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.
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.
Correlations among hadrons with the same electric charge produced in Z0 decays are studied using the high statistics data collected from 1991 through 1995 with the OPAL detector at LEP. Normalized ...factorial cumulants up to fourth order are used to measure genuine particle correlations as a function of the size of phase space domains in rapidity, azimuthal angle and transverse momentum. Both all-charge and like-sign particle combinations show strong positive genuine correlations. One-dimensional cumulants initially increase rapidly with decreasing size of the phase space cells but saturate quickly. In contrast, cumulants in two- and three-dimensional domains continue to increase. The strong rise of the cumulants for all-charge multiplets is increasingly driven by that of like-sign multiplets. This points to the likely influence of Bose–Einstein correlations. Some of the recently proposed algorithms to simulate Bose–Einstein effects, implemented in the Monte Carlo model Pythia, are found to reproduce reasonably well the measured second- and higher-order correlations between particles with the same charge as well as those in all-charge particle multiplets.
The inclusive branching ratio for the process b→τ−ν̄τX has been measured using hadronic Z decays collected by the OPAL experiment at LEP in the years 1992–2000. The result is: ...BR(b→τ−ν̄τX)=(2.78±0.18±0.51)%. This measurement is consistent with the Standard Model expectation and puts a constraint of tanβ/MH±<0.53 GeV−1 at the 95% confidence level on Type II Two Higgs Doublet Models.
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