4,217 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 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
Predictions for Anomalous tau+ tau- gamma Production at LEP 1
We calculate distributions for tau+ tau- gamma production at LEP 1 taking
into account a potentially existing anomalous magnetic moment a_tau of the tau
lepton. The existing upper limits for |a_tau| are known from the dependence of
the decay Z -> tau+ tau- gamma on (a_tau)^2 and are of the order of (1 - 5)%.
We show that such limits are also sensitive to linear terms in a_tau, which are
of equal importance at |a_tau| ~ (1 - 2)% and dominate below this value.
Contributions from an electric dipole moment d_tau do not interfere with the
electromagnetic vertex or with the anomalous magnetic moment. Appropriate
formulae are derived.Comment: 13 pages, LaTex, 5 figure
Off-shell W pair production with anomalous couplings: The CC11 process
The differential cross-sections for processes of the type e+ e- to W+ W- to l
nu q anti-q are determined with account of background contributions and of
anomalous triple gauge boson couplings. Analytic expressions for dsigma/ds1 ds2
dcostheta, where theta is the production angle of the W boson, are numerically
integrated with the Fortran package GENTLE. QED corrections are taken into
account in the leading logarithmic approximation. The importance of the various
contributions is studied for center-of-mass energies of 190 GeV, 500 GeV, and 1
TeV.Comment: 26 pages, Latex, 6 figures, 2 tables, uses feynmp and epsf styles;
tables numerically updated, 5 references added, few typos corrected, version
to be published in EPJ
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.
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