagged). TheQ 2 range for these massive photons is 4–400 GeV2, with an averageQ 2 of 8 GeV2. We have observed 48e + e −→e + e − μ + μ − events in which both final state electrons are detected. In the sample of single-tag events we have identified 34.9±6.7 events as due to the reactionγ;→τ + τ −, on an estimated background of 5.1±2.7 events. In all cases the measured event distributions agree with QED calculations.

Evidence is presented for the production of prompt Jψ mesons (not originating in b-hadron decays) in hadronic Z0 decays. Using a sample of 3.6 million hadronic events, 24 prompt Jψ candidates are identified from their decays into e+e− and μ+μ− pairs. The background is estimated to be 10.2 ± 2.0 events. The following branching ratio for prompt Jψ production is obtained: Br(Z0→ promptJψ+ X) = (1.9 +- 0.7 ± 0.5 ± 0.5) · 10−4, where the first error is statistical, the second systematic and the third error accounts for uncertainties in the prompt Jψ production mechanism.

We have measured the mean charged particle multiplicities separately for bb̄, cc̄ and light quark (uū,dd̄,ss̄) initiated events produced in e+e− annihilations at LEP. The data were recorded with the OPAL detector at eleven different energies above the Z0 peak, corresponding to the full statistics collected at LEP1.5 and LEP2. The difference in mean charged particle multiplicities for bb̄ and light quark events, δbl, measured over this energy range is consistent with an energy independent behaviour, as predicted by QCD, but is inconsistent with the prediction of a more phenomenological approach which assumes that the multiplicity accompanying the decay of a heavy quark is independent of the quark mass itself. Our results, which can be combined into the single measurement δbl=3.44±0.40(stat)±0.89(syst) at a luminosity weighted average centre-of-mass energy of 195 GeV, are also consistent with an energy independent behaviour as extrapolated from lower energy data.

From data collected around the Z resonance by the OPAL detector at LEP, a sample of B $_s^0$ decays was obtained using D $_s^-}\ell^+$ combinations, where the $D_s^-$ was fully reconstructed in the $\phi\pi^-, \mathrm{K}^{*0}\mathrm{K}^-$ and $\mathrm{K}^0_{\mathrm{S}}\mathrm{K}^-$ decay channels or partially reconstructed in the $\phi\ell^-\bar{\nu}(\mathrm{X})$ decay channel. These events were used to study $\mathrm{B_s^0}$ oscillation. The flavor (b or $\bar{b}$ ) at decay was determined from the lepton charge while the flavor at production was determined from a combination of techniques. The expected sensitivity of the experiment is 4.1 ps $^{-1}$ . The experiment was not able to resolve the oscillatory behavior, and we deduced that the B $_s^0$ oscillation frequency $\Delta m_s > 1.0$ ps $^{-1}$ at the 95% confidence level.

UML State Machines constitute an integral part of software behavior specification within the Unified Modeling Language (UML). The development of realistic software applications often results in complex and distributed models. Hence, potential errors can be very subtle and hard to locate for the developer. In this paper, we present a set of robustness rules that seek to avoid common types of errors by ruling out certain modelling constructs. Furthermore, adherence to these rules can improve model readability and maintainability. The robustness rules constitute a general Statechart style guide for different dialects, such as UML State Machines, Statemate, and Esterel Studio. Based on this style guide, an automated checking framework has been implemented as a plug-in for the prototypical Statechart modeling tool KIEL. Simple structural checks can be formulated in a compact, abstract manner in the Object Constraint Language (OCL). The framework can also incorporate checks that go beyond the expressiveness of OCL by implementing them in Java directly, which can also serve as a gateway to formal verification tools; we have exploited this to incorporate a theorem prover for more advanced checks. As a case study, we adopted the UML well-formedness rules; this confirmed that individual rules can easily be incorporated into the framework.

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.

A search for pair-produced scalar electron, muon and tau leptons has been performed using a data sample corresponding to an integrated luminosity of 10.1 pb−1 at a centre-of-mass energy of s=161 GeV collected with the OPAL detector at LEP. Such events would yield an acoplanar pair of leptons with significant missing energy in the final state. No excess of acoplanar lepton pair events has been observed in the data. Limits are presented on the production cross-sections of charged scalar leptons. Exclusion regions are also presented in the framework of the Minimal Supersymmetric Standard Model. A 95% C.L. lower limit on the right-handed selectron mass of 71.5 GeV is obtained for tan β ≥ 1.5, |μ| ≥ 200 GeV and mχ10 = 12.0 GeV. A 95% C.L. lower limit on the right-handed smuon mass of 51.0 GeV is obtained for mχ10 = 12.0 GeV, independent of tan β and μ.

