OPAL Detector Research Articles

A measurement of semileptonic B decaysto narrow orbitally-excited charm mesons

The decay chain b -> .hivin.B -> D**0l-nX, D**0 -> D*+p-, D0p+, D0 -> (Kp or K3p) is identified in a sample of 3.9 million hadronic Z decays collected with the OPAL detector at LEP. The branching ratio BR(b -> .hivin.B) x BR (.hivin.B -> D01l-nX) x BR(D01 -> D*+p-) is measured to be (2.64 +- 0.79(stat)+-0.39(syst)) x 10-3 for the JP=1+ (D01) state. For decays into the JP=2+ (D*01) state, an upper limit of 1.4 x 10-3 is placed on the branching ratio at the 95% confidence level. [on SciFinder (R)]

Tests of the standard model and constraints on new physics from measurements of fermion-pair production at 189-209 GeV at LEP

Production of events with hadronic and leptonic final states has been measured in \({\rm e^+e^-}\) collisions at centre-of-mass energies of 130–172 GeV, using the OPAL detector at LEP. Cross-sections and leptonic forward-backward asymmetries are presented, both including and excluding the dominant production of radiative Z\(\gamma\) events, and compared to Standard Model expectations. The ratio \(R_{\rm b}\) of the cross-section for \({\rm b\bar b}\) production to the hadronic cross-section has been measured. In a model-independent fit to the Z lineshape, the data have been used to obtain an improved precision on the measurement of \(\gamma\)-Z interference. The energy dependence of \(\alpha_{\mathrm{em}}\) has been investigated. The measurements have also been used to obtain limits on extensions of the Standard Model described by effective four-fermion contact interactions, to search for \(t\)-channel contributions from new massive particles and to place limits on gaugino pair production with subsequent decay of the gaugino into a light gluino and a quark pair.

Search for stable and long-lived massive charged particles in e+e− collisions at [formula omitted] GeV

A search for stable and long-lived massive particles of electric charge |Q/e|=1 or fractional charges of 2/3, 4/3, and 5/3 is reported using data collected by the OPAL detector at LEP, at centre-of-mass energies from 130 to 209 GeV. These particles are assumed to be pair-produced in e+e− collisions and not to interact strongly. No evidence for the production of these particles was observed. Model-independent upper limits on the production cross-section between 0.005 and 0.028 pb have been derived for scalar and spin-1/2 particles with charge ±1. Within the framework of the Constrained Minimal Supersymmetric Standard Model (CMSSM), this implies a lower limit of 98.0 (98.5) GeV on the mass of long-lived right- (left-)handed scalar muons and scalar taus. Long-lived charged heavy leptons and charginos are excluded for masses below 102.0 GeV. For particles with fractional charge ±2/3, ±4/3 and ±5/3, the upper limit on the production cross-section varies between 0.005 and 0.020 pb. All mass and cross-section limits are derived at the 95% confidence level and are valid for particles with lifetimes longer than 10−6 s.

Measurement of charged current triple gauge boson couplings using W pairs at LEP

Triple gauge boson couplings are measured from W-pair events recorded by the OPAL detector at LEP at centre-of-mass energies of 183 - 209 GeV with a total integrated luminosity of 680 pb(-1). Only CP-conserving couplings are considered and SU(2) x U(1) relations are used, resulting in four independent couplings, k(gamma), g(1)(z), lambdagamma and g(5)(z). Determining each coupling in a separate fit, assuming the other couplings to take their Standard Model values, we obtain k(gamma) = 0.88(-0.08)(+0.09), g(1)(z) = 0.987 (-0.033),(+0.034) lambda(gamma) = -0.060(-0.033)(+0.034) and g(5)(z) = -0.04(-0.12)(+0.13), where the errors include both statistical and systematic uncertainties. Fits are also performed allowing some of the couplings to vary simultaneously. All results are consistent with the Standard Model predictions.

Inclusive analysis of the b quark fragmentation function in Z decays at LEP

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)

Search for nearly mass-degenerate charginos and neutralinos at LEP

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.

