arXiv subjects

The OPAL collaboration

Publications and source records attributed to The OPAL collaboration.

At least 55 records · Page 3Linked to original sources

arXiv2003

Search for Stable and Long-Lived Massive Charged Particles in e+e- Collisions at sqrt(s) = 183-209 GeV

A search for stable and long-lived massive particles of electric charge |Q/e|=1 or fractional charges of 2/3, 4/3, and 5/3 is reported using data collected by the OPAL detector at LEP, at centre-of-mass energies from 130 to 209 GeV. These particles are assumed to be pair-produced in e+e- collisions and not to interact strongly. No evidence for the production of these particles was observed. Model-independent upper limits on the production cross-section between 0.005 and 0.028 pb have been derived for scalar and spin-1/2 particles with charge +-1. Within the framework of the Constrained Minimal Supersymmetric Standard Model (CMSSM), this implies a lower limit of 98.0 (98.5) GeV on the mass of long-lived right (left)- handed scalar muons and scalar taus. Long-lived charged heavy leptons and charginos are excluded for masses below 102.0 GeV. For particles with fractional charge +-2/3, +-4/3, and +-5/3, the upper limit on the production cross-section varies between 0.005 and 0.020 pb. All mass and cross-section limits are derived at the 95% confidence level and are valid for particles with lifetimes longer than 10-6s.

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arXiv2003

Test of non-commutative QED in the process e+e- -> gamma gamma at LEP

Non-commutative QED would lead to deviations from the Standard Model depending on a new energy scale Lambda_NC and a unique direction in space defined by two angles eta and xi. Here in this analysis eta is defined as the angle between the unique direction and the rotation axis of the earth. The predictions of such a theory for the process e+e- -> gamma gamma are evaluated for the specific orientation of the OPAL detector and compared to the measurements. Distributions of the polar and azimuthal scattering angles are used to extract limits on the energy scale Lambda_NC depending on the model parameter eta. At the 95% confidence level Lambda_NC is found to be larger than 141 GeV for all eta and xi. It is shown that the time dependence of the total cross-section could be used to determine the model parameter xi if there were a detectable signal. These are the first limits obtained on non-commutative QED from an e+e- collider experiment.

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arXiv2003

Bose-Einstein Correlations of pi0 Pairs from Hadronic Z0 Decays

We observed Bose-Einstein correlation in pi0 pairs produced in Z0 decays using the data sample collected by the OPAL detector at LEP 1 from 1991 to 1995. Using a static Gaussian picture for the pion emitter source, we obtain the chaoticity parameter lambda = 0.55 +- 0.10 +- 0.10 and the source radius R = (0.59 +- 0.08 +- 0.05) fm. according to the JETSET and HERWIG Monte Carlo models, the Bose-Einstein correlations in our data sample largely connect pi0s originating from the decays of different hadrons. Prompt pions formed at string break-up of cluster decays only form a small fraction of the sample.

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arXiv2003

A Measurement of Semileptonic B Decays to Narrow Orbitally-Excited Charm Mesons

The decay chain b -> Bbar -> D^{**0} l nu X, D^{**0} -> D^{*+} pi^-, D^{*+} -> D^0 pi^+, D^0 ->(Kpi or K3pi) is identified in a sample of 3.9 million hadronic Z decays collected with the OPAL detector at LEP. The branching ratio BR (b -> Bbar) x BR (Bbar -> D^0_1 l nu X) x BR (D^0_1 -> D^{*+} pi^-) is measured to be (2.64 +- 0.79 (stat) +- 0.39 (syst)) X 10^-3 for the J^P = 1^+ (D^0_1) state. For decays into the J^P = 2^+ (D^{*0}_2) state, an upper limit of 1.4 X 10^-3 is placed on the branching ratio at the 95% confidence level.

