The τ−→μ−ν̄μντ 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(τ−→μ−ν̄μντ)=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 gμ/ge=1.0005±0.0044 and gτ/ge=1.0031±0.0048, in good agreement with the Standard Model prediction of unity. A value for the Michel parameter η=0.004±0.037 has also been determined and used to find a limit for the mass of the charged Higgs boson, mH±>1.28tanβ, in the Minimal Supersymmetric Standard Model.
We have measured the mean charged particle multiplicities separately for bb̄, cc̄ and light quark (uū,dd̄,ss̄) initiated events produced in e+e− annihilations at LEP. The data were recorded with the ...OPAL detector at eleven different energies above the Z0 peak, corresponding to the full statistics collected at LEP1.5 and LEP2.
The difference in mean charged particle multiplicities for bb̄ and light quark events, δbl, 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 δbl=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.
The b quark forward–backward asymmetry has been measured using hadronic Z0 decays collected by the OPAL experiment at LEP. Z0→bb̄ 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: AFBb=0.0582±0.0153±0.0012ats=89.50GeV,AFBb=0.0977±0.0036±0.0018ats=91.26GeV,AFBb=0.1221±0.0123±0.0025ats=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 sin2θeff,eW=0.23205±0.00068.
A search is performed for production of short-lived particles in e+e−→XY, with X→γγ and Y→ff̄, for scalar X and scalar or vector Y. Model-independent limits in the range of 25–60 femtobarns are ...presented on σ(e+e−→XY)×B(X→γγ)×B(Y→ff̄) 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.
A study of the energy dependence of event shape observables is presented. The strong coupling constant
α
s
has been determined from the mean values of six event shape observables. Power corrections, ...employed for the measurement of
α
s
, have been found to approximately account for hadronisation effects.
A muon tracking system consisting of four 9 cm × 10 cm sized bulk Micromegas detectors with 128 μm amplification-gap and two 10 cm × 10 cm triple GEM detectors is foreseen for high-precision tracking ...of 140 GeV muons at the H8 beamline at CERN with a rate of up to 10 kHz and an overall resolution below 40 μm. Larger detectors with an active area of 0.5 m 2 and more are under development for detector studies in high neutron or gamma ray background environments at the Gamma Irradiation Facility at CERN and the Munich tandem accelerator. Signal studies of both detector types have been performed by recording cosmic muon and 5.9 keV X-ray signals with a single charge-sensitive preamplifier using several gas-mixtures of Ar:CO 2 . The signals were digitized using 1GHz VME based flashADCs with 2520 sampling points. The analysis of the complete signal-cycles allows for the determination of rise times, pulse heights, timing fluctuations and discrimination of background, resulting in a FWHM energy resolution of about 20% and detection efficiencies of 99% and more. Models for signal formation in both detector types will be presented. The single detector spatial resolution of 80 μm was measured using a fast Gassiplex based strip readout with readout strips of 150 μm width and a pitch of 250 μm. The Gassiplex readout, formerly used at the HERMES experiment, had to be substantially adapted. No more crosstalk or non-linearities were observed after reconfiguration of the multiplexing amplifier on the frontend boards. The observed spatial resolution is limited by multiple scattering of the cosmic muons used in the laboratory. We also report on the sensitivity to gamma- and neutron background and on the behaviour of spatial resolution as a function of background rates.
Nucl.Phys.Proc.Suppl. 64 (1998) 22-26 A study of the energy dependence of event shape observables is presented. The
strong coupling constant \alpha_s has been determined from the mean values of
six ...event shape observables. Power corrections, employed for the measurement of
\alpha_s, have been found to approximately account for hadronisation effects.
The production of charm quarks is studied in deep-inelastic electron–photon scattering using data recorded by the OPAL detector at LEP at nominal e
+e
− centre-of-mass energies from 183 to 209 GeV. ...The charm quarks have been identified by full reconstruction of charged D
★ mesons using their decays into D
0
π with the D
0 observed in two decay modes with charged particle final states, K
π and K
πππ. The cross-section
σ
D
★
for production of charged D
★ in the reaction e
+e
−→e
+e
−D
★
X is measured in a restricted kinematical region using two bins in Bjorken
x, 0.0014<
x<0.1 and 0.1<
x<0.87. From
σ
D
★
the charm production cross-section
σ(
e
+
e
−→
e
+
e
−
c
c
̄
X)
and the charm structure function of the photon
F
2,c
γ
are determined in the region 0.0014<
x<0.87 and 5<
Q
2<100 GeV
2 . For
x>0.1 the perturbative QCD calculation at next-to-leading order agrees perfectly with the measured cross-section. For
x<0.1 the measured cross-section is 43.8±14.3±6.3±2.8 pb with a next-to-leading order prediction of 17.0
+2.9
−2.3 pb.
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