arXiv subjects

The OPAL Collaboration

Publications and source records attributed to The OPAL Collaboration.

At least 73 records · Page 4Linked to original sources

arXiv1998

Bose-Einstein Correlations of Three Charged Pions in Hadronic Z^0 Decays

Bose-Einstein Correlations (BEC) of three identical charged pions were studied in 4 x 10^6 hadronic 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 +/- 0.004 (stat.) +/- 0.029 (syst.) fm and a strength of λ_3 = 0.504 +/- 0.010 (stat.) +/- 0.041 (syst.). The Coulomb correction was found to increase the λ_3 value by \~9% and to reduce r_3 by ~6%. The measured λ_3 corresponds to a value of 0.707 +/- 0.014 (stat.) +/- 0.078 (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.

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arXiv1998

An upper limit for the tau-neutrino mass from tau --> 5 pi+- nu_tau decays

An upper limit for the tau-neutrino mass has been determined from the decay tau -> 5pi+- ν_tau using data collected with the OPAL detector from 1991 to 1995 in e+e- collisions at \sqrt{s} ~ M_Z. A limit of 43.2 MeV at 95% CL is obtained using a two-dimensional method in the 5pi invariant mass and energy distribution from 22 selected events. Combining this result with OPAL's previously published measurement using tau+tau- -> 3h+- \bar{nu}_tau 3h-+ ν_tau decays, a new combined limit of m_{nu_tau} < 27.6 MeV (95% CL) is obtained.

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arXiv1998

Production of K^0_S and Lambda in Quark and Gluon Jets from Z^0 Decay

The production of K^0_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_cut = 0.005), the ratios of K^0_S and Lambda production rates in gluon and quark jets relative to the mean charged particle multiplicity were found to be 1.10 +/- 0.02 +/- 0.02 and 1.41 +/- 0.04 +/- 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 degrees, the ratios of relative production rates in gluon and quark jets were determined to be 0.94 +/- 0.07 +/- 0.07 for K^0_S and 1.18 +/- 0.10 +/- 0.17 for Lambda. The results of both analyses are compared to the predictions of Monte Carlo models.

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arXiv1998

Photon and Light Meson Production in Hadronic Z^0 Decays

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 π^0, η, ρ(770)+-, ω(782), η'(958) and a_0(980)+-. The particle multiplicities per hadronic Z^0 decay, extrapolated to the full energy range, are: = 20.97 +/- 0.02 +/- 1.15, = 9.55 +/- 0.06 +/- 0.75, = 0.97 +/- 0.03 +/- 0.11, = 2.40 +/- 0.06 +/- 0.43, = 1.04 +/- 0.04 +/- 0.14, = 0.14 +/- 0.01 +/- 0.02, = 0.27 +/- 0.04 +/- 0.10. 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.

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arXiv1998

Search for Stable and Long-Lived Massive Charged Particles with the OPAL Detector at LEP in e+e- Collisions at \sqrt{s}=130-183 GeV

A search for stable and long-lived massive particles of electric charge |Q/e| = 1 or 2/3, pair-produced in e+e- collisions at centre-of-mass energies from 130 to 183 GeV, is reported by the OPAL collaboration at LEP. No evidence for production of these particles was observed in a mass range between 45 and 89.5 GeV. Model-independent upper limits on the production cross-section between 0.05 and 0.19 pb have been derived for scalar and spin-1/2 particles with charge +/-1. Within the framework of the minimal supersymmetric model (MSSM), this implies a lower limit of 82.5 (83.5) GeV on the mass of long-lived right- (left-)handed scalar muons and scalar taus. Long-lived charged leptons and charginos are excluded for masses below 89.5 GeV. For particles with charge +/-2/3 the upper limits on the production cross-section vary between 0.05 and 0.2 pb. All limits, on masses and on cross-sections, are valid at the 95% confidence level for particles with lifetimes longer than 10^{-6} s.

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arXiv1998

An upper limit on the anomalous magnetic moment of the τlepton

Using radiative Z^0 -> τ^+ τ^- γevents collected with the OPAL detector at LEP at \sqrt{s}= M_Z during 1990-95, a direct study of the electromagnetic current at the τγvertex has been performed in terms of the anomalous magnetic form factor F_2 of the τlepton. The analysis is based on a data sample of 1429 e^+ e^- -> τ^+ τ^- γevents which are examined for a deviation from the expectation with F_2 = 0. From the non-observation of anomalous τ^+ τ^- γproduction a limit of -0.068 < F_2 < 0.065 is obtained. This can also be interpreted as a limit on the electric dipole form factor F_3 as -3.8 x 10^-16 e-cm < eF_3 < 3.6 x 10^-16 e-cm. The above ranges are valid at the 95% confidence level.

