Showing papers by "D. M. Strom published in 1999"

Measurement of the strong coupling constant αs and the vector and axial-vector spectral functions in hadronic tau decays the opal collaboration

Abstract: The spectral functions of the vector current and the axial-vector current have been measured in hadronic $\tau$ decays using the OPAL detector at LEP. Within the framework of the Operator Product Expansion a simultaneous determination of the strong coupling constant $\alpha_{\rm s}$ , the non-perturbative operators of dimension 6 and 8 and of the gluon condensate has been performed. Different perturbative descriptions have been compared to the data. The Contour Improved Fixed Order Perturbation Theory gives $\alpha_{\rm s}(m_\tau^2) = 0.348 \pm 0.009_{\rm exp} \pm 0.019_{\rm theo}$ at the $\tau$ -mass scale and $\alpha_{\rm s}(m^2_{\rm Z}) = 0.1219 \pm 0.0010_{\rm exp} \pm 0.0017_{\rm theo}$ at the ${\rm Z}^0$ -mass scale. The values obtained for $\alpha_{\rm s}(m^2_{\rm Z})$ using Fixed Order Perturbation Theory or Renormalon Chain Resummation are 2.3% and 4.1% smaller, respectively. The ‘running’ of the strong coupling between $s_0 \simeq 1.3 {\rm GeV}^2$ and $s_0 = m_\tau^2$ has been tested from direct fits to the integrated differential hadronic decay rate $R_\tau(s_0)$ . A test of the saturation of QCD sum rules at the $\tau$ -mass scale has been performed.

Search for Higgs bosons in e+e- collisions at 183 GeV

Abstract: The data collected by the OPAL experiment at \(\sqrt s=183\) GeV were used to search for Higgs bosons which are predicted by the Standard Model and various extensions, such as general models with two Higgs field doublets and the Minimal Supersymmetric Standard Model (MSSM). The data correspond to an integrated luminosity of approximately 54 pb\(^{-1}\). None of the searches for neutral and charged Higgs bosons have revealed an excess of events beyond the expected background. This negative outcome, in combination with similar results from searches at lower energies, leads to new limits for the Higgs boson masses and other model parameters. In particular, the 95% confidence level lower limit for the mass of the Standard Model Higgs boson is 88.3 GeV. Charged Higgs bosons can be excluded for masses up to 59.5 GeV. In the MSSM, \(m_{\rm h^0} > 70.5\) GeV and \(m_{\rm A^0} > 72.0\) GeV are obtained for \(\tan{\beta}>1\), no and maximal scalar top mixing and soft SUSY-breaking masses of 1 TeV. The range \(0.8 < \tan\beta < 1.9\) is excluded for minimal scalar top mixing and \(m_{\mathrm{top}} \le 175\) GeV. More general scans of the MSSM parameter space are also considered.

Measurements of flavour-dependent fragmentation functions in Z0→qq¯ events

Abstract: Fragmentation functions for charged particles in \({\rm Z}^0 \rightarrow {\rm q}\bar{\rm q}\) events have been measured for bottom (b), charm (c) and light (uds) quarks as well as for all flavours together. The results are based on data recorded between 1990 and 1995 using the OPAL detector at LEP. Event samples with different flavour compositions were formed using reconstructed \({\rm D}^{*\pm}\) mesons and secondary vertices. The \(\xi_{\rm p} =\ln(1/x_{\rm p})\) distributions and the position of their maxima \(\xi_0\) are also presented separately for uds, c and b quark events. The fragmentation function for b quarks is significantly softer than for uds quarks.

A measurement of R(b) using a double tagging method

Abstract: The fraction of ${\rm Z^0 \to b\overline b}$ events in hadronic ${\rm Z^0}$ decays has been measured by the OPAL experiment using the data collected at LEP between 1992 and 1995. The ${\rm Z^0 \to b\overline b}$ decays were tagged using displaced secondary vertices, and high momentum electrons and muons. Systematic uncertainties were reduced by measuring the b-tagging efficiency using a double tagging technique. Efficiency correlations between opposite hemispheres of an event are small, and are well understood through comparisons between real and simulated data samples. A value of \[ R_{\rm b} \equiv \frac{\sigma (\rm e^+e^- \rightarrow{\rm b\overline b} )} {\sigma (\rm e^+e^- \rightarrow hadrons)} = 0.2178 \pm 0.0011 \pm 0.0013 \] was obtained, where the first error is statistical and the second systematic. The uncertainty on $R_{\rm c}$ , the fraction of ${\rm Z^0 \to c \overline c}$ events in hadronic ${\rm Z^0}$ decays, is not included in the errors. The dependence on $R_{\rm c}$ is \[ \frac{\Delta R_{\rm b}}{R_{\rm b}} = -0.056 \frac{\Delta R_{\rm c}}{R_{\rm c}} , \] where $\Delta R_{\rm c}$ is the deviation of $R_{\rm c}$ from the value 0.172 predicted by the Standard Model. The result for $R_{\rm b}$ agrees with the value of $0.2155 \pm 0.0003$ predicted by the Standard Model.

