Previous OPAL Research Articles

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.).

Measurement of the hadronic photon structure function F2γ at LEP2

The hadronic structure function of the photon F2γ(x,Q2) is measured as a function of Bjorken x and of the photon virtuality Q2 using deep-inelastic scattering data taken by the OPAL detector at LEP at e+e− centre-of-mass energies from 183 to 209 GeV. Previous OPAL measurements of the x dependence of F2γ are extended to an average Q2 of 〈Q2〉=780 GeV2 using data in the kinematic range 0.15<x<0.98. The Q2 evolution of F2γ is studied for 12.1<〈Q2〉<780 GeV2 using three ranges of x. As predicted by QCD, the data show positive scaling violations in F2γ with F2γ(Q2)/α=(0.08±0.02+0.05−0.03)+(0.13±0.01+0.01−0.01)lnQ2, where Q2 is in GeV2, for the central x region 0.10–0.60. Several parameterisations of F2γ are in qualitative agreement with the measurements whereas the quark-parton model prediction fails to describe the data.

Measurement of the mass and width of the W boson in e +e − collisions at 189 GeV

The mass and width of the W boson are determined in e +e − collisions at LEP using 183 pb −1 of data recorded at a centre-of-mass energy s =189 GeV with the OPAL detector. The invariant mass distributions from 970 W +W −→qqqq and 1118 W +W −→qqℓ ν ℓ candidate events are used to measure the mass of the W boson, M W=80.451±0.076 (stat.)±0.049 (syst.) GeV. A direct measurement of the width of the W boson gives Γ W=2.09±0.18 (stat.)±0.09 (syst.) GeV. The results are combined with previous OPAL results from 78 pb −1 of data recorded with s from 161 to 183 GeV, to obtain: M W=80.432±0.066 (stat.)±0.045 (syst.) GeV, Γ W=2.04±0.16 (stat.)±0.09 (syst.) GeV. The consistency of the direct measurement of M W with that inferred from other measurements of electroweak parameters provides an important test of the Standard Model of electroweak interactions.

Measurement of the low- x behaviour of the photon structure function

The photon structure function F γ 2( x, Q 2) has been measured using data taken by the OPAL detector at e +e − centre-of-mass energies of 91 GeV, 183 GeV and 189 GeV, in Q 2 ranges of 1.5–30.0 GeV 2 (LEP1), and 7.0–30.0 GeV 2 (LEP2), probing lower values of x than ever before. Since previous OPAL analyses, new Monte Carlo models and new methods, such as multi-variable unfolding, have been introduced, reducing significantly the model dependent systematic errors in the measurement.

Measurement of the low-x behaviour of the photon structure function $F_2^\gamma$

The photon structure function $F_2^\gamma(x,Q^2)$ has been measured using data taken by the OPAL detector at $\rm e^+e^-$ centre-of-mass energies of 91 GeV, 183 GeVand 189 \GeV, in $Q^2$ ranges of 1.5–30.0 GeV $^2$ (LEP1), and 7.0–30.0 GeV $^2$ (LEP2), probing lower values of x than ever before. Since previous OPAL analyses, new Monte Carlo models and new methods, such as multi-variable unfolding, have been introduced, reducing significantly the model dependent systematic errors in the measurement. The results do not conclusively prove, but are completely consistent with, the presence of a rise in $F_2^\gamma$ at low-x as expected from QCD.

W +W − production cross section and W branching fractions in e +e − collisions at 189 GeV

From a data sample of 183 pb −1 recorded at a center-of-mass energy of s =189 GeV with the OPAL detector at LEP, 3068 W-pair candidate events are selected. Assuming Standard Model W boson decay branching fractions, the W-pair production cross section is measured to be σ WW=16.30±0.34(stat.)±0.18(syst.) pb. When combined with previous OPAL measurements, the W boson branching fraction to hadrons is determined to be 68.32±0.61(stat.)±0.28(syst.)% assuming lepton universality. These results are consistent with Standard Model expectations.

A measurement of the product branching ratio $f ({\rm b} \to \Lambda_{\rm b}) \cdot{\rm BR} (\Lambda_{\rm b} \to \Lambda {\rm X})$ in ${\rm Z}^0$ decays

The product branching ratio, \(f ({\rm b} \to \Lambda_{\rm b}) \cdot{\rm BR} (\Lambda_{\rm b} \to \Lambda{\rm X})\), where \(\Lambda_{\rm b}\) denotes any weakly-decaying b–baryon, has been measured using the OPAL detector at LEP. \(\Lambda_{\rm b}\)'s are selected by the presence of energetic \(\Lambda\) particles in bottom events tagged by the presence of displaced secondary vertices. A fit to the momenta of the \(\Lambda\) particles separates signal from B meson and fragmentation backgrounds. The measured product branching ratio is \(\) Combined with a previous OPAL measurement, one obtains \(\)

Search for scalar top and scalar bottom quarks at [formula omitted] = 189 GeV at LEP

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 Z 0 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.

Measurement of the W mass and width in e +e − collisions at 183 GeV

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 −→ q q q q and 300 W + W −→ q q ℓ ν ℓ candidate events are used to obtain a measurement of the mass of the W boson, M W=80.39±0.13(stat.)±0.05(syst.) GeV, assuming the Standard Model relation between M W and Γ W. A second fit provides a direct measure of the width of the W boson and gives Γ W=1.96±0.34(stat.)±0.20(syst.) GeV. These results are combined with previous OPAL results to obtain M W=80.38±0.12(stat.)±0.05(syst.) GeV and Γ W=1.84±0.32(stat.)±0.20(syst.) GeV.

