arXiv subjects

The OPAL Collaboration

Publications and source records attributed to The OPAL Collaboration.

At least 91 records · Page 5Linked to original sources

arXiv1997

Search for Unstable Heavy and Excited Leptons in e^+e^- Collisions at sqrt{s} = 170-172 GeV

We have searched for unstable neutral and charged heavy leptons, N and L^+/-, and for excited states of neutral and charged leptons, nu*, e*, mu* and tau*, 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, f/Lambda, are determined for masses up to the kinematic limit.

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arXiv1997

Measurement of triple gauge boson couplings from W^+W^- production at sqrt{s} = 172 GeV

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 q q(bar) q q(bar), q q(bar) l nu(bar)_l, and l nu(bar)_l l(bar)' nu_l' 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 +/- 0.38, alpha_W-phi = 0.00 + 0.30 - 0.28 +/- 0.11, alpha_W = 0.18 + 0.49 - 0.47 +/- 0.23, Delta(g_1^z) = -0.03 + 0.40 -0.37 +/- 0.14, Delta(kappa_gamma^(HISZ)) = 0.03 + 0.55 - 0.51 +/- 0.20, and Delta(kappa) = 0.03 + 0.49 - 0.46 +/- 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 +/- 0.10. All of these measurements are consistent with the Standard Model.

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arXiv1997

Measurement of the W Boson Mass and Production and Decay Properties in e^+e^- Collisions at sqrt{s} = 172 GeV

This paper describes the measurement of the W boson mass, M_W, and decay width, Gamma_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 W^+W^- events has been selected for an integrated luminosity of 10.36 pb^-1. The W^+W^- production cross-section is measured to be sigma_WW = 12.3 +/- 1.3(stat.) +/- 0.3(syst.) pb, consistent with the Standard Model expectation. The W^+W^- -> qq(bar) l nu and W^+W^- -> qq(bar)qq(bar) final states are used to obtain a direct measurement of Gamma_W = 1.30^{+0.62}_{-0.55}(stat.) +/- 0.18(syst.) GeV. Assuming the Standard Model relation between M_W and Gamma_W, the W boson mass is measured to be M_W = 80.32 +/- 0.30(stat.) +/- 0.09(syst.) GeV. The event properties of the fully-hadronic decays of 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} ~ 161-172 GeV, the W boson branching fraction to hadrons is determined to be 69.8^{+3.0}_{-3.2}(stat.) +/- 0.7(syst.)%, consistent with the prediction of the Standard Model. The combined mass measurement from direct reconstruction and from the W^+W^- production cross-sections measured at sqrt{s} ~ 161 and sqrt{s} ~ 172 GeV is M_W = 80.35 +/- 0.24(stat.) +/- 0.07(syst.) GeV.

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arXiv1997

Multi-photon final states in e^+e^- collisions at sqrt{s} = 130-172 GeV

The process e^+e^- -> gamma gamma (gamma) is studied using data recorded with the OPAL detector at LEP. The data sample corresponds to a total integrated luminosity of 25.38 pb^{-1} taken at centre-of-mass energies of 130-172 GeV. The measured cross-sections agree well with the expectation from QED. In a combined fit using data from all centre-of-mass energies, the angular distribution is used to obtain improved limits on the cut-off parameters: Lambda_+ > 195 GeV and Lambda_- > 210 GeV (95% CL). In addition, limits on non-standard e^+e^-gamma couplings and contact interactions, as well as a 95% CL mass limit for an excited electron, M_{e^*} > 194 GeV for an e^+e^-gamma coupling kappa = 1, are determined.

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arXiv1997

Search for the Standard Model Higgs Boson in e^+e^- Collisions at sqrt{s} = 161-172 GeV

This paper describes a search for the Standard Model Higgs boson using data from 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.

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arXiv1997

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

Gluon jets are identified in 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.

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arXiv1997

Asymmetry of Lambda Baryons in Hadronic Z^0 Decays

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 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 = 2 E_{Lambda}/\sqrt{s}) > 0.3, the longitudinal polarization has been measured to be -32.9 +/- 5.5(stat) +/- 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.