W+W- production and triple gauge boson couplings at LEP energies up to 183 GeV

Abstract: A study of W-pair production in ${\rm e^+e^-}$ annihilations at LEP is presented, based on $877\,{\rm W^+W^-}$ candidates corresponding to an integrated luminosity of 57 pb $^{-1}$ at $\sqrt s=183$ GeV. Assuming that the angular distributions of the W-pair production and decay, as well as their branching fractions, are described by the Standard Model, the W-pair production cross-section is measured to be $15.43\pm 0.61 ({\rm stat.}) \pm 0.26 ({\rm syst.})$ pb. Assuming lepton universality and combining with our results from lower centre-of-mass energies, the W branching fraction to hadrons is determined to be $67.9 \pm 1.2 ({\rm stat.})\pm 0.5 ({\rm syst.})$ %. The number of W-pair candidates and the angular distributions for each final state ( ${\rm q \bar q} \bar \ell u_\ell$ , ${\rm q \bar q q \bar q}, \bar \ell u_\ell \ell^\prime \bar u_{\ell^\prime })$ are used to determine the triple gauge boson couplings. After combining these values with our results from lower centre-of-mass energies we obtain $\Delta \kappa_\gamma = 0.11^{+0.52}_{-0.37}$ , $\Delta g^{\rm z}_1 = 0.01^{+ 0.13}_{-0.12}$ and $\lambda = -0.10^{+0.13}_{-0.12}$ , where the errors include both statistical and systematic uncertainties and each coupling is determined by setting the other two couplings to the Standard Model value. The fraction of W bosons produced with a longitudinal polarisation is measured to be $0.242 \pm 0.091({\rm stat.}) \pm 0.023 ({\rm syst.})$ . All these measurements are consistent with the Standard Model expectations.

Measurement of the Λb0 lifetime using semileptonic decays

Abstract: We report a measurement of the Bs(0) lifetime in the semileptonic decay channel Bs(0) --> Ds- mu+ nuX (and its charge conjugate), using approximately 0.4 fb(-1) of data collected with the D0 detector during 2002-2004. Using 5176 reconstructed Ds- mu+ signal events, we have measured the Bs(0) lifetime to be tau(Bs(0))=1.398+/-0.044(stat)(-0.025)(+0.028)(syst) ps. This is the most precise measurement of the Bs(0) lifetime to date.

Intermittency and correlations in hadronic Z0 decays

Abstract: A multidimensional study of local multiplicity fluctuations and multiparticle correlations of hadrons produced in Z\(^0\) decays is performed. The study is based on the data sample of more than \(4\!\times\!10^6\) events recorded with the OPAL detector at LEP. The fluctuations and correlations are analysed in terms of the normalized scaled factorial moments and cumulants up to the fifth order. The moments are observed to have intermittency-like behaviour, which is found to be more pronounced with increasing dimension. The large data sample allows for the first time a study of the factorial cumulants in \({\rm e^+e^-}\) annihilation. The analysis of the cumulants shows the existence of genuine multiparticle correlations with a strong intermittency rise up to higher orders. These correlations are found to be stronger in higher dimensions. The decomposition of the factorial moments into lower-order correlations shows that the dynamical fluctuations have important contributions from genuine many-particle correlations. The Monte Carlo models Jetset 7.4 and Herwig 5.9 are found to reproduce the trend of the measured moments and cumulants but they underestimate the magnitudes. The results are found to be consistent with QCD jet formation dynamics, although additional contributions from other mechanisms cannot be excluded.