A measurement of theτ lepton polarization and its forward-backward asymmetry at the Z0 resonance using the OPAL detector is described. The measurement is based on analyses of τ→ρντ, ττπ(K)ντ, $$\tau \to e\bar \nu _e \nu _\tau $$ , $$\tau \to \mu \bar \nu _\mu \nu _\tau $$ andτ→a1 ν τ decays from a sample of 89075 e+e−→τ + τ − candidates corresponding to an integrated luminosity of 117 pb−1. Assuming that theτ lepton decays according to V-A theory, we measure the averageτ polarization at √s=MZ to be 〈P〉=(−13.0±0.9±0.9)% and theτ polarization forward-backward asymmetry to be A pol FB =(−9.4±1.0±0.4)%, where the first error is statistical and the second systematic. These results are consistent with the hypothesis of lepton universality and, when combined, can be expressed as a measurement of sin2 θ eff lept =0.2334±0.0012 within the context of the Standard Model.

We have searched for excited states of charged and neutral leptons, e∗, μ∗, τ∗ and ν∗, in e+e− collisions at s=130 using the OPAL detector at LEP. No evidence for their existence was found. With the most common coupling assumptions, the final states from excited lepton pair production include ℓ+ℓ−γγ and ℓ+ℓ−WW (with the subsequent decay of the virtual W bosons). From the analysis of these final states, 95% confidence level lower mass limits of 66.5 GeV for e∗, 66.8 GeV for μ∗, 65.3 GeV for τ∗, 66.2 GeV for νe∗, 66.5 GeV for νμ∗ and 64.7 GeV for ντ∗ are inferred. From the analysis of γγ final states with missing energy and using alternative coupling assignments which permit photonic ν∗ decays, a 95% confidence level lower mass limit of 65.0 GeV for each ν∗ flavour is inferred. From the analysis of the ℓ+ℓ−γ, acoplanar lepton pair, and single γ final states expected from excited lepton single production, upper limits on fΛ (ratio of the coupling to the compositeness scale) are determined for excited lepton masses up to the kinematic limit.

The first prototypes of the production version of the Front-End Driver (FED) card for the CMS silicon strip tracker are about to be manufactured. The FEDs provide the off-detector processing of the tracker readout system. They digitise and zero-suppress the multiplexed analogue optical data sent on each Level-1 trigger from the ondetector APV25 pipeline chips. This paper outlines the design and describes in detail the implementation of the 96 ADC channel, 9U VME64x form factor card. In total, 440 FEDs will be required to readout the silicon strip tracker.

Searches for unstable neutral and charged heavy leptons (L0, L±) have been performed using a data sample of 2.6 pb−1 at a centre-of-mass energy of s=130 GeV and 2.6 pb−1 at 136 GeV collected with the OPAL detector at LEP during November 1995. No candidate event was observed. If an unstable Dirac neutral heavy lepton L0 decays only into eW∗, μW∗ or τW∗, the lower limits on its mass at 95% C.L. are 62.5 GeV, 63.0 GeV and 57.4 GeV, respectively. The limits are modified for a Majorana L0 to 51.4 GeV, 52.2 GeV and 44.2 GeV, respectively. For charged heavy leptons, a lower mass limit of 64.5 GeV at 95% C.L. was obtained, if L± decays into a stable heavy neutrino νl and W∗±, and ml± − mνl > 10 GeV. If L± decays through lepton flavour mixing into a massless neutrino νℓ and W∗±, the lower limit on ml± was determined to be 63.9 GeV at 95% C.L.

Reports experiments in which the variation in photon and neutron sensitivity of A-150 plastic tissue-equivalent ionisation chambers was studied by placing different ionisation chambers under vacuum, thus attempting to remove all water from the A-150 plastic. For all the ionisation chambers there were no significant problems introduced due to the absorption of water into the A-150 electrodes.

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.

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.

