Showing papers by "D. M. Strom published in 1998"

Search for anomalous production of di-lepton events with missing transverse momentum in e+e- collisions at √S = 161 and 172 GeV

Abstract: Events containing two charged leptons and significant missing transverse momentum are selected from a data sample corresponding to a total integrated luminosity of 20.6 pb\(^{-1}\) at centre-of-mass energies of 161 GeV and 172 GeV. The observed number of events, four at 161 GeV and nine at 172 GeV, is consistent with the number expected from Standard Model processes, predominantly arising from \(\mathrm{W}^+\mathrm{W}^-\) production with each W decaying l eptonically. This topology is also an experimental signature for the pair production of new particles that decay to a charged lepton accompanied by one or more invisible particles. Further event selection criteria are described that optimise the sensitivity to particular new physics channels. No evidence for new phenomena is observed and limits on the production of scalar charged lepton pairs and other new particles are presented.

Search for chargino and neutralino production at √s=170 and 172 GeV at LEP

Abstract: A search for charginos and neutralinos, predicted by supersymmetric theories, is performed using a data sample of 182.1 pb-1 taken at a centre-of-mass energy of 189 GeV with the OPAL detector at LEP. No evidence for chargino or neutralino production is found. Upper limits on chargino and neutralino pair production (X1+X1-, X10X20) cross-sections are obtained as a function of the chargino mass (mX1±), the lightest neutralino mass (mX10) and the second lightest neutralino mass (mX20). Within the Constrained Minimal Supersymmetric Standard Model framework, and for mX1± - mX10 ≥ 5 GeV, the 95% confidence level lower limits on mX1± are 93.6 GeV for tan β = 1.5 and 94.1 GeV for tan β = 35. These limits are obtained assuming a universal scalar mass m0 ≥ 500 GeV. The corresponding limits for all m0 are 78.0 and 71.7 GeV. The 95% confidence level lower limits on the lightest neutralino mass, valid for any value of tan β are 32.8 GeV for m0 ≥ 500 GeV and 31.6 GeV for all m0.

Polarization and forward-backward asymmetry of Lambda baryons in hadronic Z(0) decays

Abstract: The longitudinal polarization, the transverse polarization, and the forward-backward asymmetry of \(\Lambda\) baryons, have been measured using a sample of 4.34 million hadronic \(\mathrm{Z}^0\) decays collected with the OPAL detector at LEP between 1990 and 1995. These results are important as an aid to the understanding of hadronization mechanisms. Significant longitudinal polarization has been observed at intermediate and high momentum. For \(x_E\) (\(\equiv 2 E_{\Lambda}/\sqrt{s}) >\) 0.3, the longitudinal polarization has been measured to be \(-32.9 \pm 5.5 \) (stat) \(\pm\) 5.2 (syst)%. We have observed no transverse polarization. A significant forward-backward asymmetry has been measured and can be described by a JETSET model.

Measurement of f(c→D∗+X), f(b→D∗+X) and Γcc¯/Γhad using D∗± mesons

Abstract: The production rates of \({\rm D}^{*\pm}\) mesons in charm and bottom events at centre-of-mass energies of about 91 GeV and the partial width of primary \({\rm c\bar c}\) pairs in hadronic \({\rm Z}^0\) decays have been measured at LEP using almost 4.4 million hadronic \({\rm Z}^0\) decays collected with the OPAL detector between 1990 and 1995. Using a combination of several charm quark tagging methods based on fully and partially reconstructed \({\rm D}^{*\pm}\) mesons, and a bottom tag based on identified muons and electrons, the hadronisation fractions of charm and bottom quarks into \({\rm D}^{*\pm}\) mesons have been found to be \(\) The fraction of \({\rm c\bar c}\) events in hadronic \({\rm Z}^0\) decays, \(\Gamma_{\rm c\bar c}/\Gamma_{\rm had}=\Gamma({\rm Z}^0\to{\rm c\bar c}) / \Gamma({\rm Z}^0\to\rm hadrons)\), is determined to be \(\) In all cases the first error is statistical, and the second one systematic. The last error quoted for \(\Gamma_{\rm c\bar c}/\Gamma_{\rm had}\) is due to external branching ratios.

