A study of neutral-current four-fermion processes is performed using a data sample corresponding to 55.3 pb −1 of integrated luminosity collected by the L3 detector at LEP at an average centre-of-mass energy of 183 GeV. The neutral-current four-fermion cross sections for final states with a pair of charged leptons plus jets and with four charged leptons are measured to be consistent with the Standard Model predictions. Events with fermion pair masses close to the Z boson mass are selected in all observable final states and the ZZ production cross section is measured to be σ ZZ =0.30 +0.22 +0.07 −0.16 −0.03 pb, in agreement with the Standard Model expectation. No evidence for the existence of anomalous triple gauge boson ZZZ and ZZ γ couplings is found and limits on these couplings are set.
No description provided.
The cross section of charm production in γγ collisions σ(e + e − →e + e − c c ̄ X) is measured at LEP with the L3 detector at centre-of-mass energies from 91 GeV to 183 GeV. Charmed hadrons are identified by electrons and muons from semileptonic decays. The direct process γγ→c c ̄ is found to be insufficient to describe the data. The measured cross section values and event distributions require contributions from resolved processes, which are sensitive to the gluon density in the photon.
Total cross section for inclusive charm production.
We report the first observation of two narrow charmed strange baryons decaying to $\Xi_c^+\gamma$ and $\Xi_c^0\gamma$, respectively, using data from the CLEO II detector at CESR. We interpret the observed signals as the $\Xi_c^{+\prime}(c{su})$ and $\Xi_c^{0\prime}(c{sd})$, the symmetric partners of the well-established antisymmetric $\Xi_c^+(c[su])$ and $\Xi_c^0(c[sd])$. The mass differences $M(\Xi_c^{+\prime})-M(\Xi_c^+)$ and $M(\Xi_c^{0\prime})-M(\Xi_c^0)$ are measured to be $107.8\pm 1.7\pm 2.5$ and $107.0\pm 1.4\pm 2.5 MeV/c^2$, respectively.
The data for two resonances are combined together.
CONST(NAME=EPS) is the parameter of the Peterson fragmentation function (C.Peterson et al., PR D27, 105 (1983)) D(N)/D(Z) = FD(Z) = const * (1/Z)*1/(1 - (1/Z)-CONST(NAME=EPS)/(1-Z))**2. The data for two resonances are combined together.
A study of W-pair production in e+e- annihilations at Lep2 is presented, based on 877 W+W- candidates corresponding to an integrated luminosity of 57 pb-1 at sqrt(s) = 183 GeV. Assuming that the angular distributions of the W-pair production and decay, as well as their branching fractions, are described by the Standard Model, the W-pair production cross-section is measured to be 15.43 +- 0.61 (stat.) +- 0.26 (syst.) pb. Assuming lepton universality and combining with our results from lower centre-of-mass energies, the W branching fraction to hadrons is determined to be 67.9 +- 1.2 (stat.) +- 0.5 (syst.)%. The number of W-pair candidates and the angular distributions for each final state (qqlnu,qqqq,lnulnu) are used to determine the triple gauge boson couplings. After combining these values with our results from lower centre-of-mass energies we obtain D(kappa_g)=0.11+0.52-0.37, D(g^z_1)=0.01+0.13-0.12 and lambda=-0.10+0.13-0.12, where the errors include both statistical and systematic uncertainties and each coupling is determined setting the other two couplings to the Standard Model value. The fraction of W bosons produced with a longitudinal polarisation is measured to be 0.242+-0.091(stat.)+-0.023(syst.). All these measurements are consistent with the Standard Model expectations.
Total W+ W- cross section measurement. The DSYS error corresponds to the total systematic error.
Cross section for W+ W- production in different decay channels. The DSYS error corresponds to the total systematic error.
