3,536 research outputs found

### Event Shapes and Power Corrections in e+e- Annihilation

The effects of the hadronisation of partons on the distribution of event
shape observables are associated with corrections which are suppressed by
reciprocal powers of the energy scale of the process. The correction is
determined by one non-calculable parameter \alpha_0 for which an universal
value of 0.5 +/- 20% is found from the investigation of the distribution of
event shape observables and their mean values measured in e^+e^- annihilation.Comment: 4 pages, LaTeX, 4 .eps-files included, sprocl.sty and wrapfig.sty
included, contribution to the Proceedings of the DIS 2000 conference, April
25-30, Liverpool, U.

### Determination of alpha_S at 500 GeV from Event Shapes and Jet Rates

The potential of the TESLA linear e+e- collider to determine the strong
coupling constant, alpha_S, at 500 GeV is investigated. Experimental
complications due to background from W- and Z-pairs, top-production, initial
state photon radiation and from beamstrahlung are considered. The hadronic
event selection procedures used by the experiments at LEP II are reviewed for
the applicability at TESLA. An estimate of the various error contributions to
the total uncertainty of an $\alpha_S$ determination is presented. It confirms
that hadronisation effects are diminished while the uncertainty from the choice
of the renormalisation scale will dominate. Fits of the ln(R)-matched second
order (O(alpha_S^2)) and resummed calculation (NLLA) to six observables are
used to estimate the error contributions. This yields the expectation of the
precision for alpha_S(500 GeV) of +-0.0025.Comment: 10 pages, LaTeX2e, 8 .eps-files include

### alpha_S Evolution from 35 GeV to 202 GeV and Flavour Independence

Determinations of the strong coupling constant alpha_S at centre-of-mass
energies of 192 through 202 GeV at LEP are presented. The energy evolution of
alpha_S is in agreement with the prediction of QCD. The combined investigation
of
OPAL and JADE data in the energy range of 35 through 189 GeV yields
alpha_S(m_Z)=0.1187^{+0.0034}_{-0.0019}. The strenght of the strong coupling is
flavour independent if quark mass effects are taken into account.Comment: 5 pages, LaTeX2e, 7 .eps-file included, Moriond.sty included,
contribution to the Proceedings of the QCD Moriond conference, March 18-25,
Les Arcs, Franc

### Combined QCD analysis of e^+ e^- data at sqrt(s) = 14 to 172 GeV

A study of the energy dependence of event shape observables is presented. The
strong coupling constant \alpha_s has been determined from the mean values of
six event shape observables. Power corrections, employed for the measurement of
\alpha_s, have been found to approximately account for hadronisation effects.Comment: 6 pages, uses espcrc2.sty (included) and epsfig.sty, LaTeX, 8
.eps-file

### Tests of Power Corrections to Event Shape Distributions from e+e- Annihilation

A study of differential event shape distributions using e+e- data at
centre-of-mass energies of 35 to 183 GeV is presented. We investigated
non-perturbative power corrections for the thrust, C-parameter, total and wide
jet broadening observables. We observe a good description of the distributions
by the combined resummed QCD calculations plus power corrections from the
dispersive approach. The single non-perturbative parameter \alpha_0 is measured
to be \alpha_0 (2 GeV) = 0.502 +- 0.013 (stat.) ^{+0.046)_{-0.032} (exp. syst.)
^{+0.074}_{-0.053} (theo. syst.) and is found to be universal for the
observables studied within the given systematic uncertainties. Using revised
calculations of the power corrections for the jet broadening variables,
improved consistency of the individual fit results is obtained. Agreement is
also found with results extracted from the mean values of event shape
distributions.Comment: 19 pages, LaTeX2e, 8 .eps-files included, paper contributed to the
EPS-HEP99 conference in Tampere, Finlan