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arXiv2003

Di-Jet Production in Photon-Photon colisions at sqrt(s)ee from 189 to 209 GeV

Di-jet producion is studied in collisions of quasi-real photons at e+e- centre- of-mass energies sqrt(s)ee from 189 to 209 GeV at LEP. The data were collected with the OPAL detector. Jets are reconstructed using an inclusive k_t clustering algorithm for all cross-section measurements presented. A cone jet algorithm is used in addition to study the different structure of the jets resulting from either of the algorithms. The inclusive di-jet cross-section is measured as a function of the mean transverse energy Etm(jet) of the two leading jets, and as a functiuon of the estimated fraction of the photon momentum carried by the parton entering the hard sub-process, xg, for different regions of Etm (jet). Angular distribution in di-jet events are measured and used to demonstrate the dominance of quark and gluon initiated processes in different regions of phase space. Furthermore the inclusive di-jet cross-section as a function of |eta(jet)| and |delta eta (jet)| is presented where eta(jet) is the jet pseudo-rapidity. Different regions of the xg+ -xg- -space are explored to study and control the influence of an underlying event. The results are compared to next-to-leading order perturbative QCD calculations and to the predictions of the leading order Monte Carlo generator PYTHIA.

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arXiv2002

A Measurement of the tau -> mu nu nu Branching Ratio

The tau -> mu nu nu branching ratio has been measured using data collected from 1990 to 1995 by the OPAL detector at the LEP collider. The resulting value of B(tau -> mu nu nu) = 0.1734 +- 0.0009(stat) +- 0.0006(syst) has been used in conjunction with other OPAL measurements to test lepton universality, yielding the coupling constant ratios gmu/ge=1.0005+-0.0044 and gt/ge=1.0031+-0.0048, in good agreement with the Standard Model prediction of unity. A value for the Michel parameter eta = 0.004 +- 0.037 has also been determined and used to find a limit for the mass of the charged Higgs boson mH+- > 1.25 tan beta, in the Minimal Supersymmetric Standard Model.

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arXiv2002

Charged particle multiplicities in heavy and light quark initiated events above the Z0 peak

We have measured the mean charged particle multiplicities separately for bbbar, ccbar and light quark (uubar, ddbar, ssbar) initiated events produced in e+e- annihilations at LEP. The data were recorded with the OPAL detector at eleven different energies above Z0 peak, corresponding to the full statistics collected at LPE1.5 and LEP2. The difference in mean charged and particle multiplicities for bbbar and light quark events, delta_bl, measured over this energy range is consistent with an energy independent behaviour, as predicted by QCD, but is inconsistent with the prediction of a more phenomenological approach which assumes that the multiplicity accompanying the decay of a heavy quark is independent of the quark mass itself. Our results, which can be combined into the single measurement delta_bl = 3.44+-0.40(stat)+-0.89(syst) at a luminosity weighted average centre-of mass energy of 195 GeV, are also consistent with an energy independent behaviour as extrapolated from lower energy data.

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arXiv2002

Search for Nearly Mass-Degenerate Charginos and Neutralinos at LEP

A search was performed for charginos with masses close to the mass of the lightest neutralino in e+e- collisions at centre-of-mass energies of 189-209 GeV recorded by the OPAL detector at LEP. Events were selected if they had an observed high-energy photon from initial state radiation, reducing the dominant background from two-photon scattering to a negligible level. No significant excess over Standard Model expectations has been observed in the analysed data set corresponding to an integrated luminosity of 570pb-1. Upper limits were derived on the chargino pair-productin cross-section, and lower limits on the chargino mass were derived in the context of the Minimal Supersymmetric Extension of the Standard Model for the gravity and anomaly mediated Supersymmetry breaking scenarios.