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arXiv1998

A Search for Neutral Higgs Bosons in the MSSM and Models with Two Scalar Field Doublets

A search is described for the neutral Higgs bosons h^0 and A^0 predicted by models with two scalar field doublets and, in particular, the Minimal Supersymmetric Standard Model (MSSM). The search in the Z^0 h^0 and h^0 A^0 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 Z^0 resonance to set limits on m_h and m_A in general models with two scalar field doublets and in the MSSM. For example, in the MSSM, for tan(beta) > 1, minimal and maximal scalar top quark mixing and soft SUSY-breaking masses of 1 TeV, the 95% confidence level limits m_h > 59.0 GeV and m_A > 59.5 GeV are obtained. For the first time, the MSSM parameter space is explored in a detailed scan.

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arXiv1998

Search for an Excess in the Production of Four-Jet Events from e^+e^- Collisions at \sqrt{s} = 130-184 GeV

Events with four distinct jets from e^+e^- collisions, collected by the OPAL detector at centre-of-mass energies between 130 and 184 GeV, are analysed for a peak in the sum of dijet masses. This search is motivated by the ALEPH Collaboration's observation of a clear excess of events with dijet mass sums close to 105 GeV in data taken at centre-of-mass energies of 130 and 136 GeV in 1995. We have observed no significant excess of four-jet events compared to the Standard Model expectation for any dijet mass sum at any energy. Our observation is inconsistent with the excess observed by ALEPH in 1995. Upper limits are determined on the production cross-section as a function of the dijet mass sum.

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arXiv1998

Production of f_0(980), f_2(1270) and ϕ(1020) in hadronic Z^0 decay

Inclusive production of the f_0(980), f_2(1270) and ϕ(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 π+π- and K+K- mass spectra. Fragmentation functions are reported for the three states. Total inclusive rates are measured to be 0.141 +/- 0.007 +/- 0.011 f_0, 0.155 +/- 0.011 +/- 0.018 f_2, and 0.091 +/- 0.002 +/- 0.003 ϕmesons 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 ϕ, and with the Lund string model of hadron production. All measurements are consistent with the hypothesis that the f_0 is a conventional qq(bar) scalar meson.

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arXiv1998

Determination of the Production Rate of D^*0 Mesons and of the Ratio V/(V+P) in Z^0 -> cc(bar) Decays

In e^+e^- 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 -> cc(bar) decays. The production ratio is determined to be P^eff_V = V/(V+P) = 0.57 +/- 0.05. The relative primary production of vector and pseudoscalar mesons, P^prim_V, is studied in the context of the production and decay of orbitally excited charmed resonances. The first measurement of the inclusive D^*+_s production rate in hadronic Z^0 decays is presented.

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arXiv1998

Search for charged Higgs bosons in e^+e- collisions at sqrt{s} = 130 - 172 GeV

A search is described to detect charged Higgs bosons via the process e^+e^- -> H^+H^-, using data collected by the OPAL detector at center-of-mass energies of 130-172 GeV with a total integrated luminosity of 25 pb^-1. The decay channels are assumed to be H^+ -> qq'(bar) and H^+ -> τ^+ ν_τ. No evidence for charged Higgs boson production is observed. The lower limit for its mass is determined to be 52 GeV at 95% confidence level, independent of the H^+ -> τ^+ ν_τ branching ratio.

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arXiv1998

Measurements of the B^0_s and Λ_b Lifetimes

This paper presents updated measurements of the lifetimes of the B^0_s meson and the Λ_b baryon using 4.4 million hadronic Z^0 decays recorded by the OPAL detector at LEP from 1990 to 1995. A sample of B^0_s decays is obtained using D_s^- \ell^+ combinations, where the D_s^- is fully reconstructed in the ϕπ^-, K^*0 K^- and K^- K^0_S decay channels and partially reconstructed in the ϕ\ell^- ν(bar) X decay mode. A sample of Λ_b decays is obtained using Λ_c^+ \ell^- combinations, where the Λ_c^+ is fully reconstructed in its decay to a p K^- π^+ final state and partially reconstructed in the Λ\ell^+ νX decay channel. From 172 +/- 28 D_s^- \ell^+ combinations attributed to B^0_s decays, the measured lifetime is τ(B^0_s) = 1.50 +0.16 -0.15 +/- 0.04 ps, where the errors are statistical and systematic, respectively. From the 129 +\- 25 \Lamda_c^+ \ell^- combinations attributed to Λ_b decays, the measured lifetime τ(Λ_b) = 1.29 +0.24 -0.22 +/- 0.06 ps, where the errors are statistical and systematic, respectively.

