110 research outputs found
Formation of the in Two-Photon Collisions at LEP
The two-photon width of the meson has been
measured with the L3 detector at LEP. The is studied in the decay
modes , KK, KK,
KK, , , and
using an integrated luminosity of 140 pb at GeV and
of 52 pb at GeV. The result is
(BR) keV. The dependence of the cross section is studied for
GeV. It is found to be better described by a Vector Meson
Dominance model form factor with a J-pole than with a -pole. In addition,
a signal of events is observed at the mass. Upper limits
for the two-photon widths of the , , and are also
given
Study of Z Boson Pair Production in e+e- Collisions at LEP at \sqrt{s}=189 GeV
The pair production of Z bosons is studied using the data collected by the L3
detector at LEP in 1998 in 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 (stat.) +/- 0.04 (syst.) 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 (stat.) +/- 0.02
(syst.) pb. Both results are in agreement with the Standard Model predictions.
Limits on anomalous couplings between neutral gauge bosons are derived from
these measurements
Search for Scalar Leptons in e+e- collisions at \sqrt{s}=189 GeV
We report the result of a search for scalar leptons in 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 Ge
Search for Charginos with a Small Mass Difference with the Lightest Supersymmetric Particle at \sqrt{s} = 189 GeV
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, in the case of heavy scalar leptons, chargino mass limits are
obtained for any \tilde{\chi}^{+-}_1 - \tilde{\chi}^0_1 mass difference
Direct Observation of Longitudinally Polarised W Bosons
The three different helicity states of W bosons, produced in the reaction
e+e- -> W+W- -> l nu q q~ are studied using leptonic and hadronic W decays at
sqrt{s}=183GeV and 189GeV. The W polarisation is also measured as a function of
the scattering angle between the W- and the direction of the e- beam. The
analysis demonstrates that W bosons are produced with all three helicities, the
longitudinal and the two transverse states. Combining the results from the two
center-of-mass energies and with leptonic and hadronic W decays, the fraction
of longitudinally polarised W bosons is measured to be 0.261 +/- 0.051(stat.)
+/- 0.016(syst.) in agreement with the expectation from the Standard Model
Search for Low Scale Gravity Effects in e+e- Collisions at LEP
Recent theories propose that quantum gravity effects may be observable at LEP
energies via gravitons that couple to Standard Model particles and propagate
into extra spatial dimensions. The associated production of a graviton and a
photon is searched for as well as the effects of virtual graviton exchange in
the processes: e+e- -> gamma gamma, ZZ, WW, mu mu, tau tau, qq and ee No
evidence for this new interaction is found in the data sample collected by the
L3 detector at LEP at centre-of-mass energies up to 183 GeV. Limits close to 1
TeV on the scale of this new scenario of quantum gravity are set
Search for Charged Higgs Bosons in e+e- Collisions at \sqrt{s} = 189 GeV
A search for pair-produced charged Higgs bosons is performed with the L3
detector at LEP using data collected at a centre-of-mass energy of 188.6 GeV,
corresponding to an integrated luminosity of 176.4 pb^-1. Higgs decays into a
charm and a strange quark or into a tau lepton and its associated neutrino are
considered. The observed events are consistent with the expectations from
Standard Model background processes. A lower limit of 65.5 GeV on the charged
Higgs mass is derived at 95 % confidence level, independent of the decay
branching ratio Br(H^{+/-} -> tau nu)
Production of Single W Bosons at LEP
We report on the observation of single W boson production in a data sample collected by the L3 detector at LEP2. The signal consists of large missing energy final states with a single energetic lepton or two hadronic jets. The cross-section is measured to be at the centre of mass energy \sqrt{s}=172 \GeV{}, consistent with the Standard Model expectation. From this measurement the following limits on the anomalous WW gauge couplings are derived at 95\% CL: and
Measurement of the Running of the Fine-Structure Constant
Small-angle Bhabha scattering data recorded at the Z resonance and large-angleBhabha scattering data recorded at \textrm{Ge\kern -0.1em V} bythe L3 detector at LEP are used to measure the running of the effective fine-structure constant for spacelike momentum transfers. The results are\begin{eqnarray*} \alpha^{-1}(-2.1 \mathrm{Ge\kern -0.1em V}^{2}) - \alpha^{-1}(-6.25 \mathrm{Ge\kern -0.1em V}^{2}) & = & 0.78 \pm 0.26 \\ \alpha^{-1}(-12.25 \mathrm{Ge\kern -0.1em V}^{2}) - \alpha^{-1}(-3434 \mathrm{Ge\kern -0.1em V}^{2}) & = & 3.80 \pm 1.29, \\\end{eqnarray*}in agreement with theoretical predictions.Bhabha scattering data recorded at \sqrt{s}=189 GeV by the L3 detector at LEP are used to measure the running of the effective fine-structure constant for spacelike momentum transfers. The results are alpha^-1(-2.1 GeV^2) - alpha^-1(-6.25 GeV^2) = 0.78 +/- 0.26 alpha^-1(-12.25 GeV^2) - alpha^-1(-3434 GeV^2) = 3.80 +/- 1.29, in agreement with theoretical predictions.Small-angle Bhabha scattering data recorded at the Z resonance and large-angle Bhabha scattering data recorded at s =189 GeV by the L3 detector at LEP are used to measure the running of the effective fine-structure constant for spacelike momentum transfers. The results are α â1 (â2.1 GeV 2 )âα â1 (â6.25 GeV 2 )=0.78±0.26 α â1 (â12.25 GeV 2 )âα â1 (â3434 GeV 2 )=3.80±1.29, in agreement with theoretical predictions
Measurements of Cross Sections and Forward-Backward Asymmetries at the Z Resonance and Determination of Electroweak Parameters
We report on measurements of hadronic and leptonic cross sections and
leptonic forward-backward asymmetries performed with the L3 detector in the
years 1993-95. A total luminosity of 103 pb^-1 was collected at centre-of-mass
energies \sqrt{s} ~ m_Z and \sqrt{s} ~ m_Z +/- 1.8 GeV which corresponds to 2.5
million hadronic and 245 thousand leptonic events selected. These data lead to
a significantly improved determination of Z parameters. From the total cross
sections, combined with our measurements in 1990-92, we obtain the final
results: m_Z = 91189.8 +/- 3.1 MeV, Gamma_Z = 2502.4 +/- 4.2 MeV, Gamma_had =
1741.1 +/- 3.8 MeV, Gamma_l = 84.14 +/- 0.17 MeV. An invisible width of
Gamma_inv = 499.1 +/- 2.9 MeV is derived which in the Standard Model yields for
the number of light neutrino species N_nu = 2.978 +/- 0.014. Adding our results
on the leptonic forward-backward asymmetries and the tau polarisation, the
effective vector and axial-vector coupling constants of the neutral weak
current to charged leptons are determined to be \bar{g}_V^l = -0.0397 +/-
0.0017 and \bar{g}_A^l = -0.50153 +/- 0.00053.Including our measurements of the
Z -> b \bar{b} forward-backward and quark charge asymmetries a value for the
effective electroweak mixing angle of sin^2\bar{\theta}_W = 0.23093 +/- 0.00066
is derived. All these measurements are in good agreement with the Standard
Model of electroweak interactions. Using all our measurements of electroweak
observables an upper limit on the mass of the Standard Model Higgs boson of m_H
< 133 GeV is set at 95% confidence level.Comment: 97 pages, 37 figure
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