Showing papers by "Gregor Herten published in 1993"

Measurements of\(B^0 - \bar B^0 \) mixing,\({{\Gamma (Z^0 \to b\bar b)} \mathord{\left/ {\vphantom {{\Gamma (Z^0 \to b\bar b)} {\Gamma (Z^0 \to hadrons)}}} \right. \kern-\nulldelimiterspace} {\Gamma (Z^0 \to hadrons)}}\) and semileptonic branching ratios forb-flavoured hadrons in hadronicZ0 decays

Abstract: From a sample of about 450 000 hadronicZ0 decays, measurements of the average\(B^0 - \bar B^0 \) mixing parameter, χ, the branching fraction ofZ0 bosons into hadrons containing bottom quarks,\({{\Gamma (Z^0 \to b\bar b)} \mathord{\left/ {\vphantom {{\Gamma (Z^0 \to b\bar b)} {\Gamma (Z^0 \to hadrons)}}} \right. \kern- ulldelimiterspace} {\Gamma (Z^0 \to hadrons)}}\), the average semileptonic branching ratios for such hadrons,B(b→l) andB(b→c→l), and the mean scaled energy of these hadrons, 〈xE〉, are presented. The measurements were obtained using a simultaneous fit to single-lepton and dilepton events collected with the OPAL detector at LEP, including both electrons and muons. The results are where the errors are statistical and systematic, respectively, in each case. The result forB)b→c→l) excludes decays of the typeb→→l.

Measurement of the average b hadron lifetime in Z 0 decays

Abstract: A sample of 2610 electron candidates and 2762 muon candidates identified in hadronicZ0 decays has been used to measure the averageb hadron lifetime. These data were recorded with the OPAL detector during 1990 and 1991. Maximum likelihood fits to the distributions of the lepton impact parameters yield an averageb hadron lifetime of $$\tau _b = 1523 \pm 34 \pm 38fs$$ , 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 theb hadrons produced inZ0 decays.

A study of muon pair production and evidence for tau pair production in photon-photon collisions at LEP

Abstract: 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.

A measurement of the forward-backward asymmetry of {Mathematical expression} and {Mathematical expression} at centre of mass energies on and near the Z0 peak using D*± mesons

Abstract: A measurement is presented of the forwardbackward asymmetry of the processes\(e^ + e^ + \to c\bar c\) and\(e^ + e^ + \to b\bar b\) at centre of mass energies near 91 GeV Decays of theZ0 into charm and bottom quarks are identified using 6507 chargedD* mesons, reconstructed in the decaysD*±→D0π±→(K∓π±)π±,D*±→D0π±→(K∓π±π0)π± andD*±→D0π±→(K∓π±π∓π±)π± Thec quark asymmetry on theZ0 peak is measured to be $$A_{FB}^c = 0052 \pm 0028(stat) \pm 0012(sys)$$ The energy dependence of the asymmetry has been investigated at energies close to theZ0 peak Consistency with the predictions of the Standard Model is found

The forward-backward asymmetry of {Mathematical expression} using leptons in hadronic Z0 decays

Abstract: The forward-backward asymmetries of $$e^ + e^ - \to Z^0 \to b\bar b and e^ + e^ - \to Z^0 \to c\bar c$$ have been measured by the OPAL Collaboration using samples of hadronicZ 0 decays in which electron or muon candidates were observed. The asymmetries were measured simultaneously in a two parameter fit, which used the distributions of the track momentum and transverse momentum component with respect to the associated jet to distinguish lepton candidates from different sources. From a sample of 360 000 hadronic events with centre-of-mass energies within ±0.5 GeV of theZ 0 mass and mean energy 91.24. GeV, the values obtained for the $$b\bar b$$ asymmetry before and after correcting, for the effect of $$B^0 \overline {B^0 } $$ mixing, and for the $$c\bar c$$ asymmetry are, respectively: $$\begin{gathered} A_{FB}^{b,mix} = 0.070 \pm 0.014(stat) \pm 0.005(sys), \hfill \\ A_{FB}^b = 0.092 \pm 0.018(stat) \pm 0.007(sys) \hfill \\ \pm 0.003(mix), \hfill \\ A_{FB}^c = 0.014 \pm 0.030(stat) \pm 0.020(sys). \hfill \\ \end{gathered} $$ The measurement of the $$b\bar b$$ asymmetry is confirmed by a measurement using only leptons with high transverse momentum. This sample is highly enriched in semileptonic decays ofb hadrons. Measurements using high transverse momentum leptons were also performed using samples of events with centre-of-mass energies further from theZ 0 mass. After correcting for the effect of $$B^0 \overline {B^0 } $$ mixing, the results are: $$\begin{gathered} A_{FB}^b (\langle \sqrt s \rangle = 89.66GeV) \hfill \\ = 0.071 \pm 0.054(stat) \pm 0.007(sys) \pm 0.002(mix), \hfill \\ A_{FB}^b (\langle \sqrt s \rangle = 92.75GeV) \hfill \\ = 0.131 \pm 0.047(stat) \pm 0.012(sys) \pm 0.004(mix). \hfill \\ \end{gathered} $$

A measurement of Γ (Z0→b {Mathematical expression}/Γ(Z0→hadrons) using an impact parameter technique

Abstract: The fractional partial width of theZ0 tob quarks,\(\Gamma _{b\bar b} /\Gamma _{had} \), has been measured by OPAL using an impact parameter technique. The method has been developed using 130 000 hadronic events collected by OPAL in 1990. We find: $$\frac{{\Gamma _{b\bar b} }}{{\Gamma _{had} }} = 0.222 \pm 0.007(stat) \pm 0.008(sys).$$ The measurement assumes the relative rates of theZ0 touds andc quarks given by the Standard Model. Varying the charm fraction from the Standard Model value of\(\Gamma _{c\bar c} /\Gamma _{had} = 0.171\) changes the result by an additional\(\Delta \Gamma _{b\bar b} /\Gamma _{b\bar b} = - 0.135 \times \Delta \Gamma _{c\bar c} /\Gamma _{c\bar c} \).

A test of the flavour independence of the strong interaction for five flavours

Abstract: We report on a measurement of the ratio of the strong coupling constants\(\alpha _s (M_{Z^0 } )\) for various data samples with different quark flavour compositions obtained frome+e− annihilation at LEP. We select events with muons, electrons, andD*± mesons from charm and bottom decays, events with highly energeticKs0 mesons originating predominantly from strange quarks, and events with highly energetic stable charged particles from a combination of up, down and strange quarks. From the jet production rates in these events we obtain from these samples the relative value of αs for the individual quark flavours as $$\begin{gathered} \frac{{\alpha _s^b }}{{\alpha _s^{udsc} }} = 1.017 \pm 0.036,\frac{{\alpha _s^c }}{{\alpha _s^{udsb} }} = 0.918 \pm 0.115, \hfill \\ \frac{{\alpha _s^s }}{{\alpha _s^{udcb} }} = 1.158 \pm 0.164,\frac{{\alpha _s^{uds} }}{{\alpha _s^{cb} }} = 1.038 \pm 0.221, \hfill \\ \end{gathered}$$ with quadratically combined statistical and systematic errors. We also compare the αs values for quarks of diffrent weak isospin, charge and generation. No evidence for a dependence of αs on any of these quark properties is observed. Finally, combining all samples and determining the αsf/αsincl ratio for each flavour, we do not find any dependence of the strong coupling constant on flavour.