From a sample of 146900 hadronicZ 0 decays recorded by the OPAL detector at LEP, we have studied the azimuthal correlations of particles in hadronic events. It is expected that these correlations are sensitive to interference effects in QCD. We have compared the data to QCD Monte Carlo models which include and which do not include interference effects. We find that the distributions of azimuthal correlations are not reproduced by the parton shower models we have tested unless interference effects are included, no matter which hadronisation scheme is used.

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.

We describe measurements of inclusive branching ratios of the tau lepton to KS0 and charged K∗ (892) using Z0→τ+τ− candidates collected with the OPAL detector at LEP during 1990–1992. From a total of 61 854 tau candidates we find: Br(τ−→KS0X−ντ) = 0.97 ± 0.09(stat.) ± 0.06(syst.) %Br(τ−→K∗−≥0h0ντ) = 1.94 ± 0.27(stat.) ± 0.15(syst.) % where X− refers to any configuration of particles with charge −1 and h0 is any neutral hadron other than the KS0.

The string effect in QCD is investigated using data from the OPAL detector at LEP. By comparing the charged particle flow in three-jet multihadronic events with that in events with two jets and a hard isolated photon, the sensitivity to particular models is reduced. A comparison with various Monte Carlo models is presented. The difference in particle flows in the interquark region is found to be well reproduced by a leading order calculation of soft gluon emission, in the spirit of the Local Parton Hadron Duality hypothesis.

A pretrigger system is described for running the OPAL detector at the LEP e+ e− collider with more bunches than originally foreseen. A large number of low threshold pretrigger signals are formed by several independent components of the detector, and combined by a custom-built VME-based central pretrigger logic. Flexibility, high efficiency and high redundancy in all physics channels are all achieved with low additional deadtime, without any compromise to the trigger performance.

Using data recorded at centre-of-mass energies around 183 and 189 GeV with the OPAL detector at LEP, the fundamental coupling of the charm quark to the W boson has been studied. The ratio RcW≡Γ(W→cX)/Γ(W→hadrons) has been measured from jet properties, lifetime information, and leptons produced in charm decays. A value compatible with the Standard Model expectation of 0.5 is obtained: RcW=0.481±0.042(stat.)±0.032(syst.). By combining this result with measurements of the W boson total width and hadronic branching ratio, the magnitude of the CKM matrix element |Vcs| is determined to be |Vcs|=0.969±0.058.

The properties of hadronic Z0 decays with final state photons, measured with OPAL at LEP, have been compared with predictions from two different matrix element calculations ofO(αα s ). Two calculations, GNJETS and EEPRAD, have been investigated which use different schemes to restrict the phase space around the poles of the cross section. Assuming the E0-JADE jet definition, both calculations describe the data well in large regions of phase space fory cut values around 0.06. For very large and very small jet-photon masses some deviations from the predictions have been found, indicating the importance of higher order corrections. Significant differences between the calculations are only apparent in the predicted rate of 1-jet plus photon events. The rate is higher in GNJETS which reproduces the data better than EEPRAD.

A new technique has been used to determine a limit on theν τ mass using Z0→τ + τ − events selected from 140 pb−1 of data collected with the OPAL detector at LEP during the period 1990–1994. Using Z0→τ + τ − events in which bothτ-leptons decay to three charged particles, a novel likelihood analysis is applied to the 2-dimensional distribution of missing mass squared and missing energy, from which we obtain a limit ofMν gt <35.3 MeV at 95% confidence level. Combining our new result with OPAL’s previous published measurement fromτ→ 5πν τ decays, we obtain the new 95% confidence level limit ofMν τ <29.9 MeV.

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.

This document specifies the functionality which will be required of the final Front End Drivers (FEDs) for the Tracker, and explains the motivation behind each of these requirements. Furthermore, it lists all the input/output signals which these FEDs must deal with. Also discussed is the issue of what subset of this functionality will be in the first version of the final FED (FF1), which will be used for rod/petal testing in 2002. Not addressed in this document are details of the implementation. This will be described in a separate ‘FED Specifications’ document.

Three-jet variables constructed from multi-hadronic events produced byZ 0 decays are compared to theoretical calculations assuming a vector gluon or a hypothetical scalar gluon. The data yield conclusive direct evidence for the former case. The distributions of the reduced energy of the second-most energetic jet and of the cosine of the Ellis-Karliner angle are chosen to demonstrate this effect.