arXiv subjects

The OPAL Collaboration

Publications and source records attributed to The OPAL Collaboration.

At least 55 records · Page 3Linked to original sources

arXiv1998

Search for Anomalous Photonic Events with Missing Energy in e+e- Collisions at Ecm = 130, 136 and 183 GeV

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.

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arXiv1998

Measurement of the Semileptonic Branching Ratio of Charm Hadrons Produced in Z to ccbar Decays

The inclusive charm hadron semileptonic branching fractions B(c to e) and B(c to mu) in Z to ccbar events have been determined using 4.4 million hadronic Z decays collected with the OPAL detector at LEP. A charm-enriched sample is obtained by selecting events with reconstructed D*+- mesons. Using leptons found in the hemisphere opposite that of the D*+- mesons, the semileptonic branching fractions of charm hadrons are measured to be B(c to e) = 0.103 +-0.009 +0.009 -0.008 and B(c to mu) = 0.090 +-0.007 +0.007 -0.006 where the first errors are in each case statistical and the others systematic. Combining these measurements, an inclusive semileptonic branching fraction of charm hadrons of B(c to l) = 0.095 +-0.006 +0.007 -0.006 is obtained.

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arXiv1998

A Measurement of Rb using a Double Tagging Method

The fraction of Z to bbbar events in hadronic Z decays has been measured by the OPAL experiment using the data collected at LEP between 1992 and 1995. The Z to bbbar 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 Rb = 0.2178 +- 0.0011 +- 0.0013 was obtained, where the first error is statistical and the second systematic. The uncertainty on Rc, the fraction of Z to ccbar events in hadronic Z decays, is not included in the errors. The dependence on Rc is Delta(Rb)/Rb = -0.056*Delta(Rc)/Rc where Delta(Rc) is the deviation of Rc from the value 0.172 predicted by the Standard Model. The result for Rb agrees with the value of 0.2155 +- 0.0003 predicted by the Standard Model.

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arXiv1998

Search for Chargino and Neutralino Production at sqrt{s} = 181-184 GeV at LEP

A search for charginos and neutralinos, predicted by supersymmetric theories, has been performed using a data sample of 57 pb-1 at centre-of-mass energies of 181-184 GeV taken with the OPAL detector at LEP. No evidence for chargino or neutralino production has been found. Upper limits on chargino and neutralino pair production cross-sections are obtained as a function of the chargino mass, the lightest neutralino mass, and the second lightest neutralino mass. For large chargino masses the limits have been improved with respect to the previous analyses at lower centre-of-mass energies. Exclusion regions at 95% confidence level of parameters of the Constrained Minimal Supersymmetric Standard Model are determined for the case of a large universal scalar mass, m_0, implying heavy scalar fermions, and for the case of a small m_0 resulting in light scalar fermions and giving the worst-case limits. Within this framework and for the lightest chargino-neutralino mass difference greater than 5 GeV the 95% C.L. lower limits on the lightest chargino mass for m_0 = 500 GeV are 90.0 and 90.2 GeV for tan(beta) = 1.5 and 35 respectively. These limits for all m_0 (the worst-case) are 69.1 and 65.2 GeV for tan(beta) = 1.5 and 35 respectively. Exclusion regions are also presented for neutralino masses, including an absolute lower limit at 95% C.L. for the mass of the lightest neutralino of 30.1 GeV for m_0 = 500 GeV (24.2 GeV for all m_0), with implications for experimental searches for the lightest neutralino as a dark matter candidate.

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arXiv1998

Search for Acoplanar Lepton Pair Events in e+e- Collisions at sqrt{s} = 161, 172 and 183 GeV

A selection of di-lepton events with significant missing transverse momentum has been performed using a total data sample of 77.0 pb-1 at e+e- centre-of-mass energies of 161 GeV, 172 GeV and 183 GeV. The observed numbers of events: four at 161 GeV, nine at 172 GeV, and 78 at 183 GeV, are consistent with the numbers expected from Standard Model processes, which arise predominantly from W+W- production with each W decaying leptonically. This topology is an experimental signature also 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 apparent and model independent limits on the production cross-section times branching ratio squared for various new physics processes are presented. Assuming a 100% branching ratio for the decay of a right-handed charged slepton to a charged lepton and the lightest neutralino, we exclude at 95% CL: right-handed smuons with masses below 65 GeV for smuon - neutralino mass differences greater than 2 GeV and right-handed staus with masses below 64 GeV for stau - neutralino mass differences greater than 10 GeV. Right-handed selectrons are excluded at 95% CL for masses below 77 GeV for selectron - neutralino mass differences greater than 5 GeV within the framework of the Minimal Supersymmetric Standard Model assuming mu < -100 GeV and tan{beta} = 1.5.