Search for anomalous photonic events with missing energy in e+e- collisions at √s = 130, 136 and 183 GeV

Abstract: Photonic events with large missing energy have been observed in $\mathrm e^+e^-$ collisions at centre-of-mass energies of 130, 136 and 183 GeV collected in 1997 using the OPAL detector at LEP. Results are presented for event topologies with a single photon and missing transverse energy or with an acoplanar photon pair. Cross-section measurements are performed within the kinematic acceptance of each selection. These results are compared with the expectations from the Standard Model process ${\rm e}^+{\rm e}^-\to u\overline{ u}$ + photon(s). No evidence is observed for new physics contributions to these final states. Using the data at $\sqrt{s}$ = 183 GeV, upper limits on ${\sigma({\rm e^+e^-}\to{\rm XY})\cdot{\rm BR}(\rm X \to Y \gamma)}$ and ${\sigma({\rm e^+e^-}\to{\rm XX})\cdot{\rm BR}^2(\rm X \to Y \gamma)}$ are derived for the case of stable and invisible Y. These limits apply to single and pair production of excited neutrinos ( $\rm X = u^*, Y = u$ ), to neutralino production ( $\rm X={{\tilde{\chi}}^{0}}_{2}, Y={{\tilde{\chi}}^{0}}_{1}$ ) and to supersymmetric models in which $\rm X = {{\tilde{\chi}}^{0}}_{1}$ and $\rm Y=\tilde{\mathrm{G}}$ is a light gravitino.

A measurement of the product branching ratio f(b → Λb) · BR(Λb → ΛX) in Z0 decays

Abstract: 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 anomalous photonic events with missing energy in e+ e- collisions at S**(1/2) = 130-GeV, 136-GeV and 183-GeV

Abstract: Photonic events with large missing energy have been observed in e+e- collisions at centre-of-mass energies of 130, 136 and 183 GeV collected in 1997 using the OPAL detector at LEP. Results are presented for event topologies with a single photon and missing transverse energy or with an acoplanar photon pair. Cross-section measurements are performed within the kinematic acceptance of each selection. These results are compared with the expectations from the Standard Model process e+e- to nunubar + photon(s). No evidence is observed for new physics contributions to these final states. Using the data at Ecm = 183 GeV, upper limits on sigma(e+e- to X.Y)*BR(X to Y gamma) and sigma(e+e- to X.X)*BR(X to Y gamma)**2 are derived for the case of stable and invisible Y. These limits apply to single and pair production of excited neutrinos (X = nu*, Y = nu), to neutralino production (X = Chi_2^0, Y = Chi_1^0) and to supersymmetric models in which X = chi_1^0 and Y is a light gravitino.

Measurement of the Michel parameters in leptonic tau decays

Abstract: The Michel parameters of the leptonic $\tau$ decays are measured using the OPAL detector at LEP. The parameters $\rho_\ell$ , $\xi_\ell$ , $(\xi\delta)_\ell$ (with $\ell ={\rm e}, \mu$ ) and $\eta_\mu$ are extracted from the energy spectra of the charged decay leptons and from their energy–energy correlations. A new method involving a global likelihood fit of Monte Carlo generated events with complete detector simulation and background treatment has been applied to the data recorded at center-of-mass energies close to $\sqrt{s} =m_{\rm Z^0}$ corresponding to an integrated luminosity of 155 pb $^{-1}$ . If e- $\mu$ universality is assumed and inferring the $\tau$ polarization from neutral current data, the measured Michel parameters are: \[ \begin{array}{l@{\quad}l} \rho = 0.781 \pm 0.028 \pm 0.018, & \xi = 0.98 \pm 0.22 \pm 0.10, \eta = 0.027 \pm 0.055 \pm 0.005, & \xi\delta = 0.65 \pm 0.14 \pm 0.07, \end{array} \] where the value of $\eta$ has been constrained using the published OPAL measurements of the leptonic branching ratios and the $\tau$ lifetime. Limits on non-standard coupling constants and on the masses of new gauge bosons are obtained. The results are in agreement with the V–A prediction of the Standard Model.

Inclusive production of charged hadrons and K0 s mesons in photon-photon collisions

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

Measurement of Tau Branching Ratios to Five Charged Hadrons

Abstract: The branching ratios of the decay of the \(\tau\) lepton to five charged hadrons have been measured with the OPAL detector at LEP using data collected between 1991 and 1995 at \({\rm e^+e^-}\) centre-of-mass energies close to the \({\rm Z}^0\) resonance. The branching ratios are measured to be \(\)\(\) where the first error is statistical and the second systematic.