Determination of the production rate of D*0 mesons and of the ratio V/(V+P) in $$Z^0 \to c\bar c$$ decays

In e+e2212 collisions at centre-of-mass energies around 91 GeV, D*0 mesons have been reconstructed using data collected with the OPAL detector at LEP. The hadronisation fraction has been measured to be f(c→D*0)=0.218±0.054±0.045±0.007, where the errors correspond to the statistical and systematic errors specific to this analysis, and to systematic uncertainties from externally measured branching fractions, respectively. Together with previous OPAL measurements of the hadronisation fractions of other charmed mesons, this value is used to investigate the relative production of observed vector and pseudoscalar charmed mesons in \(Z^0 \to c\bar c\) decays. The production ratio is determined to be PeffV = V/(V+P)=0.57±0.05. The relative primary production of vector and pseudoscalar mesons, Pvrim, is studied in the context of the production and decay of orbitally excited charmed resonances. The first measurement of the inclusive Ds*+ production rate in hadronic Z0 decays is presented.

Search for the standard model Higgs boson in ${\rm e^+e^-}$ collisions at $\sqrt s = 161$ –172 GeV

This paper describes a search for the Standard Model Higgs boson using data from \({\rm e^+e^-}\) collisions collected at center-of-mass energies of 161, 170 and 172 GeV by the OPAL detector at LEP. The data collected at these energies correspond to integrated luminosities of 10.0, 1.0 and 9.4 pb\(^{-1}\), respectively. The search is sensitive to the main final states from the process in which the Higgs boson is produced in association with a fermion anti-fermion pair, namely four jets, two jets with missing energy, and two jets produced together with a pair of electron, muon or tau leptons. One candidate event is observed, in agreement with the Standard Model background expectation. In combination with previous OPAL searches at center-of-mass energies close to the Z\(^0\) resonance and the revised previous OPAL searches at 161 GeV , we derive a lower limit of 69.4 GeV for the mass of the Standard Model Higgs boson at the 95% confidence level.

Search for chargino and neutralino production at

A search for charginos and neutralinos, predicted by supersymmetric theories, has been performed using a data sample of $10.3$ pb $^{-1}$ at centre-of-mass energies of $\sqrt{s} = $ 170 and 172 GeV with the OPAL detector at LEP. No evidence for these particles has been found. The results are combined with those from previous OPAL chargino and neutralino searches at lower energies to obtain limits. Exclusion regions at 95% C.L. of parameters of the Minimal Supersymmetric Standard Model are determined. Within this framework, for $\tan\beta \ge 1.0$ , lower mass limits are placed on the lightest chargino and the three lightest neutralinos. The 95% C.L. lower mass limit on the lightest chargino, assuming that it is heavier than the lightest neutralino by more than 10 GeV, is 84.5 GeV for the case of a large universal scalar mass ( $m_0 >$ 1 TeV) and 65.7 GeV for the smallest $m_0$ compatible with current limits on the sneutrino mass and slepton cross-sections. The lower limit on the lightest neutralino mass at 95% C.L. for $\tan \beta \geq 1.0$ is 24.7 GeV for $m_0 = 1$ TeV and 13.3 GeV for the minimum $m_0$ scenario. These mass limits are higher for increasing values of $\tan\beta$ . The interpretation of the limits in terms of gluino and scalar quark mass limits is also given.

Search for the Standard Model Higgs boson in e +e − collisions at [formula omitted

This letter describes a search for the Standard Model Higgs boson using data from e +e − collisions collected at a centre-of-mass energy of 161 GeV by the OPAL detector at LEP. The ata correspond to an integrated luminosity of 10.0 pb −1. The search is sensitive to the main final states from the process where the Higgs boson is produced in association with a fermion anti-fermion pair, namely four jets, two jets with missing energy, and two jets produced together with a pair of electrons, muons or tau leptons. Two candidate events are observed, in agreement with Standard Model background expectations. In combination with previous OPAL searches at centre-of-mass energies close to the Z 0 resonance, we derive a lower limit of 65.0 GeV for the mass of the Standard Model Higgs boson, at the 95% confidence level.

Improved measurement of the lifetime of the τ lepton

A new measurement of the τ lifetime is presented. It uses data collected with the Opal detector during 1994, which almost doubles the size of the Opal τ sample. Two statistically independent techniques are used: an impact parameter analysis of one-prong decay tracks and a fit to the decay length distribution of three-prong decays. The lifetime obtained from the 1994 data by combining the results of these methods is τ τ = 289.7 ± 2.5 (stat)± 1.5 (sys) fs. When combined with the previous Opal τ lifetime measurement the improved τ lifetime is τ τ = 289.2 ± 1.7 (stat.) ± 1.2 (sys.) fs.

Search for the minimal standard model Higgs boson

A search for the Minimal Standard Model Higgs boson (H 0) 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 ν ν) H 0, H 0→ q q ̄ 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.

Search for the minimal standard model Higgs boson in e +e − collisions at LEP

A search for the minimal standard model Higgs boson (H 0) 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 −, v v or τ +τ −) H 0, H 0→( q q or τ +τ −) for Higgs boson masses above 25 GeV/ c 2. 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< m H 0 <44GeV/ c 2 at the 95% confidence level.