Test of non-commutative QED in the process e+e−→γγ at LEP

Non-commutative QED would lead to deviations from the Standard Model depending on a new energy scale ΛNC and a unique direction in space defined by two angles η and ξ. In this analysis, η is defined as the angle between the unique direction and the rotation axis of the earth. The predictions of a tree level calculation for the process e+e−→γγ are evaluated for the specific orientation of the OPAL detector and compared to the measurements. Distributions of the polar and azimuthal photon angles are used to extract limits on the energy scale ΛNC depending on the model parameter η. It is shown that the time dependence of the total cross-section could be used to determine the model parameter ξ if there were a detectable signal. This is the first experimental study of non-commutative QED at an e+e− collider.

A measurement of the [formula omitted] branching ratio

The ρ − → μ − ν μν τ branching ratio has been measured using data collected from 1990 to 1995 by the OPAL detector at the LEP collider. The resulting value of B( τ − → μ − τ μτ τ) = 0.1734 ± 0.0009(stat) ± 0.0005(syst) has been used in conjunction with other OPAL measurements to test lepton universality, yielding the coupling constant ratios g μ / g e = 1.0005±0.0043 and g τ / g e = 1.0031 ± 0.0047, in good agreement with the standard Model prediction of unity, and also to determine a value for the Michel parameter η = 0.004± 0.036. This is subsequently used to find a model-dependent limit of the mass for the charged Higgs boson, m H ± > 1.30 tan β, in the Minimal Supersymmetric Standard Model framework.

Measurements of αs from QCD event shapes at OPAL and LEP

New preliminary measurements of the strong coupling as are presented, using hadronic decays observed in the OPAL detector at LEP. Previous OPAL measurements are updated to incorporate the latest theoretical predictions. New techniques for the estimation of theoretical uncertainties are discussed. The preliminary combined measurement, from all event shape fits performed by the four LEP collaborations at all centre-of-mass energies is α s = 0.1198 ± 0.0003 (stat.) ± 0.0047 (syst.).

Search for charged Higgs bosons with the OPAL detector at LEP

Search for charged Higgs bosons with the OPAL detector at LEP

Measurement of the cross-section for the process $\gamma\gamma\to p\bar{p}$ at $\sqrt{s_{ee}} = 183-189 $ GeV at LEP

The exclusive production of proton-antiproton pairs in the collisions of two quasi-real photons has been studied using data taken at \(\sqrt{s_{ee}} = 183 \) GeV and 189 GeV with the OPAL detector at LEP. Results are presented for \(p\bar{p}\) invariant masses, W, in the range 2.15<W<3.95 GeV. The cross-section measurements are compared with previous data and with recent analytic calculations based on t he quark-diquark model.

Decay-mode independent searches for new scalar bosons with the OPAL detector at LEP

This paper describes topological searches for neutral scalar bosons S0 produced in association with a Z0 boson via the Bjorken process \(e^+e^- \to{\rm S}^0{}{\rm Z}^0\) at centre-of-mass energies of 91 GeV and 183-209 GeV. These searches are based on studies of the recoil mass spectrum of \({\rm Z}^0 \to e^+e^-\) and \(\mu^+ \mu^-\) events and on a search for \({\rm S}^0{\rm Z}^0\) with \({\rm Z}^0 \to \nu\bar{\nu}\) and S\(^0 \to e^+ e^-\) or photons. They cover the decays of the S0 into an arbitrary combination of hadrons, leptons, photons and invisible particles as well as the possibility that it might be stable.

Improvement measurements of the b quark mass at LEP

An improved determination of the b-quark mass at the M z scale, m b(M z) , has been obtained by the Delphi and Opal experiments at LEP. The normalized three-jet rate of b-quarks with respect to light quarks, R 3 bl , has been measured with different jet clustering algorithms and compared with the massive NLO calculations to extract m b(M z) . For Delphi, the increase in statistics and a better understanding of the flavour tagging, hadronization process and the gluon splitting rates leads to a precision of 380 MeV. The effect of the b-quark mass on the four-jet rate has also been studied by Delphi.