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arXiv2002

Measurement of Neutral-Current Four-Fermion Production at LEP2

Four-fermion final states qqbar e+e- and qqbar mu+mu- from neutral-current interactions in e+e- collisions are studied in the OPAL detector at LEP at centre-of-mass energies from 183 GeV to 209 GeV. The data analysed correspond to a total integrated luminosity of about 650 pb-1 recorded from 1997 to 2000. Corresponding to the acceptance of the OPAL detector, a signal definition is applied requiring both leptons to have a scattering angle satisfying cos0 <0.95. Further requirements are made on the invariant masses of the fermion pairs. The extracted cross-sections for the processes e+e- -> qqbar e+e- and e+e- qqbar mu+mu- are consistent with the expectations from the Standard Model.

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arXiv2002

Multi-Photon Production in e+e- collisions at sqrt(s) = 181 - 209 GeV

The process e+e- -> gamma gamma (gamma) is studied using data collected by the OPAL detector at LEP between the years 1997 and 2000. The data set corresponds to an integrated luminosity of 672.3pb-1 at centre-of-mass energies lying between 181 GeV and 209 GeV. Total and differential cross- sections are determined and found to be good agreement with the predictions of QED. Fits to the observed angular distributions are used to set limits on parameters from several models of physics beyond the Standard Model such as cut-off parameters, contact interactions of the type e+e-gamma gamma, gravity in extra spatial dimensions and excited electrons. In events with three photons in the final state the mass spectrum of photon pairs is investigated. No narrow resonance X-> gamma gamma is found and limits are placed on the product of the Xgamma production cross-section and branching ratio.

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arXiv2002

Measurement of the b quark forward-backward asymmetry around the Z0 peak using an inclusive tag

The b quark forward-backward asymmetry has been measured using hadronic Z0 decays collected by the OPAL experiment at LEP. Z0 -> bbbar decays were selected using a combination of secondary vertex and lepton tags, and the sign of the b quark charge was determined using an inclusive tag based on jet, vertex and kaon charges. The results, corrected to the quark level, are: A^b_FB = 0.0582 +- 0.0153 +- 0.0012 at sqrt(s) = 89.50 GeV A^b_FB = 0.0977 +- 0.0036 +- 0.0018 at sqrt(s) = 91.26 GeV A^b_FB = 0.1221 +- 0.0123 +- 0.0025 at sqrt(s) = 92.91 GeV where the first error is statistical and the second systematic in each case. Within the framework of the Standard Model, the result is interpreted as a measurement of the effective weak mixing angle for electrons of sin2theta_W^eff,e = 0.23205 +- 0.00068.

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arXiv2002

Search for the Standard Model Higgs Boson with the OPAL Detector at LEP

This paper summarises the search for the Standard Model Higgs boson in e+e- collisions at centre-of-mass energies up to 209 GeV performed by the OPAL Collaboration at LEP. The consistency of the data with the background hypothesis and various Higgs boson mass hypotheses is examined. No indication of a signal is found in the data and a lower bound of 112.7GeV/C^2 is obtained on the mass of the Standard Model Higgs boson at the 95% CL.

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arXiv2002

Search for a low mass CP-odd Higgs boson in e+e- collisions with the OPAL detector at LEP2

We have analysed the data collected by OPAL at centre-of-mass energies between 189 and 209 GeV searching for Higgs boson candidates from the process e+e- -> h0Z0 followed by the decay of h0 -> A0A0 where A0 is the CP-odd Higgs boson. The search is done in the region where the A0 mass, mA, is below the production threshold for bbbar, and the CP-even Higgs boson mass mh is within the range 45-86 GeV/c^2. In this kinematic range, the decay of h0 -> A0A0 may be dominant and previous Higgs boson searches have very small sensitivities. This search can be interpreted within any model that predicts the existence of at least one scalar and one pseudoscalar Higgs boson. No excess of events is observed above the expected Standard Model backgrounds. Model-independent limits on the cross-section for the process e+e- -> h0Z0 are derived assuming 100% decays of the h0 into A0A0 and 100% decays of the A0A0 into each of the following final states: ccbarccbar, gggg, tau+tau-tau+tau-, ccbargg ggtau+tau- and ccbartau+tau-. The results are also interpreted in the CP-conserving no-mixing MSSM scenario, where the region 45 <= mh <= 85 GeV/c^2 and 2<= mA <= 9.5 GeV/c^2 is excluded.

