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.
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 charginos nearly mass-degenerate with the lightest supersymmetric particle is performed using the 176 pb
−1 of data collected at 189 GeV in 1998 with the L3 detector. Mass differences ...between the chargino and the lightest supersymmetric particle below 4 GeV 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 at LEP2, in the case of heavy scalar leptons, chargino mass limits are obtained for any
χ
̃
±
1−
χ
̃
0
1
mass difference.
We report on measurements of $\mathrm{e^{+}e^{-}}$ annihilation into hadronsand lepton pairs.The data have been collected with the L3 detector at LEP at centre-of-massenergies between 130 and ...189~\textrm{Ge\kern -0.1em V}.Using a total integrated luminosity of 243.7~pb$^{-1}$, 25864 hadronic and 8573lepton-pair events are selected for the measurement of cross sections and leptonicforward-backward asymmetries. The results are in good agreement with Standard Model predictions.
Searches for scalar top and scalar bottom quarks, as well as for mass-degenerate scalar quarks of the first two families are performed at 189 GeV centre-of-mass energy with the L3 detector at LEP. No ...signals are observed. Model-independent limits on the scalar top production cross sections are determined for the decay modes $\rm \tilde{t}_1 \rightarrow c \tilde{\chi}_1^0$ and $\rm \tilde{t}_1 \rightarrow b \ell \tilde{\nu}$. For scalar quarks of the other flavours $\tilde{\mathrm{q}} \to \mathrm{q} \tilde{\chi}_1^0$ decays are considered. Within the framework of the Minimal Supersymmetric Standard Model mass limits are set at 95\% C.L. for these particles. Indirect limits on the gluino mass are also derived.
We report on measurements of e
+e
− annihilation into hadrons and lepton pairs. The data have been collected with the L3 detector at LEP at centre-of-mass energies between 130 and 189 GeV. Using a ...total integrated luminosity of 243.7 pb
−1, 25864 hadronic and 8573 lepton-pair events are selected for the measurement of cross sections and leptonic forward-backward asymmetries. The results are in good agreement with Standard Model predictions.
We report on measurements of the inclusive production rate of
Σ
+ and
Σ
0 baryons in hadronic Z decays collected with the L3 detector at LEP. The
Σ
+ baryons are detected through the decay
Σ
+→p
π
0, ...while the
Σ
0 baryons are detected via the decay mode
Σ
0→
Λγ. The average numbers of
Σ
+ and
Σ
0 per hadronic Z decay are measured to be:
N
Σ
+
+
N
Σ
̄
+
=0.114±0.011
stat
±0.009
syst
N
Σ
0
+
N
Σ
̄
0
=0.095±0.015
stat
±0.013
syst
.
These rates are found to be higher than the predictions from Monte Carlo hadronization models and analytical parameterizations of strange baryon production.
A first measurement of the cross section of the process e
+e
−→Z
γγ is reported using a total integrated luminosity of 231
pb
−1 collected with the L3 detector at centre-of-mass energies of
182.7
GeV
...and
188.7
GeV
. By selecting hadronic events with two isolated photons the e
+e
−→Z
γγ cross section is measured to be
0.49
+0.20
−0.17±0.04
pb
at
182.7
GeV
and
0.47±0.10±0.04
pb
at
188.7
GeV
. The measurements are consistent with Standard Model expectations. Limits on Quartic Gauge Boson Couplings
a
0/
Λ
2 and
a
c
/
Λ
2 of
−0.009
GeV
−2<a
0/Λ
2<0.008
GeV
−2
and
−0.007
GeV
−2<a
c/Λ
2<0.013
GeV
−2
are derived at 95% confidence level.
We report the result of a search for scalar leptons 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 are set on the production cross sections for these new particles. 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 scalar electron of 65.5 GeV.
The pair production of Z bosons is studied using the data collected by the L3 detector at LEP in 1998 in $\rm e^+e^-$ collisions at a centre--of--mass energy of 189\,GeV. All the visible final states ...are considered and the cross section of this process is measured to be $0.74^{+0.15}_{-0.14}\,{(\rm stat.)}\,{\pm 0.04}\,{(\rm syst.)}\, \mathrm{pb}$. Final states containing b quarks are enhanced by a dedicated selection and their production cross section is found to be $0.18^{+0.09}_{-0.07}\,{(\rm stat.)}\,\pm 0.02\,{\rm (syst.)}\, \mathrm{pb}$. Both results are in agreement with the Standard Model predictions. Limits on anomalous couplings between neutral gauge bosons are derived from these measurements
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