Search for the standard model Higgs boson in e+e- collisions at v = 161-172 GeV

Abstract: 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.

Investigation of CP violation in B0 → J/ψKS0 decays at LEP

Abstract: An investigation of CP violation was performed using a total of 24 candidates for B0→J/ψK0SB0→J/ψKS0 decay, with a purity of about 60%. These events were selected from 4.4 million hadronic Z0Z0 decays recorded by the OPAL detector at LEP. An analysis procedure, involving techniques to reconstruct the proper decay times and tag the produced b-flavours, B0B0 or B¯0B¯0 , has been developed to allow a first direct study of the time dependent CP asymmetry that, in the Standard Model, is sin2βsin⁡2β . The result is where the first error is statistical and the second systematic. This result is used to determine probabilities for different values of sin2βsin⁡2β in the physical region from −1−1 to +1.

Photon and light meson production in hadronic Z\(^0\)decays

Abstract: The inclusive production rates and differential cross-sections of photons and mesons with a final state containing photons have been measured with the OPAL detector at LEP. The light mesons covered by the measurements are the $\pi^0$ , $\eta$ , $\rho(770)^{\pm}$ , $\omega(782)$ , $\eta'(958)$ and a $_0(980)^{\pm}$ . The particle multiplicities per hadronic Z $^0$ decay, extrapolated to the full energy range, are: \begin{eqnarray} \langle n_{\gamma} \rangle & = & 20.97 \pm$ 0.02 \pm1.15 , onumber \\ \langle n_{\pi^0} \rangle & = & 9.55 \pm 0.06 \pm 0.75 , onumber \\ \langle n_{\eta} \rangle & = & 0.97 \pm 0.03 \pm 0.11 , onumber\\ \langle n_{\rho^{\pm}} \rangle & = & 2.40 \pm 0.06 \pm 0.43 , onumber \\ \langle n_{\omega} \rangle & = & 1.04 \pm 0.04 \pm 0.14 , onumber \\ \langle n_{\eta'} \rangle & = & 0.14 \pm 0.01 \pm 0.02 , onumber \\ \langle n_{{\mathrm a}_0^{\pm}} \rangle & = & 0.27 \pm 0.04 \pm 0.10 , onumber \end{eqnarray} where the first errors are statistical and the second systematic. In general, the results are in agreement with the predictions of the JETSET and HERWIG Monte Carlo models.

Measurement of the quark to photon fragmentation function through the inclusive production of prompt photons in hadronic Z(0) decays

Abstract: The inclusive production of prompt photons with energy above 10 GeV is measured using the OPAL detector in hadronic ${\rm Z^0}$ decays at LEP. Minimal isolation cuts were imposed upon the prompt photons. The production rate and energy spectrum of prompt photons are found to be in agreement with QCD predictions for the quark-to-photon fragmentation function.

Production of f0(980), f2(1270) and φ(1020) in hadronic Z0 decay

Abstract: Inclusive production of the \({\rm f_0}(980)\), \({\rm f}_2(1270)\) and \(\phi(1020)\) resonances has been studied in a sample of 4.3 million hadronic \({\rm Z}^0\) decays from the OPAL experiment at LEP. A coupled channel analysis has been used for the \({\rm f}_0\) in simultaneous fits to the resonances in inclusive \(\pi^+\pi^-\) and \({\rm K^+K^-}\) mass spectra. Fragmentation functions are reported for the three states. Total inclusive rates are measured to be \(0.141 \pm 0.007 \pm 0.011\)\({\rm f}_0\), \(0.155\pm0.011\pm0.018\)\({\rm f}_2\) and \(0.091\pm0.002\pm0.003\)\(\phi\) mesons per hadronic \({\rm Z}^0\) decay. The production properties of the \({\rm f}_0\), including those in three-jet events, are compared with those of the \({\rm f}_2\) and \(\phi\), and with the Lund string model of hadron production. All measurements are consistent with the hypothesis that the \({\rm f_0}(980)\) is a conventional \({\rm q\bar q}\) scalar meson.