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arXiv1998

Measurement of the Longitudinal Cross-Section using the Direction of the Thrust Axis in Hadronic Events at LEP

In the process e+e- to hadrons, one of the effects of gluon emission is to modify the 1+cos(theta)**2 form of the angular distribution of the thrust axis, an effect which may be quantified by the longitudinal cross-section. Using the OPAL detector at LEP, we have determined the longitudinal to total cross-section ratio to be 0.0127+-0.0016+-0.0013 at the parton level, in good agreement with the expectation of QCD computed to Order(alpha_s**2) Comparisions at the hadron level with Monte Carlo models are presented. The dependence of the longitudinal cross-section on the value of thrust has also been studied, and provides a new test of QCD.

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arXiv1998

A Study of Parton Fragmentation in Hadronic Z0 Decays Using Lambda-Lambdabar Correlations

The correlated production of Lambda and Lambdabar baryons has been studied using 4.3 million multihadronic Zo decays recorded with the OPAL detector at LEP. Di-lambda pairs were investigated in the full data sample and for the first time also in 2-jet and 3-jet events selected with the k_t algorithm. The distributions of rapidity differences from correlated Lambda-Lambdabar pairs exhibit short-range, local correlations and prove to be a sensitive tool to test models, particularly for 2-jet events. The JETSET model describes the data best but some extra parameter tuning is needed to improve agreement with the experimental results in the rates and the rapidity spectra simultaneously. The recently developed modification of JETSET, the MOdified Popcorn Scenarium (MOPS), and also HERWIG do not give satisfactory results. This study of di-lambda production in 2- and 3-jet events supports the short-range compensation of quantum numbers.

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arXiv1998

Search for Scalar Top and Scalar Bottom Quarks at sqrt(s) = 183 GeV at LEP

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 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 Z 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. Assuming a relatively light scalar neutrino, 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.

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arXiv1998

Production of Chi_c2 Mesons in Photon-Photon Collisions at LEP

We present an observation at LEP of the production of chi_c2 mesons in the collisions of two quasi-real photons using the OPAL detector. The chi_c2 mesons are reconstructed in the decay channel Chi_c2 to J/psi gamma to l+l- gamma (with l= e,mu) using all data taken at e+e- centre-of-mass energies of 91 and 183 GeV, corresponding to integrated luminosities of 167 and 55 pb-1 respectively. The two-photon width of the Chi_2 is determined to be 1.76 +- 0.47 +- 0.37 +- 0.15 keV where the first error is statistical, the second is systematic and the third comes from branching ratio uncertainties.

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arXiv1998

Tests of the Standard Model and Constraints on New Physics from Measurements of Fermion-pair Production at 183 GeV at LEP

Cross-sections for hadronic, b-bbar and lepton pair final states in e+e- collisions at sqrt(s) = 183 GeV, measured with the OPAL detector at LEP, are presented and compared with the predictions of the Standard Model. Forward-backward asymmetries for the leptonic final states have also been measured. Cross-sections and asymmetries are also presented for data recorded in 1997 at sqrt(s) = 130 and 136 GeV. The results are used to measure the energy dependence of the electromagnetic coupling constant alpha_em, and to place limits on new physics as described by four-fermion contact interactions or by the exchange of a new heavy particle such as a leptoquark, or of a squark or sneutrino in supersymmetric theories with R-parity violation.

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arXiv1998

Measurement of the Strong Coupling Constant and the Vector and Axial-Vector Spectral Functions in Hadronic Tau Decays

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_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_s(mtau**2) = 0.348 +- 0.009 +- 0.019 at the tau-mass scale and alpha_s(mz**2) = 0.1219 +- 0.0010 +- 0.0017 at the Z-mass scale. The values obtained for alpha_s(mz**2) 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 ~1.3 GeV**2 and s_0 = mtau**2 has been tested from direct fits to the integrated differential hadronic decay rate R_tau. A test of the saturation of QCD sum rules at the tau-mass scale has been performed.