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arXiv1997

Measurement of the Photon Structure Function F_2^gamma at Low x

Deep inelastic electron-photon scattering is studied using e+e- data collected by the OPAL detector at centre-of-mass energies sqrt{s_ee} ~ M_{Z^0}. The photon structure function F_2^gamma(x,Q^2) is explored in a Q^2 range of 1.1 to 6.6 GeV/c^2 at lower x values than ever before. To probe this kinematic region events are selected with a beam electron scattered into one of the OPAL luminosity calorimeters at scattering angles between 27 and 55 mrad. A measurement is presented of the photon structure function F_2^gamma(x,Q^2) at = 1.86 GeV^2 and 3.76 GeV^2 in five logarithmic x bins from 0.0025 to 0.2.

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arXiv1997

Tests of the Standard Model and Constraints on New Physics from Measurements of Fermion-pair Production at 130-172 GeV at LEP

Production of events with hadronic and leptonic final states has been measured in e^+e^- collisions at centre-of-mass energies of 130-172 GeV, using the OPAL detector at LEP. Cross-sections and leptonic forward-backward asymmetries are presented, both including and excluding the dominant production of radiative Z γevents, and compared to Standard Model expectations. The ratio R_b of the cross-section for bb(bar) production to the hadronic cross-section has been measured. In a model-independent fit to the Z lineshape, the data have been used to obtain an improved precision on the measurement of γ-Z interference. The energy dependence of α_em has been investigated. The measurements have also been used to obtain limits on extensions of the Standard Model described by effective four-fermion contact interactions, to search for t-channel contributions from new massive particles and to place limits on chargino pair production with subsequent decay of the chargino into a light gluino and a quark pair.

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arXiv1997

Measurement of f(c -> D^+ X), f(b -> D^+ X) and Gamma_cc(bar)/Gamma_had using D^*+/- Mesons

The production rates of D^*+/- mesons in charm and bottom events at centre-of-mass energies of about 91 GeV and the partial width of primary cc(bar) pairs in hadronic Z^0 decays have been measured at LEP using almost 4.4 million hadronic 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 D^*+/- mesons, and a bottom tag based on identified muons and electrons, the hadronisation fractions of charm and bottom quarks into D^*+/- mesons have been found to be: f(b -> D^*+ X) = 0.173 +/- 0.016 +/- 0.012 and f(c -> D^*+ X) = 0.222 +/- 0.014 +/- 0.014 The fraction of cc(bar) events in hadronic Z^0 decays, Gamma_cc(bar)/Gamma_had = Gamma(Z^0 -> cc(bar))/Gamma(Z^0 -> hadrons), is determined to be Gamma_cc(bar)/Gamma_had = 0.180 +/- 0.011 +/- 0.012 +/- 0.006 In all cases the first error is statistical, and the second one systematic. The last error quoted for Gamma_cc(bar)/Gamma_had is due to external branching ratios.

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arXiv1997

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

The inclusive production of prompt photons with energy above 10 GeV is measured using the OPAL detector in hadronic Z^0 decays at LEP. In contrast to previous measurements, the prompt photons were not required to be isolated. The production rate and energy spectrum are found to be in agreement with QCD predictions for the quark-to-photon fragmentation function.

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arXiv1997

Search for Chargino and Neutralino Production at sqrt{s} = 170 and 172 GeV at LEP

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) >= 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 and slepton masses. The lower limit on the lightest neutralino mass at 95% C.L. for tan(beta) >= 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.

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arXiv1997

A Measurement of the B_s^0 Lifetime using Reconstructed D_s^- Mesons

We report a measurement of the B^0_s meson lifetime from B_s^0 -> D^-_s X decays, where D_s^- mesons are reconstructed in the D_s^- -> phi pi^- and D_s^- -> K^{*0} K^- 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 measure tau(B_s^0) = 1.72 + 0.20 - 0.19 + 0.18 - 0.17 ps, where the errors are statistical and systematic, respectively.

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arXiv1997

Measurement of the Q^2 evolution of the photon structure function F_2^gamma

New measurements are presented of the photon structure function F_2^gamma(Q) at four values of Q^2 between 9 and 59 GeV/c^2 based on data collected with the OPAL detector at centre-of-mass energies of 161-172 GeV, with a total integrated luminosity of 18.1 pb^-1. The evolution of F_2^gamma with Q^2 in bins of x is determined in the Q^2 range from 1.86 to 135 GeV/c^2 using data taken at centre-of-mass energies of 91 GeV and 161-172 GeV. F_2^gamma is observed to increase with Q^2 with a slope of 1/alpha_em dF_2^gamma/dln(Q^2) = 0.10 +0.05 -0.03 measured in the range 0.1 < x < 0.6.

