A search for chargino, neutralino and scalar lepton pair-production in e+e- collisions at the centre-of-mass energy of 189 GeV is performed under the assumptions that R-parity is not conserved in ...decays and only one of the coupling constants lambda_ijk, lambda'_ijk or lambda''_ijk is non-negligible. No signal is found in a data sample corresponding to an integrated luminosity of 176.4 pb-1. Limits on the production cross sections, on the Minimal Supersymmetric Standard Model parameters and on the masses of the supersymmetric particles are derived.
A search for charginos nearly mass-degenerate with the lightestsupersymmetric particle isperformed using the 176 {pb$^{-1}$ of data collected at 189 {Ge\kern -0.1em V} in 1998 with the L3detector. ...Mass differences between the chargino and the lightest supersymmetric particlebelow 4 {Ge\kern -0.1em V} are considered.The presence of a high transverse momentum photon is required to single out the signal from the photon-photon interaction background. No evidence for charginos is found and upper limits on the cross section for chargino pair production are set. For the first time, in the case ofheavy scalar leptons, chargino mass limits are obtained for any $\tilde{\chi}^{\pm}_1 - \tilde{\chi}^0_1$ mass difference.
We report on measurements of the inclusive production rate of $\Sigma^+$ and $\Sigma^0$ baryons in hadronic Z decays collected with the L3 detector at LEP. The $\Sigma^+$ baryons are detected through ...the decay $\Sigma^+ \rightarrow {\rm p} \pi^0$, while the $\Sigma^0$ baryons are detected via the decay mode $\Sigma^0 \rightarrow \Lambda \gamma$. The average numbers of $\Sigma^+$ and $\Sigma^0$ per hadronic Z decay are measured to be: \begin{eqnarray*} \left N_{\Sigma^+} \right + \left N_{\bar{\Sigma}^+} \right & = & 0.114 \pm 0.011_{\mbox{\it \small stat}} \pm 0.009_{\mbox{\it \small syst}} \\ \left N_{\Sigma^0} \right + \left N_{\bar{\Sigma}^0} \right & = & 0.095 \pm 0.015_{\mbox{\it \small stat}} \pm 0.013_{\mbox{\it \small syst}} \ \mbox{.} \end{eqnarray*} These rates are found to be higher than the predictions from Monte Carlo hadronization models and analytical parameterizations of strange baryon production.
The structure functions of real and virtual photons are derivedfrom cross section measurements of the reaction$\rm e^+e^-\rightarrow e^+e^- + \hbox{hadrons}$ at LEP.The reaction is studied at ...$\sqrt{\rm {s}} \simeq 91$ GeV with the L3detector. One of the final state electrons is detected at a large angle relative to the beam direction, leading to $Q^2$ values between40 GeV$^2$ and 500 GeV$^2$.The other final state electron is either undetected or it is detected ata four-momentum transfer squared $P^2$ between 1 GeV$^2$ and 8 GeV$^2$.These measurements are compared with predictions of the Quark PartonModel and other QCD based models.
The process $e+e- ->$ W+W-gamma is analysed using the data collected with the L3 detector at LEP at a centre-of-mass energy of 188.6GeV, corresponding to an integrated luminosity of 176.8pb$^{-1}$. ...Based on a sample of 42 selected W+W- candidates containing an isolated hard photon, the W+W-gamma cross section, defined within phase-space cuts, is measured to be: sigma_WWgamma = 290 +/- 80 +/- 16 fb, consistent with the Standard Model expectation. Including the process e+e- -> nu nu gamma gamma, limits are derived on anomalous contributions to the Standard Model quartic vertices W+W- gamma gamma and W+W-Z gamma at 95% CL: -0.043 GeV^-2 a_0/Lambda^2 0.043 GeV^-2 0.08 GeV^-2 a_c/Lambda^2 0.13 GeV^-2 0.41 GeV^-2 a_n/Lambda^2 0.37 GeV^-2.
A first measurement of the cross section of the process$\rm e^+e^-\rightarrow Z\gamma\gamma$ is reported using a total integrated luminosity of 231\,pb$^{-1}$collected with the L3 detector at ...centre-of-mass energiesof 182.7\,GeV and 188.7\,GeV. By selectinghadronic events with two isolated photonsthe $\rm e^+e^-\rightarrow Z\gamma\gamma$cross section is measured to be$ 0.49^{+0.20}_{-0.17}\pm0.04\,{\mathrm{pb}}$ at 182.7\,GeV and $0.47 \pm 0.10\pm0.04\,{\mathrm{pb}}$ at 188.7\,GeV. Themeasurements are consistent with Standard Model expectations.Limits on Quartic Gauge Boson Couplings $a_0/\Lambda^2$ and $a_c/\Lambda^2$ of-0.009\, GeV$^{-2}$ $ a_0/\Lambda^2 $ 0.008\,GeV$^{-2}$ and-0.007\,GeV$^{-2}$ $ a_c/\Lambda^2 $ 0.013\,GeV$^{-2}$ are derived at95\(onfidence level.
We present a combined measurement of $\Rb = \Gamma(\mathrm{Z \rightarrow b\overline{b}}) / \Gamma(\mathrm{Z} \rightarrow\mbox{hadro ns})$ and the semileptonic branching ratio of b quarks in Z decays, ...$\Brbl$, using double-tag methods. Two analyses are performed on one million hadronic Z decays collected in 1994 and 1995. The first analysis exploits the capabilities of the silicon microvertex detector. The tagging of b-events is based on the large impact parameter of tracks from weak b-decays with respect to the $\mathrm{e^+e^-}$ collision point. In the second analysis, a high-$p_t$ lepton tag is used to enhance the b-component in the sample and its momentum spectrum is used to constrain the model dependent uncertainties in the semileptonic b-decay. The analyses are combined in order to provide precise determinations of $\Rb$ and $\Brbl$: Rb = 0.2174 \pm 0.0015\mbox{(stat.)} \pm 0.0028\mbox{(sys.)}; $\Brbl = (10.16 \pm 0.13\mbox{(stat.)} \pm 0.30\mbox{(sys.)})\%.
A search for heavy neutrinos that are isosinglets under the standard $SU(2)_L$ gauge group is made at center-of-mass energies 130 $< \sqrt{s} <$ 189 GeV with the L3 detector at LEP. Such heavy ...neutrinos are expected in many extensions of the Standard Model. The search is performed for the first generation heavy singlet neutrino, $\mathrm{N}_e$, through the decay mode $\mathrm{N}_e \rightarrow \mathrm{e} + \mathrm{W}$. We set upper limits on the mixing parameter between the heavy and light neutrino for the heavy neutrino mass range from 80 GeV to 185 GeV.
Aims.Several experiments have reported observations on possible correlations between the flux of high energy muons and intense solar flares. If confirmed, these observations would have significant ...implications for acceleration processes in the heliosphere able to accelerate protons and other ions to energies of at least tens of GeV. Methods.The solar flare of the 14 of July 2000 offered a unique opportunity for the L3+C experiment to search for a correlated enhancement in the flux of muons using the L3 precision muon spectrometer. Its capabilities for observing a directional excess in the flux of muons above 15 GeV (corresponding to primary proton energies above 40 GeV) are presented along with observations made on the 14th of July 2000. Results.We report an excess which appeared at a time coincident with the peak increase of solar protons observed at lower energies. The probability that the excess is a background fluctuation is estimated to be 1%. No similar excess of the muon flux was observed up to 1.5 h after the solar flare time.
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