Multiplicity distributions of gluon and quark jets and tests of QCD analytic predictions

Abstract: Gluon jets are identified in ${\rm e^+e^-}$ hadronic annihilation events by tagging two quark jets in the same hemisphere of an event. The gluon jet is defined inclusively as all the particles in the opposite hemisphere. Gluon jets defined in this manner have a close correspondence to gluon jets as they are defined for analytic calculations, and are almost independent of a jet finding algorithm. The charged particle multiplicity distribution of the gluon jets is presented, and is analyzed for its mean, dispersion, skew, and curtosis values, and for its factorial and cumulant moments. The results are compared to the analogous results found for a sample of light quark (uds) jets, also defined inclusively. We observe differences between the mean, skew and curtosis values of gluon and quark jets, but not between their dispersions. The cumulant moment results are compared to the predictions of QCD analytic calculations. A calculation which includes next-to-next-to-leading order corrections and energy conservation is observed to provide a much improved description of the data compared to a next-to-leading order calculation without energy conservation. There is agreement between the data and calculations for the ratios of the cumulant moments between gluon and quark jets.

Production of f(0)(980), f(2)(1270) and phi(1020) in hadronic Z(0) decay

Abstract: Inclusive production of the f(0)(980), f(2)(1270) and phi(1020) resonances has been studied in a sample of 4.3 million hadronic Z(0) decays from the OPAL experiment at LEP. A coupled channel analysis has been used for the f(0) in simultaneous fits to the resonances in inclusive pi(+)pi(-) and K+K- mass spectra. Fragmentation functions are reported for the three states. Total inclusive rates are measured to be 0.141 plus or minus 0.007 plus or minus 0.011 f(0), 0.155 plus or minus 0.011 plus or minus 0.018 f(2) and 0.091 plus or minus 0.002 plus or minus 0.003 phi mesons per hadronic Z(0) decay. The production properties of the f(0), including those in three-jet events, are compared with those of the f(2) and phi, and with the Lund string model of hadron production. All measurements are consistent with the hypothesis that the f(0)(980) is a conventional q(q)over-bar scalar meson.

A search for neutral Higgs bosons in the MSSM and models with two scalar field doublets

Abstract: A search is described for the neutral Higgs bosons h0 and A0 predicted by models with two scalar field doublets and, in particular, the Minimal Supersymmetric Standard Model (MSSM). The search in the Z0h0 and h0A0 production channels is based on data corresponding to an integrated luminosity of 25 pb−1 from e+e− collisions at centre-of-mass energies between 130 and 172 GeV collected with the OPAL detector at LEP. The observation of a number of candidates consistent with Standard Model background expectations is used in combination with earlier results from data collected at the Z0 resonance to set limits on mh and ma in general models with two scalar field doublets and in the MSSM. For example, in the MSSM, for tan β > 1, minimal and maximal scalar top quark mixing and soft SUSY-breaking masses of 1 TeV, the 95% confidence level limits mh > 59.0 GeV and ma > 59.5 GeV are obtained. For the first time, the MSSM parameter space is explored in a detailed scan.

Determination of the production rate of D *0 mesons and of the ratio V/(V+P) in Z0 → cc̄ decays

Abstract: 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.

Measurement of the W boson mass and W + W - production and decay properties in e + e - collisions at √ s = 172 GeV