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arXiv2002

Measurement of the Cross-Section for the Process GammaGamma -> Ppbar at sqrt(s)_ee=183-189 GeV at LEP

The exclusive production of proton-antiproton pairs in the collisions of two quasi-real photons had been studied using data taken at sqrt(s)_ee=183 GeV and 189 GeV with the OPAL detector at LEP. Results are presented for Ppbar invariant masses, W, in the range 2.15 W< <3.95 GeV. The cross-section measurements are compared with previous data and with recent analytic calculations based on the quark-diquark model.

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arXiv2002

Charged Particle Momentum Spectra in e+e- annihilation at sqrt(s) = 192-209 GeV

Charged particle momentum distributions are studied in the reaction e+e- -> hadrons, using data collected with the OPAL detector at centre-of-mass energies from 192 GeV to 209 GeV. The data correspond to an average centre-of- mass energy of 201.7 GeV and a total integrated luminosity of 433 pb-1. The measured distributions and derived quantities, in combination with corresponding results obtained at lower centre-of-mass energies, are compared to QCD predictions in various theoretical approaches to study the energy dependence of the strong interaction and to test QCD as the theory describing it. In general, a good agreement is found between the measurements and the corresponding QCD predictions.

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arXiv2002

Search for Scalar Top and Scalar Bottom Quarks at LEP

Searches for a scalar top quark and a scalar bottom quark have been performed using a data sample of 438 pb-1 at centre-of-mass energies of sqrt(s) = 192 - 209 GeV collected with the OPAL detector at LEP. No evidence for a signal was found. The 95% confidence level lower limit on the scalar top quark mass is 97.6 GeV if the mixing angle between the supersymmetric partners of the left- and right-handed states of the top quark is zero. When the scalar top quark decouples from the Z0 boson, the lower limit is 95.7 GeV. 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 decaying into a bottom quark, a charged lepton and a scalar neutrino has also been studied. The lower limit on the scalar top quark mass is 93.0 GeV for this decay mode, if the mass difference between the scalar top quark and the scalar neutrino is greater than 10 GeV and if the mixing angle of the scalar top quark is zero. From a search for the scalar bottom quark, a mass limit of 96.6 GeV was obtained if the mass difference between the scalar bottom quark and the lightest neutralino is larger than 10 GeV.

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arXiv2002

Search for Associated Production of Massive States Decaying into Two Photonsin e+e- Annihilations at sqrt(s) = 88-209 GeV

A search is performed for production of short-lived particles in e+e- -> XY, with X -> gamma gamma and Y -> ffbar, for scalar X and scalar or vector Y. Model-independent limits in the range of 25-60 femtobarns are presented on sigma (e+e- -> XY) x B(X -> ffbar) for centre-of-mass energies in the range 205-207 GeV. The data from all LEP centre-of-mass energies 88-209 GeV are also interpreted in the context of fermiophobic Higgs boson models, for which a lower mass limit of 105.5 GeV is obtained for a "benchmark" fermiophobic Higgs boson.

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arXiv2002

Search for Charged Excited Leptons in e+e- Collisions at sqrt(s) = 183-209 GeV

A search for charged excited leptons decaying into a lepton and photon has been performed using approximately 680 pb-1 of e+e- collision data collected by the OPAL detector at LEP at centre-of-mass energies between 183 GeV and 209 GeV. No evidence for their existence was found. Upper limits on the product of the cross-section and the branching fraction are inferred. Using results from the search for singly produced excited leptons, upper limits on the ratio of the excited lepton coupling constant to the compositeness scale are calculated. From pair production searches, 95% confidence level lower limits on the masses of excited electrons, muons and taus are determined to be 103.2 GeV.

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