Hadronic event shape distributions from e super(+)e super(-) annihilation measured by the OPAL experiment at centre-of-mass energies between 91 GeV and 209 GeV are used to determine the strong ...coupling alpha sub(S). The results are based on QCD predictions complete to the next-to-next-to-leading order (NNLO), and on NNLO calculations matched to the resummed next-to-leading-log-approximation terms (NNLO+NLLA). The combined NNLO result from all variables and centre-of-mass energies is while the combined NNLO+NLLA result is The completeness of the NNLO and NNLO+NLLA results with respect to missing higher order contributions, studied by varying the renormalization scale, is improved compared to previous results based on NLO or NLO+NLLA predictions only. The observed energy dependence of alpha sub(S) agrees with the QCD prediction of asymptotic freedom and excludes the absence of running.
A search is made for charged Higgs bosons predicted by Two-Higgs-Doublet extensions of the Standard Model (2HDM) using electron-positron collision data collected by the OPAL experiment at
, ...corresponding to an integrated luminosity of approximately 600 pb
−1
. Charged Higgs bosons are assumed to be pair-produced and to decay into
,
τν
τ
or AW
±
. No signal is observed. Model-independent limits on the charged Higgs-boson production cross section are derived by combining these results with previous searches at lower energies. Under the assumption
, motivated by general 2HDM type II models, excluded areas on the
plane are presented and charged Higgs bosons are excluded up to a mass of 76.3 GeV at 95 % confidence level, independent of the branching ratio BR(H
±
→
τν
τ
). A scan of the 2HDM type I model parameter space is performed and limits on the Higgs-boson masses
and
m
A
are presented for different choices of tan
β
.
Hadronic event shape distributions from e
+
e
−
annihilation measured by the OPAL experiment at centre-of-mass energies between 91 GeV and 209 GeV are used to determine the strong coupling
α
S
. The ...results are based on QCD predictions complete to the next-to-next-to-leading order (NNLO), and on NNLO calculations matched to the resummed next-to-leading-log-approximation terms (NNLO + NLLA). The combined NNLO result from all variables and centre-of-mass energies is
while the combined NNLO + NLLA result is
The completeness of the NNLO and NNLO + NLLA results with respect to missing higher order contributions, studied by varying the renormalization scale, is improved compared to previous results based on NLO or NLO + NLLA predictions only. The observed energy dependence of
α
S
agrees with the QCD prediction of asymptotic freedom and excludes the absence of running.
Four-fermion final states
q
qe
+
e
−
and
q
q
μ
+μ
−
from neural current interactions in e
+e
− collisions are studied in the OPAL detector at LEP at centre-of-mass energies from 183 GeV to 209 GeV. ...The data analysed corresponds to a total integrated luminosity of about 650 pb
−1 recorded from 1997 to 2000. Corresponding to the acceptance of the OPAL detector, a signal definition is applied requiring both leptons to have a scattering angle |cos θ | < 0.95. Further requirements are made on the invariant masses of the fermions pairs. The extracted cross-sections for the processes
e
+e
− → q
qe
+e
−
and
e
+e
− → q
qμ
+μ
−
are consistent with the expectations from the Standard Model.
Measurement of αs with radiative hadronic events Ainsley, C.; Åkesson, P.F.; Alexander, G. ...
European physical journal. C, Particles and fields,
2008/1, Letnik:
53, Številka:
1
Journal Article
Recenzirano
Odprti dostop
Hadronic final states with a hard isolated photon are studied using data taken at centre-of-mass energies around the mass of the Z boson with the OPAL detector at LEP. The strong coupling α
s
is ...extracted by comparing data and QCD predictions for event shape observables at average reduced centre-of-mass energies ranging from 24 GeV to 78 GeV, and the energy dependence of α
s
is studied. Our results are consistent with the running of α
s
as predicted by QCD and show that within the uncertainties of our analysis event shapes in hadronic Z decays with hard and isolated photon radiation can be described by QCD at reduced centre-of-mass energies. Combining all values from different event shape observables and energies gives α
s
(M
Z
)=0.1182±0.0015(stat.)±0.0101(syst.).