Abstract: This paper describes the measurement of the W boson mass, \(M_W\), and decay width, \(\Gamma_{\rm W}\), from the direct reconstruction of the invariant mass of its decay products in W pair events collected at a mean centre-of-mass energy of \(\sqrt s=172.12\) GeV with the OPAL detector at LEP. Measurements of the W pair production cross-section, the W decay branching fractions and properties of the W decay final states are also described. A total of 120 candidate \({\rm W^+W^-}\) events has been selected for an integrated luminosity of 10.36 pb\(^{-1}\). The \({\rm W^+W^-}\) production cross-section is measured to be \(\sigma_{\rm WW}=12.3\pm1.3{\rm (stat.)}\pm0.3{\rm (syst.)}\) pb, consistent with the Standard Model expectation. The \({\rm W^+W^-\to q\bar q \ell \bar u_\ell}\) and \({\rm W^+W^-} \to{\rm q \overline q q \overline q}\) final states are used to obtain a direct measurement of \(\Gamma_{\rm W} =1.30^{+0.70}_{-0.55}{\rm (stat.)}\pm0.18{\rm (syst.)}\) GeV. Assuming the Standard Model relation between \(M_{\rm W}\) and \(\Gamma_{\rm W}\), the W boson mass is measured to be \(M_{\rm W}=80.32\pm0.30{\rm (stat.)}\pm0.09{\rm (syst.)}\) GeV. The event properties of the fully-hadronic decays of \({\rm W^+W^-}\) events are compared to those of the semi-leptonic decays. At the current level of precision there is no evidence for effects of colour reconnection in the observables studied. Combining data recorded by OPAL at \(\sqrt s\sim161\)–172 GeV, the W boson branching fraction to hadrons is determined to be \(69.8^{+3.0}_{-3.2}{\rm (stat.)}\pm0.7{\rm (syst.)}\)%, consistent with the prediction of the Standard Model. The combined mass measurement from direct reconstruction and from the \({\rm W^+W^-}\) production cross-sections measured at \(\sqrt s\sim161\) and \(\sqrt s\sim172\) GeV is \(M_{\rm W} = 80.35 \pm 0.24{\rm (stat.)}\pm0.07{\rm (syst.)}\) GeV.

Search for anomalous production of photonic events with missing energy in e+e- collisions at √s = 130-172 GeV

Abstract: Photonic events with large missing energy have been observed in \(\mathrm{e^+e^-}\) collisions at centre-of-mass energies of 130, 136, 161 and 172 GeV using the OPAL detector at LEP Results are presented based on search topologies designed to select events with a single photon and missing transverse energy or events with a pair of acoplanar photons In both search topologies, cross-section measurements are performed within the kinematic acceptance of the selection These results are compared with the expectations from the Standard Model processes \({\rm e^+e^-}\to u\overline u \gamma (\gamma )\) (single-photon) and \({\rm e^+e^-}\to u\overline u \gamma \gamma (\gamma )\) (acoplanar-photons) No evidence is observed for new physics contributions to these final states Upper limits on \(\sigma({\rm e^+e^- \to XY)\cdot BR(X\to Y\gamma })\) and \(\sigma({\rm e^+e^- \to XX)\cdot BR^2(X\to Y\gamma })\) are derived for the case of stable and invisible Y These limits apply to single and pair production of excited neutrinos (\({\rm X}= u^*,{\rm Y}= u\)), to neutralino production (\({\rm X}=\tilde\chi^0_2, {\rm Y}=\tilde\chi^0_1\)), and to supersymmetric models in which \({\rm X}= \tilde\chi^0_1\) and \({\rm Y}=\tilde{\rm G}\) is a light gravitino For the latter scenario, the results of the acoplanar-photons search are used to provide mode l-dependent lower limits on the mass of the lightest neutralino

Search for unstable heavy and excited leptons in e+e− collisions at ∝s = 170-172 GeV

Abstract: We have searched for unstable neutral and charged heavy leptons, N and L±, and for excited states of neutral and charged leptons, ν* e*, μ* and τ*, in e+e− collisions at centre-of-mass energies of 170 and 172 GeV using the OPAL detector at LEP. No evidence for their existence was found. From the analysis of charged-current decays of pair-produced unstable heavy leptons, and of charged-current and photonic decays of pair-produced excited leptons, lower limits on their masses are derived. From the analysis of charged-current and photonic decays of singly-produced excited leptons, upper limits on the ratio of the coupling to the compositeness scale, ϕ/λ, are determined for masses up to the kinematic limit.