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arXiv1997

Spin alignment of leading K*(892)^0 mesons in hadronic Z^0 decays

Helicity density matrix elements for inclusive K*(892)^0 mesons from hadronic Z^0 decays have been measured over the full range of K^*0 momentum using data taken with the OPAL experiment at LEP. A preference for occupation of the helicity zero state is observed at all scaled momentum x_p values above 0.3, with the matrix element rho_00 rising to 0.66 +/- 0.11 for x_p > 0.7. The values of the real part of the off-diagonal element rho_1-1 are negative at large x_p, with a weighted average value of -0.09 +/- 0.03 for x_p > 0.3, in agreement with new theoretical predictions based on Standard Model parameters and coherent fragmentation of the qq(bar) system from the Z^0 decay. All other helicity density matrix elements measured are consistent with zero over the entire x_p range. The K^*0 fragmentation function has also been measured and the total rate determined to be 0.74 +/- 0.02 +/- 0.02 K*(892)^0 mesons per hadronic Z^0 decay.

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arXiv1997

An upper limit on the branching ratio for tau decays into seven charged particles

We have searched for decays of the tau lepton into seven or more charged particles, using data collected with the OPAL detector from 1990 to 1995 in e^+e^- collisions at sqrt(s) ~ M_Z. No candidate events were found and an upper limit on the branching ratio for tau decays into seven charged particles of 1.8 x 10^-5 at the 95% confidence level was determined.

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arXiv1997

Measurement of the Branching Fractions and Forward-Backward Asymmetries of the Z^0 into Light Quarks

Using approximately 4.3 million hadronic Z^0 decays collected with the OPAL detector at LEP between 1990 and 1995, we measure the branching fractions of the Z^0 into up-type and down-type light quarks, R_q, and the forward-backward asymmetries, A_FB(q), using high-momentum stable particles as a tag. Adopting a method that employs double tagged events to determine the flavour tagging efficiencies, and assuming the flavour independence of strong interactions and SU(2) isospin symmetry, we measure: R_{d,s}/(R_d + R_u + R_s) = 0.371 +/- 0.016(stat.) +/- 0.016(syst.) and A_FB(d,s) = 0.068 +/- 0.035(stat.) +/- 0.011(syst.), assuming the branching fractions and forward-backward asymmetries of down and strange quarks to be equal. The results are essentially free of assumptions based on hadronisation models. These results are in agreement with the Standard Model expectations and are used to infer the left and right handed couplings of strange/down quarks to the Z^0, yielding g_L^{d,s} = -0.44^{+0.13}_{-0.09} and g_R^{d,s} = +0.13^{+0.15}_{-0.17}. The results for the up quark, R_u/(R_d + R_u + R_s) = 0.258 +/- 0.031(stat.) +/- 0.032(syst.) and A_FB(u)= 0.040 +/- 0.067(stat.) +/- 0.028(syst.), are fully negatively correlated and almost completely positively correlated, respectively, with the corresponding down-type results.

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arXiv1997

A Study of B Meson Oscillations Using Hadronic Z^0 Decays Containing Leptons

An event sample enriched in semileptonic decays of b hadrons is selected using an inclusive lepton selection from approximately 3.0 million hadronic Z^0 decays collected with the OPAL detector. This sample is used to investigate B meson oscillations by reconstructing a proper decay time for the parent of each lepton, using a jet charge method to estimate the production flavour of this parent, and using the lepton charge to tag the decay flavour. We measure the mass difference between the two B^0_d mass eigenstates Delta(m_d) = 0.444 +/- 0.029 + 0.020 - 0.017 ps^-1. For the B^0_s system, we find Delta(m_s) > 3.1 ps^-1 at the 95% confidence level. This limit varies only a little if alternative limit setting approaches are adopted. Regions at higher Delta(m_s) are also excluded with some methods for setting the limit. By studying the charge symmetry of the B^0_d mixing structure, we are able to constrain possible CP and CPT violating effects. We measure the CP violation parameter Re(epsilon_B) = -0.006 +/- 0.010 +/- 0.006 and the indirect CPT violating parameter Im(delta_B) = -0.020 +/- 0.016 +/- 0.006. If we invoke CPT symmetry, then we obtain Re(epsilon_B) = 0.002 +/- 0.007 +/- 0.003.

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