We search for anomalous trilinear gauge couplings in the ZZ
γ and Z
γγ vertices using data collected with the L3 detector at LEP at a centre-of-mass energy
s
=189
GeV
. No evidence is found and ...limits on these couplings and on new physics scales are derived from the analysis of the process e
+e
−→Z
γ.
A search for a Higgs boson decaying into invisible particles is performed using the datacollected at LEP by the L3 experiment at centre-of-mass energies of$183\gev{}$ and $189\gev{}$. The integrated ...luminosities are respectively 55.3~\pb{} and 176.4~\pb{}.The observed candidates are consistent with the expectations from StandardModel processes.In the hypothesis that the production cross section of this Higgs bosonequals the Standard Model one and the branching ratio into invisibleparticles is $100\%$, a lower masslimit of $89.2\gev{}$ is set at $95\%$ confidence level.
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.
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 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 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 search for excited leptons, $\rm e^*,~\mu^*, ~\tau^*, ~\nu_e^*, ~\nu_{\mu}^*$ and $\nu_{\tau}^*$, was performed with the L3 detector at LEP using data collected at a centre--of--mass energy of 189 ...GeV, corresponding to an integrated luminosity of $\rm 176~pb^{-1}$. No evidence of their production is observed. From the searches for pair produced excited leptons, lower mass limits are set close to the kinematic limit. From the searches for singly produced excited leptons, upper limits on their couplings are derived in the mass range up to $189$ GeV.
We report the result of a search for charginos and neutralinos, in $\rm e^+e^-$ collisions at 189 GeV centre-of-mass energy at LEP. No evidence for such particles is found in a data sample of 176 ...pb$^{-1}$. Improved upper limits for these particles are set on the production cross sections. New exclusion contours in the parameter space of the Minimal Supersymmetric Standard Model are derived, as well as new lower limits on the masses of these supersymmetric particles. Under the assumptions of common gaugino and scalar masses at the GUT scale, we set an absolute lower limit on the mass of the lightest neutralino of 32.5 GeV and on the mass of the lightest chargino of 67.7 GeV.
The measurements of
R
b
=σ(
e
+
e
−
→
b
b
̄
)/σ(
e
+
e
−
→
q
q
̄
)
and of the b quark forward–backward charge asymmetry,
A
fb
b, at centre-of-mass energies above the Z pole are described. The ...measurement of
R
b is performed at
s
between 130 and 189
GeV
using a b-tagging method that exploits the relatively large decay length of b-hadrons. The measurement of
A
fb
b is performed using the large statistics event sample collected at
s
=189
GeV
with a lepton-tag analysis based on the selection of prompt muons and electrons. The results at
s
=189
GeV
are
R
b
=0.163±0.013
(stat.)±0.005
(syst.)
,
A
fb
b
=0.61±0.18
(stat.)±0.09
(syst.)
.
A search for a Higgs boson decaying into invisible particles is performed using the data collected at LEP by the L3 experiment at centre-of-mass energies of
183
GeV
and
189
GeV
. The integrated ...luminosities are respectively 55.3 pb
−1 and 176.4 pb
−1. The observed candidates are consistent with the expectations from Standard Model processes. In the hypothesis that the production cross section of this Higgs boson equals the Standard Model one and the branching ratio into invisible particles is 100%, a lower mass limit of
89.2
GeV
is set at 95% confidence level.
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