Measurement of the one-prong hadronic tau branching ratios at LEP

Abstract: The branching ratios of the $\tau^-\rightarrow h^- u_\tau$ $\tau^- \rightarrow h^- \pi^0 u_\tau$ and $\tau^- \rightarrow h^- \geq 2 \pi^0 u_\tau $ decays have been measured using the 1991–1995 data recorded with the OPAL detector at LEP. These branching ratios are measured simultaneously using three selection criteria and are found to be \[ \begin{array} {l c c} {\rm BR}(\tau^- \rightarrow h^- u_\tau) & = & (11.98 \pm 0.13 \pm 0.16) \%\\ {\rm BR}(\tau^-\rightarrow h^- \pi^0 u_\tau) & = & (25.89 \pm 0.17 \pm 0.29) \%\\ {\rm BR}(\tau^- \rightarrow h^- \geq 2 \pi^0 u_\tau ) & = & ( 9.91 \pm 0.31 \pm 0.27) \% \end{array} \] where the first error is statistical and the second is systematic.

Bose-Einstein correlations of three charged pions in hadronic Z0 decays

Abstract: Bose-Einstein Correlations (BEC) of three identical charged pions were studied in $4 \times 10^6$ hadronic ${\rm Z}^0$ decays recorded with the OPAL detector at LEP. The genuine three-pion correlations, corrected for the Coulomb effect, were separated from the known two-pion correlations by a new subtraction procedure. A significant genuine three-pion BEC enhancement near threshold was observed having an emitter source radius of $r_3 = 0.580 \pm 0.004 \mbox{ (stat.)} \pm 0.029 \mbox{ (syst.)} \mbox{ fm}$ and a strength of $\lambda_3 = 0.504 \pm 0.010 \mbox{ (stat.)}\pm 0.041 \mbox{ (syst.)}$ . The Coulomb correction was found to increase the $\lambda_3$ value by $\sim $ 9% and to reduce $r_3$ by $\sim $ 6%. The measured $\lambda_3$ corresponds to a value of $0.707 \pm 0.014 \mbox{ (stat.)} \pm 0.078 \mbox{ (syst.)}$ when one takes into account the three-pion sample purity. A relation between the two-pion and the three-pion source parameters is discussed.

Measurement of triple gauge boson couplings from W+W- production at √s = 172GeV

Abstract: We present measurements of triple gauge boson coupling parameters using data recorded by the OPAL detector at LEP2, at a centre-of-mass energy of 172 GeV. A total of 120 W-pair candidates has been selected in the \({\rm q \bar q q \bar q}\), \({\rm q\bar q} \ell \bar u_\ell\) and \(\ell \bar u_\ell \bar\ell^\prime u_{\ell^\prime }\) decay channels, for an integrated luminosity of 10.4 pb\(^{-1}\). We use these data to determine several different anomalous coupling parameters using the measured cross-section and the distributions of kinematic variables. We measure \(\alpha_{B\phi} = 0.35^{+1.29}_{-1.07} \pm 0.38\), \(\alpha_{W\phi} = 0.00^{+0.30}_{-0.28} \pm 0.11\), \(\alpha_W = 0.18^{+0.49}_{-0.47} \pm 0.23\), \(\Delta g^{\rm z}_1 = -0.03^{+0.40}_{-0.37} \pm 0.14\), \(\Delta\kappa_\gamma ^{(HISZ)} = 0.03^{+0.55}_{-0.51} \pm 0.20\), and \(\Delta \kappa = 0.33^{+0.49}_{-0.46} \pm 0.21\). Combining the \(\alpha_{W\phi}\) result with our previous result obtained from the 161 GeV data sample we measure \(\alpha_{W\phi} = -0.08^{+0.28}_{-0.25} \pm 0.10\). All of these measurements are consistent with the Standard Model.

A measurement of the B-s(0) lifetime using reconstructed D-s(-) mesons

Abstract: We report a measurement of the B-s(0) meson lifetime from B-s(0) --> Ds-X decays, where D-s(-) mesons are reconstructed in the D-s(-) --> phi pi(-) and D-s(-) --> K*K-0(-) decay channels. From approximately 3.7 million hadronic Z(0) decays recorded by the OPAL detector at LEP a sample is selected containing 911 +/- 83 candidates, of which 519 +/- 136 are estimated to be from B-s(0) meson decays. Fitting the distribution of the distance from the beam spot to the decay vertex of the D-s(-) candidates with an unbinned likelihood function we measuretau(B-s(0)) = 1.72(-0.19-0.17)(+0.20+0.18) ps,where the errors are statistical and systematic, respectively.