Data collected around $\sqrt{s}=91$ GeV by the OPAL experiment at the LEP e super(+)e super(-) collider are used to study the mechanism of baryon formation. As the signature, the fraction of super(-) ...hyperons whose baryon number is compensated by the production of a $\overline{\Sigma,\overline{\Lambda}$ or $\overline{\Xi$ antihyperon is determined. The method relies entirely on quantum number correlations of the baryons, and not rapidity correlations, making it more model independent than previous studies. Within the context of the JETSET implementation of the string hadronization model, the diquark baryon production model without the popcorn mechanism is strongly disfavored with a significance of 3.8 standard deviations including systematic uncertainties. It is shown that previous studies of the popcorn mechanism with $\Lambda \overline{\Lambda}$ and $\mathrm{p}\pi \overline{\mathrm{p}}$ correlations are not conclusive, if parameter uncertainties are considered.
Data collected around
GeV by the OPAL experiment at the LEP e
+
e
−
collider are used to study the mechanism of baryon formation. As the signature, the fraction of Σ
−
hyperons whose baryon number ...is compensated by the production of a
or
antihyperon is determined. The method relies entirely on quantum number correlations of the baryons, and not rapidity correlations, making it more model independent than previous studies. Within the context of the JETSET implementation of the string hadronization model, the diquark baryon production model without the popcorn mechanism is strongly disfavored with a significance of 3.8 standard deviations including systematic uncertainties. It is shown that previous studies of the popcorn mechanism with
and
correlations are not conclusive, if parameter uncertainties are considered.
The measurement of small–angle Bhabha scattering is used to determine the luminosity at the OPAL interaction point for the LEP I data recorded between 1993 and 1995. The measurement is based on the ...OPAL Silicon-Tungsten Luminometer which is composed of two calorimeters encircling the LEP beam pipe, on opposite sides of the interaction point. The luminometer detects electrons from small–angle Bhabha scattering at angles between 25 and 58 mrad. At LEP center-of-mass energies around the Z\(^0\), about half of all Bhabha electrons entering the detector fall within a 79 nb fiducial acceptance region. The electromagnetic showers generated in the stack of 1 radiation length tungsten absorber plates are sampled by 608 silicon detectors with 38,912 radial pads of 2.5 mm width. The fine segmentation of the detector, combined with the precise knowledge of its physical dimensions, allows the trajectories of incoming 45 GeV electrons or photons to be determined with a total systematic error of less than 7 microns. We have quantified all significant sources of systematic experimental error in the luminosity determination by direct physical measurement. All measured properites of the luminosity event sample are found to be in agreement with current theoretical expectations. The total systematic measurement uncertainty is \(3.4 \times 10^{-4}\), significantly below the theoretical error of \(5.4 \times 10^{-4}\) currently assigned to the QED calculation of the Bhabha acceptance, and contributes negligibly to the total uncertainty in the OPAL measurement of \(\Gamma_{\rm inv}/ \Gamma_{\ell^+\ell^-}\), a quantity of basic physical interest which depends crucially on the luminosity measurement.
In the Compact Muon Solenoid (CMS) experiment, muon detection in the forward direction is accomplished by cathode strip chambers (CSC). These detectors identify muons, provide a fast muon trigger, ...and give a precise measurement of the muon trajectory. There are 468 six-plane CSCs in the system. The efficiency of finding muon trigger primitives (muon track segments) was studied using 36 CMS CSCs and cosmic ray muons during the Magnet Test and Cosmic Challenge (MTCC) exercise conducted by the CMS experiment in 2006. In contrast to earlier studies that used muon beams to illuminate a very small chamber area (
<
0.01
m
2
), results presented in this paper were obtained by many installed CSCs operating in situ over an area of
≈
23
m
2
as a part of the CMS experiment. The efficiency of finding two-dimensional trigger primitives within six-layer chambers was found to be
99.93
±
0.03
%
. These segments, found by the CSC electronics within 800
ns after the passing of a muon through the chambers, are the input information for the Level-1 muon trigger and, also, are a necessary condition for chambers to be read out by the Data Acquisition System.
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