Bose–Einstein correlations of π0 pairs from hadronic Z0 decays

We observe Bose–Einstein correlations in π0 pairs using back-to-back two jet hadronic events from Z0 decays in the data sample collected by the OPAL detector at LEP 1 from 1991 to 1995. Using a static Gaussian picture for the pion emitter source, we obtain the chaoticity parameter λ=0.55±0.10±0.10 and the source radius R=(0.59±0.08±0.05) fm. According to the JETSET and HERWIG Monte Carlo models, the Bose–Einstein correlations in our data sample largely connect π0s originating from the decays of different hadrons. Prompt pions formed at string break-ups or cluster decays only form a small fraction of the sample.

Search for the Standard Model Higgs boson with the OPAL detector at LEP

This paper summarises the search for the Standard Model Higgs boson in e+e- collisions at centre-of-mass energies up to 209 GeV performed by the OPAL Collaboration at LEP. The consistency of the data with the background hypothesis and various Higgs boson mass hypotheses is examined. No indication of a signal is found in the data and a lower bound of 112.7 Gev/c2 is obtained on the mass of the Standard Model Higgs boson at the 95% CL.

Measurement of neutral-current four-fermion production at LEP2

Four-fermion final states q qe + e − and q q μ +μ − from neural current interactions in e +e − collisions are studied in the OPAL detector at LEP at centre-of-mass energies from 183 GeV to 209 GeV. The data analysed corresponds to a total integrated luminosity of about 650 pb −1 recorded from 1997 to 2000. Corresponding to the acceptance of the OPAL detector, a signal definition is applied requiring both leptons to have a scattering angle |cos θ | < 0.95. Further requirements are made on the invariant masses of the fermions pairs. The extracted cross-sections for the processes e +e − → q qe +e − and e +e − → q qμ +μ − are consistent with the expectations from the Standard Model.

Measurement of the mass of the W boson in ee collisions using the fully leptonic channel

A novel method of determining the mass of the W boson in the channel is presented and applied to 667 pb-1 of data recorded at center-of-mass energies in the range 183-207 GeV with the OPAL detector at LEP. The measured energies of charged leptons and the results of a new procedure based on an approximate kinematic reconstruction of the events are combined to give: where the first error is statistical and the second is systematic. The systematic error is dominated by the uncertainty on the lepton energy, which is calibrated using data, and the parameterization of the variables used in the fitting, which is obtained using Monte Carlo events. Both of these are limited by statistics.

Multi-photon production in ee collisions at $\sqrt{s} = $ 181-209 GeV

The process $\mathrm{e}^+\mathrm{e}^- \to\gamma\gamma(\gamma)$ is studied using data collected by the OPAL detector at LEP between the years 1997 and 2000. The data set corresponds to an integrated luminosity of 672.3 pb-1 at centre-of-mass energies lying between 181 GeV and 209 GeV. Total and differential cross-sections are determined and found to be in good agreement with the predictions of QED. Fits to the observed angular distributions are used to set limits on parameters from several models of physics beyond the Standard Model such as cut-off parameters, contact interactions of the type $\mathrm{e}^+\mathrm{e}^- \gamma\gamma$ , gravity in extra spatial dimensions and excited electrons. In events with three photons in the final state the mass spectrum of photon pairs is investigated. No narrow resonance $X\to\gamma\gamma$ is found and limits are placed on the product of the $\rm X \gamma$ production cross-section and branching ratio.

A measurement of the [formula omitted] branching ratio

The τ−→μ−ν̄μντ branching ratio has been measured using data collected from 1990 to 1995 by the OPAL detector at the LEP collider. The resulting value of B(τ−→μ−ν̄μντ)=0.1734±0.0009(stat)±0.0006(syst) has been used in conjunction with other OPAL measurements to test lepton universality, yielding the coupling constant ratios gμ/ge=1.0005±0.0044 and gτ/ge=1.0031±0.0048, in good agreement with the Standard Model prediction of unity. A value for the Michel parameter η=0.004±0.037 has also been determined and used to find a limit for the mass of the charged Higgs boson, mH±>1.28tanβ, in the Minimal Supersymmetric Standard Model.

Charged particle multiplicities in heavy and light quark initiated events above the Z0 peak

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.