New measurements at a centre-of-mass energy s ≃183 GeV of the hadronic photon structure function F γ 2 ( x ) in the Q 2 interval, 9 GeV 2 ≤ Q 2 ≤30 GeV 2 , are presented. The data, collected in 1997 with the L3 detector, correspond to an integrated luminosity of 51.9 pb −1 . Combining with the data taken at a centre-of-mass energy of 91 GeV, the evolution of F γ 2 with Q 2 is measured in the Q 2 range from 1.2 GeV 2 to 30 GeV 2 . F γ 2 shows a linear growth with ln Q 2 ; the value of the slope α −1 d F γ 2 ( Q 2 )/dln Q 2 is measured in two x bins from 0.01 to 0.2 and is somewhat higher than predicted.
Measured values of F2/ALPHA as a function of x. The second systematic error (DSYS) is that due to the model dependence and is the difference between the results obtained with PHOJET and TWOGAM. The full systematic error is the quadrature sum of the two systematic errors.
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 $e^+e^-$ $\rightarrow \nu \bar{\nu +}$ photon(s). No evidence is observed for new physics contributions to these final states. Using the data at $\sqrt{s} = 183$ GeV, upper limits on $\sigma$ ($e^+ e^-$ $\rightarrow$ X.Y)*BR(X $\to \textrm{Y}_{\gamma}$) and $\sigma$ ($e^+ e^-$ $\rightarrow$ X.X)*BR$^2$ (X $\to \textrm{Y}_{\gamma}$) are derived for the case of stable and invisible Y. These limits apply to single and pair production of excited neutrinos $(\textrm{X} = \nu^*, \textrm{Y} = \nu)$, to neutralino production $(\textrm{X} = \overline{\chi}^0_2, \textrm{Y} = \overline{\chi}^0_1)$ and to supersymmetric models in which $X = \overline{\chi}^0_1$ and $Y=\overline{\textrm{G}}$ is a light gravitino.
No description provided.
No description provided.
The data for sqrt(s) = 130 and 136 GeV are combination of present data and previous one (see EPJ C2, 607), the data for sqrt(s)=161 and 172 GeV is from thesame publication.
We present results obtained from a study of the structure of hadronic events recorded by the L3 detector at a centre-of-mass energy of 183 GeV. The data sample corresponds to an integrated luminosity of 55.3 pb −1 . The distributions of event shape variables and the energy dependence of their mean values are measured. From a comparison with resummed O ( α s 2 ) QCD calculations, we determine the strong coupling constant α s (183 GeV )=0.1086 ± 0.0026 (exp) ± 0.0054 (th) . The charged particle multiplicity distribution and momentum spectrum are studied and the energy dependence of the peak position of the ξ (=−ln x p ) distribution is compared with lower energy measurements and QCD expectations.
These data are superceded by the analysis presented in Acciarri et al PL B489,65.
Using a data sample collected with the CLEO II detector at CESR, we have searched for dipion transitions between pairs of $\Upsilon$ resonances at energies near the $\Upsilon(4S)$. We obtain upper limits $B(\Upsilon(4S)\to \Upsilon(2S)\pi^+\pi^-) < 3.9 \times 10^{-4}$ and $B(\Upsilon(4S)\to \Upsilon(1S)\pi^+\pi^-) < 1.2 \times 10^{-4}$. We also observe the transitions $\Upsilon(3S)\to \Upsilon(1S)$, $\Upsilon(3S)\to \Upsilon(2S)$, and $\Upsilon(2S)\to \Upsilon(1S)$, from which we measure the cross-sections for the radiative processes $e^+e^- \to \Upsilon(3S)\gamma$ and $e^+e^- \to \Upsilon(2S)\gamma$.
The cross sections are averaged from the ones obtained for E+ E- --> GAMMA UPSI(nS) < PI+ PI- UPSI(mS) < MU+ MU- > > and E+ E- --> GAMMA UPSI(nS) < PI+ PI-UPSI(mS) < E+ E- > > channels with n=2,3, m=1,2.
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.
Second systematic error depends on Rc=Delta(R_c)/R_c ratio, where Delta(R_c) is the deviation of R_c from the value 0.172 predicted by the Standard Model.
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.
Average multipicity of LAMBDA pairs in hadronic events.
Average multipicity of LAMBDA pairs in 2-Jet events.
Average multipicity of LAMBDA pairs in 3-Jet events.
Forum