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arXiv1998

Measurement of the Michel Parameters in Leptonic Tau Decays

The Michel parameters of the leptonic tau decays are measured using the OPAL detector at LEP. The Michel parameters 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(Z) corresponding to an integrated luminosity of 155 pb-1 during the years 1990 to 1995. If e-mu universality is assumed and inferring the tau polarization from neutral current data, the measured Michel parameters are extracted. 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.

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arXiv1998

Search for Higgs Bosons and New Particles Decaying into Two Photons at sqrt(s) = 183 GeV

A search for the resonant production of high mass photon pairs associated with a leptonic or hadronic system has been performed using a data sample of 57.7 pb-1 collected at an average center-of-mass energy of 182.6 GeV with the OPAL detector at LEP. No evidence for contributions from non-Standard Model physics processes was observed. The observed candidates are used to place limits on BR (H to gamma gamma) assuming a Standard Model production rate for Higgs boson masses up to 92 GeV, and on the production cross section for a scalar resonance decaying into di-photons up to a mass of 170 GeV. Upper limits on the product of cross section and branching ratios, sigma(e+e- to XY) * BR(X to gamma gamma) * BR(Y to f fbar) as low as 70fb are obtained over the M(X) range 10 - 170 GeV for the case where 10 < M(Y) < 160 GeV and M(X)+M(Y) > 90 GeV, independent of the nature of Y provided it decays to a fermion pair and has negligible width. Higgs scalars which couple only to gauge bosons at Standard Model strength are ruled out up to a mass of 90.0 GeV at the 95% confidence level. Limits are also placed on non-minimal Higgs sectors having triplet representations.

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arXiv1998

Multi-photon production in e+e- collisions at sqrt(s) = 183 GeV

The process e+e- to gamma gamma (gamma) is studied using data recorded with the OPAL detector at LEP. The data sample corresponds to a total integrated luminosity of 56.2 pb-1 taken at a centre-of-mass energy of 183 GeV. The measured cross-section agrees well with the expectation from QED. A fit to the angular distribution is used to obtain improved limits at 95% CL on the QED cut-off parameters: Lambda+ > 233 GeV and Lambda- > 265 GeV as well as a mass limit for an excited electron, M(e*) > 227 GeV assuming equal e*egamma and eegamma couplings. No evidence for resonance production is found in the invariant mass spectrum of photon pairs. Limits are obtained for the cross-section times branching ratio for a resonance decaying into two photons.

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arXiv1998

Measurement of Tau Branching Ratios to Five Charged Hadrons

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 e+e- centre-of-mass energies close to the Z resonance. The branching ratios are measured to be BR(tau- to 3h-2h+nutau) = 0.091+-0.014+-0.005% BR(tau- to 3h-2h+pi0nutau) = 0.027+-0.018+-0.007% where the first error is statistical and the second systematic.

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arXiv1998

Inclusive Production of Charged Hadrons and K_s Mesons in Photon-Photon Collisions

The production of charged hadrons and K_s mesons in the collisions of quasi-real photons has been measured using the OPAL detector at LEP. The data were taken at e+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 K_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 10-125 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.

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arXiv1998

Measurement of the Average Polarization of b Baryons in Hadronic Z^0 Decays

In the Standard Model, b quarks produced in e^+e^- annihilation at the Z^0 peak have a large average longitudinal polarization of -0.94. Some fraction of this polarization is expected to be transferred to b-flavored baryons during hadronization. The average longitudinal polarization of weakly decaying b baryons, , is measured in approximately 4.3 million hadronic Z^0 decays collected with the OPAL detector between 1990 and 1995 at LEP. Those b baryons that decay semileptonically and produce a Λbaryon are identified through the correlation of the baryon number of the Λand the electric charge of the lepton. In this semileptonic decay, the ratio of the neutrino energy to the lepton energy is a sensitive polarization observable. The neutrino energy is estimated using missing energy measurements. From a fit to the distribution of this ratio, the value = -0.56^{+0.20}_{-0.13} +/- 0.09 is obtained, where the first error is statistical and the second systematic.

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arXiv1998

Measurements of Flavour Dependent Fragmentation Functions in Z^0 -> qq(bar) Events

Fragmentation functions for charged particles in Z -> qq(bar) 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 D* mesons and secondary vertices. The ξ_p = ln(1/x_E) distributions and the position of their maxima ξ_max are also presented separately for uds, c and b quark events. The fragmentation function for b quarks is significantly softer than for uds quarks.

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