We report on a study of radiative Bhabha and quasi-real Compton scattering at centre-of-mass energies between 50 GeV and 170 GeV, and 20 GeV and 140 GeV, respectively, using the L3 detector at LEP. The analysis is based on data corresponding to an integrated luminosity of 232.2 pb −1 . A total of 2856 radiative Bhabha and 4641 Compton scattering events are collected. Total and differential cross sections for both reactions are presented and found to be in good agreement with QED expectations. Our measurement of Compton scattering at the highest energies obtained so far is used to derive exclusion limits on the coupling λ for the on-shell production of an excited electron e ★ decaying into a γ e pair in the mass range 20 GeV
Measured cross sections for radiative Bhabha scattering events.
Measured cross sections for the quasi-real Compton scattering 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 \xi_p = ln(1/x_E) distributions and the position of their maxima \xi_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.
Fragmentation function for 'uds-quark' events.
Fragmentation function for 'c-quark' events.
Fragmentation function for 'b-quark' events.
The inclusive production rates and differential cross-sections of photons and mesons with a final state containing photons have been measured with the OPAL detector at LEP. The light mesons covered by the measurements are the \pi^0, \eta, \rho(770)+-, \omega(782), \eta'(958) and a_0(980)+-. The particle multiplicities per hadronic Z^0 decay, extrapolated to the full energy range, are: <n_\gamma> = 20.97 +/- 0.02 +/- 1.15, <n_\pi^0> = 9.55 +/- 0.06 +/- 0.75, <n_\eta> = 0.97 +/- 0.03 +/- 0.11, <n_\rho^+-> = 2.40 +/- 0.06 +/- 0.43, <n_\omega> = 1.04 +/- 0.04 +/- 0.14, <n_\eta> = 0.14 +/- 0.01 +/- 0.02, <n_a_0+-> = 0.27 +/- 0.04 +/- 0.10. where the first errors are statistical and the second systematic. In general, the results are in agreement with the predictions of the JETSET and HERWIG Monte Carlo models.
Particle multiplicities per hadronic decay extrapolated to the full energy range.
Photon fragmentation function.
Photon fragmentation function.
The production of K^0_S mesons and Lambda baryons in quark and gluon jets has been investigated using two complementary techniques. In the first approach, which provides high statistical accuracy, jets were selected using different jet finding algorithms and ordered according to their energy. Production rates were determined taking into account the dependences of quark and gluon compositions as a function of jet energy as predicted by Monte Carlo models. Selecting three-jet events with the k_perp (Durham) jet finder (y_cut = 0.005), the ratios of K^0_S and Lambda production rates in gluon and quark jets relative to the mean charged particle multiplicity were found to be 1.10 +/- 0.02 +/- 0.02 and 1.41 +/- 0.04 +/- 0.04, respectively, where the first uncertainty is statistical and the second is systematic. In the second approach, a new method of identifying quark jets based on the collimation of energy flow around the jet axis is introduced and was used to anti-tag gluon jets in symmetric (Y-shaped) three-jet events. Using the cone jet finding algorithm with a cone size of 30 degrees, the ratios of relative production rates in gluon and quark jets were determined to be 0.94 +/- 0.07 +/- 0.07 for K^0_S and 1.18 +/- 0.10 +/- 0.17 for Lambda. The results of both analyses are compared to the predictions of Monte Carlo models.
Ratios of relative yields.
Ratios of absolute rates.
A determination of the number of light neutrino families performed by measuring the cross section of single photon production in e + e − collision near the Z resonance is reported. From an integrated luminosity of 100 pb −1 , collected during the years 1991–94, we have observed 2091 single photon candidates with an energy above 1 GeV in the polar angular region 45°< θ γ <135°. From a maximum likelihood fit to the single photon cross section, the Z decay width into invisible particles is measured to be Γ inv =498±12 (stat) ±12 (sys) MeV . Using the Standard Model couplings of neutrinos to the Z, the number of light neutrino species is determined to be N ν =2.98±0.07(stat)±0.07(sys).
No description provided.
Inclusive production of the f_0(980), f_2(1270) and \phi(1020) resonances has been studied in a sample of 4.3 million hadronic Z^0 decays from the OPAL experiment at LEP. A coupled channel analysis has been used for the f_0 in simultaneous fits to the resonances in inclusive \pi+\pi- and K+K- mass spectra. Fragmentation functions are reported for the three states. Total inclusive rates are measured to be 0.141 +/- 0.007 +/- 0.011 f_0, 0.155 +/- 0.011 +/- 0.018 f_2, and 0.091 +/- 0.002 +/- 0.003 \phi mesons per hadronic Z^0 decay. The production properties of the f_0, including those in three-jet events, are compared with those of the f_2 and \phi, and with the Lund string model of hadron production. All measurements are consistent with the hypothesis that the f_0 is a conventional qq(bar) scalar meson.
Total inclusive production rates.
Fragmentation functions. Additional systematic errors of 7.6 PCT for F0, 11.6 PCT for F2 and 3.5 PCT for PHI. The uncorrelated systematic errors for F0 and F2 are negligible in comparison to the other errors.
We have searched for first generation scalar leptoquark (LQ) pairs in the enu+jets channel using ppbar collider data (integrated luminosity= 115 pb^-1) collected by the DZero experiment at the Fermilab Tevatron during 1992-96. The analysis yields no candidate events. We combine the results with those from the ee+jets and nunu+jets channels to obtain 95% confidence level (CL) upper limits on the LQ pair production cross section as a function of mass and of beta, the branching fraction to a charged lepton. Comparing with the next-to-leading order theory, we set 95% CL lower limits on the LQ mass of 225, 204, and 79 GeV/c^2 for beta=1, 1/2, and 0, respectively.
The cross section values are extracted with the assumption that BR(LQ --> EQUARK) = 1/2.
We present a measurement of the average b-hadron lifetime τ b at the e + e − collider LEP. Using hadronic Z decays collected in the period from 1991 to 1994, two independent analyses have been performed. In the first one, the b-decay position is reconstructed as a secondary vertex of hadronic b-decay particles. The second analysis is an updated measurement of τ b using the impact parameter of leptons with high momentum and high transverse momentum. The combined result is τ b =[1549±9 (stat) ±15 (syst)] fs . In addition, we measure the average charged b-decay multiplicity 〈 n b 〉 and the normalized average b-energy 〈 x E 〉 b at LEP to be 〈n b 〉=4.90±0.04 (stat)±0.11 (syst), 〈x E 〉 b =0.709±0.004 (stat+syst).
No description provided.
No description provided.
The formation of the η ′ in the reaction e + e − →e + e − η ′→e + e − π + π − γ has been measured by the L3 detector at a centre-of-mass energy of 91 GeV . The radiative width of the η ′ has been found to be Γ γγ =4.17±0.10 (stat.) ±0.27 (sys.) keV . The Q 2 dependence of the η ′ formation cross section has been measured for Q 2 ≤10 GeV 2 and the η ′ electromagnetic transition form factor has been determined. The form factor can be parametrised by a pole form with Λ=0.900±0.046 (stat.) ±0.022 (sys.) GeV . It is also consistent with recent non-perturbative QCD calculations.
Cross section of etaprime production.
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.
No description provided.
No description provided.
The second syst. errors results due to extranal branching ratios. Charge conjugated states are implied. FD is considered as a quark fragmentation fraction. Sqrt(s(E+ E-)) = 91.2 GeV.
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