### Tests of Power Corrections for Event Shapes in e+e- Annihilation

A study of perturbative QCD calculations combined with power corrections to
model hadronisation effects is presented. The QCD predictions are fitted to
differential distributions and mean values of event shape observables measured
in e+e- annihilation at centre-of-mass energies from 14 to 189 GeV. We
investigate the event shape observables thrust, heavy jet mass, C-parameter,
total and wide jet broadening and differential 2-jet rate and observe a good
description of the data by the QCD predictions. The strong coupling constant
alpha_S(M_Z) and the free parameter of the power correction calculations
alpha_0(2 GeV) are measured to be alpha_S(M_Z) = 0.1171 +/- 0.0032/0.0020 and
alpha_0(2 GeV) = 0.513 +/- 0.066/0.045. The predicted universality of alpha_0
is confirmed within the uncertainties of the measurements.Comment: 28 pages, LaTeX2e, 21 .eps-files included, accepted by Eur. Phys. J.

### C-Parameter and Jet Broadening at PETRA Energies

e^+e^- annihilation data recorded by the JADE detector at PETRA were used to
measure the C-parameter for the first time at \sqrt{s}= 35 and 44 GeV. The
distributions were compared to a resummed QCD calculation.
In addition, we applied extended resummed calculations to the total and wide
jet broadening variables, B_T and B_W. We combined the results on \alpha_s with
those of our previous study of differential 2-jet rate, thrust, and heavy jet
mass, obtaining
\alpha_s(35 GeV) = 0.1448 +0.0117 -0.0070 and \alpha_s(44 GeV) = 0.1392
+0.0105 -0.0074.
Moreover power corrections to the mean values of the observables mentioned
above were investigated considering the Milan factor and the improved
prediction for the jet broadening observables.
Our study, which considered e^+e^- data of five event shape observables
between \sqrt{s}= 14 and 183 GeV, yielded \alpha_s(M_{Z^0})=0.1177 +0.0035
-0.0034.Comment: 14 pages, LaTeX2e, 9 .eps-files included, abbreviated version of the
paper contributed to the ICHEP'98 conference in Vancouver, submitted to Phys.
Lett.

### Constraining Light Colored Particles with Event Shapes

Using recently developed techniques for computing event shapes with
Soft-Collinear Effective Theory, LEP event shape data is used to derive strong
model-independent bounds on new colored particles. In the effective field
theory computation, colored particles contribute in loops not only to the
running of alpha_s but also to the running of hard, jet and soft functions.
Moreover, the differential distribution in the effective theory explicitly
probes many energy scales, so event shapes have strong sensitivity to new
particle thresholds. Using thrust data from ALEPH and OPAL, colored adjoint
fermions (such as a gluino) below 51.0 GeV are ruled out to 95% confidence
level. This is nearly an order-of-magnitude improvement over the previous
model-independent bound of 6.3 GeV.Comment: 4 pages, 2 figure

### Jet fragmentation in e^+e^- annihilation

A short review of theoretical and experimental results on fragmentation in
e^+e^- annihilation is presented. Starting with an introduction of the concept
of fragmentation functions in e^+e^- annihilation, aspects of scaling
violation, multiplicities, small and large x, longitudinal, gluon, light and
heavy quark fragmentation are summarized

### Development of Muon Drift-Tube Detectors for High-Luminosity Upgrades of the Large Hadron Collider

The muon detectors of the experiments at the Large Hadron Collider (LHC) have
to cope with unprecedentedly high neutron and gamma ray background rates. In
the forward regions of the muon spectrometer of the ATLAS detector, for
instance, counting rates of 1.7 kHz/square cm are reached at the LHC design
luminosity. For high-luminosity upgrades of the LHC, up to 10 times higher
background rates are expected which require replacement of the muon chambers in
the critical detector regions. Tests at the CERN Gamma Irradiation Facility
showed that drift-tube detectors with 15 mm diameter aluminum tubes operated
with Ar:CO2 (93:7) gas at 3 bar and a maximum drift time of about 200 ns
provide efficient and high-resolution muon tracking up to the highest expected
rates. For 15 mm tube diameter, space charge effects deteriorating the spatial
resolution at high rates are strongly suppressed. The sense wires have to be
positioned in the chamber with an accuracy of better than 50 ?micons in order
to achieve the desired spatial resolution of a chamber of 50 ?microns up to the
highest rates. We report about the design, construction and test of prototype
detectors which fulfill these requirements

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