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arXiv1998

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

The branching ratios of the τ^- -> h^- ν_τ, τ^- -> h^- π^0 ν_τ, τ^- -> h^- >=2 π^0 ν_τdecays 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 BR(τ^- -> h^- ν_τ) = (11.98 +/- 0.13 +/- 0.16)% BR(τ^- -> h^- π^0 ν_τ) = (25.89 +/- 0.17 +/- 0.29)% BR(τ^- -> h^- >=2 π^0 ν_τ) = ( 9.91 +/- 0.31 +/- 0.27)% where the first error is statistical and the second is systematic.

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arXiv1998

Search for the B_c Meson in Hadronic Z^0 Decays

A search for decays of the B_c meson was performed using data collected from 1990-1995 with the OPAL detector on or near the Z peak at LEP. The decay channels B_c^+ -> J/psi pi^+, B_c^+ -> J/psi a_1^+ and B_c^+ -> J/psi ell^+ nu were investigated, where ell denotes an electron or a muon. Two candidates are observed in the mode B_c^+ -> J/psi pi^+, with an estimated background of (0.63 +/- 0.20) events. The weighted mean of the masses of the two candidates is (6.32 +/- 0.06) GeV/c^2, which is consistent with the predicted mass of the B_c meson. One candidate event is observed in the mode B_c^+ -> J/psi ell^+ nu with an estimated background of (0.82 +/- 0.19) events. No candidate events are observed in the B_c^+ -> J/psi a_1^+ decay mode, with an estimated background of (1.10 +/- 0.22) events. Upper bounds at the 90% confidence level are set on the production rates for these processes.

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arXiv1998

Search for Anomalous Production of Photonic Events with Missing Energy in e+e- Collisions at sqrt{s} = 130-172 GeV

Photonic events with large missing energy have been observed in 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 e+e- -> nu nu(bar) gamma (gamma) (single-photon) and e+e- -> νν(bar) gamma gamma (gamma) (acoplanar-photons). No evidence is observed for new physics contributions to these final states. Upper limits on sigma(e+e- -> XY)*Br(X -> Y gamma) and sigma(e+e- -> XX)*Br**2(X -> Y gamma) are derived for the case of stable and invisible Y. These limits apply to single and pair production of excited neutrinos (X = ν^*, Y = ν), to neutralino production (X = χ^0_2, Y = χ^0_1), and to supersymmetric models in which X = χ^0_1 and Y = \tilde{G} is a light gravitino. For the latter scenario, the results of the acoplanar-photons search are used to provide model-dependent lower limits on the mass of the lightest neutralino.

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arXiv1998

Investigation of CP violation in B^0 -> J/psi K^0_S decays at LEP

An investigation of CP violation was performed using a total of 24 candidates for B^0 -> J/psi K^0_S decay, with a purity of about 60%. These events were selected from 4.4 million hadronic Z^0 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, B^0 or B^0(bar), has been developed to allow a first direct study of the time dependent CP asymmetry that, in the Standard Model, is sin(2beta). The result is sin(2beta) = 3.2_{-2.0}^{+1.8} +/- 0.5, where the first error is statistical and the second systematic. This result is used to determine probabilities for different values of sin(2beta) in the physical region from -1 to +1.

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arXiv1997

Search for a Massive Di-photon Resonance at sqrt{s} = 91-172 GeV

A search for the resonant production of high mass photon pairs associated with a leptonic or hadronic system has been performed using a total data sample of 25.7 pb^-1 taken at centre-of-mass energies between 130 GeV and 172 GeV with the OPAL detector at LEP. The observed number of events is consistent with the expected number from Standard Model processes. The observed candidates are combined with search results from sqrt{s} ~ M_Z to place limits on Br(H^0 -> gamma gamma) within the Standard Model for Higgs boson masses up to 77 GeV, and on the production cross section of any scalar resonance decaying into di-photons. Upper limits on Br(H^0 -> gamma gamma) x sigma(e^+e^- -> H^0 Z^0) of 290 - 830 fb are obtained over 40 < M_H < 160 GeV. Type-I two-Higgs-doublet scalars which couple only to gauge bosons are ruled out up to a mass of 76.5 GeV at the 95% confidence level.

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arXiv1997

Search for Anomalous Production of Di-lepton Events with Missing Transverse Momentum in e^+e^- Collisions at sqrt{s} = 161 and 172 GeV

Events containing a pair of 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 W+W- production with each W decaying leptonically. 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.

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