Di-Jet production in photon-photon collisions at root s(ee)=161 and 172 GeV

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

Search for scalar top and scalar bottom quarks at √s = 183 GeV at LEP

Abstract: Searches for a scalar top quark and a scalar bottom quark have been performed using a total data sample of 56.8 pb\(^{-1}\) at a centre-of-mass energy of \(\sqrt{s} = \)183 GeV collected with the OPAL detector at LEP. No candidate events were observed. Combining this result with those obtained at lower centre-of-mass energies, the 95% C.L. lower limit on the scalar top quark mass is 85.0 GeV if the mixing angle between the supersymmetric partners of the left- and right-handed states of the top quark is zero. The lower limit is 81.3 GeV, even if the scalar top quark decouples from the \({\mathrm Z}^{0}\) boson. 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 in which it decays into a bottom quark, a charged lepton and a scalar neutrino was also studied. From a similar analysis, a mass limit on the light scalar bottom quark was set at 82.7 GeV for a mass difference between the scalar bottom quark and the lightest neutralino larger than 7 GeV, and at 84.0 GeV for the mass difference larger 10 GeV and the lightest neutralino heavier than 30 GeV. These limits were obtained assuming that the scalar bottom quark decays into a bottom quark and the lightest neutralino, and that a mixing angle between the supersymmetric partners of the left- and right-handed states of the bottom quark is zero.

Measurements of the QED structure of the photon

Abstract: The structure of both quasi-real and highly virtual photons is investigated using the reaction \({\rm e^+e^-}\rightarrow{\rm e^+e^-}\mu^+\mu^-\), proceeding via the exchange of two photons. The results are based on the complete OPAL dataset taken at \({\rm e^+e^-}\) centre-of-mass energies close to the mass of the Z boson. The QED structure function \(F^\gamma _2\) and the differential cross-section \({\rm d}\sigma/{\rm d}x\) for quasi-real photons are obtained as functions of the fractional momentum x from the muon momentum which is carried by the struck muon in the quasi-real photon for values of \(Q^2\) ranging from 1.5 to 400 GeV\(^2\). The differential cross-section \({\rm d}\sigma/{\rm d}x\) for highly virtual photons is measured for \(1.5 P^2\). Based on azimuthal correlations the QED structure functions \(F^\gamma _{\rm A}\) and \(F^\gamma _{\rm B}\) for quasi-real photons are determined for an average \(Q^2\) of 5.4 GeV\(^2\).

Bose-Einstein correlations in e(+)e(-) -> W(+)W(-) at 172 and 183 GeV

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

Di-Jet production in photon-photon collisions at √see=161 and 172 GeV

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

Production of K0S and Λ in quark and gluon jets from Z0 decay

Abstract: The production of \({\rm K}^0_{\rm S}\) mesons and \(\Lambda\) baryons in quark and gluon jets has been investigated using two complementary techniques. In the first approach, which provides high statistical accuracy, jets were selected using different jet finding algorithms and ordered according to their energy. Production rates were determined taking into account the dependences of quark and gluon compositions as a function of jet energy as predicted by Monte Carlo models. Selecting three-jet events with the \(k_{\perp}\) (Durham) jet finder (\(y_{\mathrm{cut}} = 0.005\)), the ratios of \({\rm K}^0_{\rm S}\) and \(\Lambda\) production rates in gluon and quark jets relative to the mean charged particle multiplicity were found to be \(1.10 \pm 0.02 \pm 0.02\) and \(1.41 \pm 0.04 \pm 0.04\), respectively, where the first uncertainty is statistical and the second is systematic. In the second approach, a new method of identifying quark jets based on the collimation of energy flow around the jet axis is introduced and was used to anti-tag gluon jets in symmetric (Y-shaped) three-jet events. Using the cone jet finding algorithm with a cone size of 30°, the ratios of relative production rates in gluon and quark jets were determined to be \(0.94 \pm 0.07 \pm 0.07\) for \(\mathrm{K^0_S}\) and \(1.18 \pm 0.10 \pm 0.17\) for \(\Lambda\). The results of both analyses are compared to the predictions of Monte Carlo models.

A study of B0s meson oscillation using hadronic Z0 decays containing leptons

Abstract: A sample of \({\rm Z}^0\) decays containing b-flavoured hadrons is tagged using leptons, and events having precise proper time measurements are selected. These events are used to study \({\rm B}^0_{\rm s}\) oscillations. The flavour (b or \(\rm \bar{b}\)) at decay is determined from the lepton charge while the flavour at production is determined from jet charge or the charge of a second lepton, where available. The experiment was not able to resolve the oscillatory behaviour, and we deduce that the \({\rm B}^0_{\rm s}\) oscillation frequency \(\Delta m_{\rm s} > 5.2\) ps\(^{-1}\) at the 95% confidence level.

Measurement of the semileptonic branching ratio of charm hadrons produced in Z0→cc¯ decays

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