4,774 research outputs found
Chirality of tensor perturbations for complex values of the Immirzi parameter
In this paper we generalise previous work on tensor perturbations in a de
Sitter background in terms of Ashtekar variables to cover all complex values of
the Immirzi parameter gamma (previous work was restricted to imaginary gamma).
Particular attention is paid to the case of real gamma. Following the same
approach as in the imaginary case, we can obtain physical graviton states by
invoking reality and torsion free conditions. The Hamiltonian in terms of
graviton states has the same form whether gamma has a real part or not; however
changes occur for the vacuum energy and fluctuations. Specifically, we observe
a gamma dependent chiral asymmetry in the vacuum fluctuations only if gamma has
an imaginary part. Ordering prescriptions also change this asymmetry. We thus
present a measurable result for CMB polarisation experiments that could shed
light on the workings of quantum gravity.Comment: 6 pages, 1 figur
Study of moments of event shapes in e+e- annihilation using JADE data
Data from e+e- annihilation into hadrons collected by the JADE experiment at
centre-of-mass energies between 14 GeV and 44 GeV were used to study moments of
event shape distributions. The data were compared with Monte Carlo models and
with predictions from QCD NLO order calculations. The strong coupling constant
measured from the moments is alpha_S(M_Z) = 0.1286 +/- 0.0007 (stat) +/- 0.0011
(expt) +/- 0.0022 (had) +/- 0.0068 (theo), alpha_S(M_Z) = 0.1286 +/- 0.0072
(total error), consistent with the world average. However, systematic
deficiencies in the QCD NLO order predictions are visible for some of the
higher moments.Comment: JADE note 147 submitted as contributed paper to ICHEP 2004, corrected
statistical error of 6 observable average and several typo
Measurement of the Strong Coupling Constant alpha_S from the Four-Jet Rate in e+e- Annihilation using JADE data
Data from e+e- annihilation into hadrons collected by the JADE experiment at
centre-of-mass energies between 14 GeV and 44 GeV were used to study the
four-jet rate as a function of the Durham algorithm's resolution parameter
y_cut. The four-jet rate was compared to a QCD NLO order calculations including
NLLA resummation of large logarithms. The strong coupling constant measured
from the four-jet rate is alpha_S(M_Z) = 0.1169 +/- 0.0004 (stat) +/- 0.0012
(expt) +/- 0.0021 (had) +/- 0.0007 (theo), alpha_S(M_Z) = 0.1169 +/- 0.0026
(total error) in agreement with the world average.Comment: JADE note 146 submitted as contributed paper to ICHEP 200
New Fermions at ee Colliders: II.~Signals and Backgrounds
We discuss the production, at high--energy ee linear colliders, of
new heavy fermions predicted by extensions of the Standard Model. We analyze in
great details the various signals and the corresponding backgrounds for both
pair production and single production in association with ordinary fermions.
Concentrating on new leptons, we use a model detector for ee collisions
at a center of mass energy of 500 GeV, to illustrate the discovery potential of
the Next Linear Colliders.Comment: 22 pages + 9 figures (not included: available by mail from A.
Djouadi), Preprint UdeM-LPN-TH-93-15
Measurement of the strong coupling alpha_S from the three-jet rate in e+e- - annihilation using JADE data
We present a measurement of the strong coupling alpha_S using the three-jet
rate measured with the Durham algorithm in e+e- -annihilation using data of the
JADE experiment at centre-of-mass energies between 14 and 44 GeV. Recent
theoretical improvements provide predictions of the three-jet rate in e+e-
-annihilation at next-to-next-to-leading order. In this paper a measurement of
the three-jet rate is used to determine the strong coupling alpha_s from a
comparison to next-to-next-to-leading order predictions matched with
next-to-leading logarithmic approximations and yields a value for the strong
coupling alpha_S(MZ) = 0.1199+- 0.0010 (stat.) +- 0.0021 (exp.) +- 0.0054
(had.) +- 0.0007 (theo.) consistent with the world average.Comment: 27 pages, 8 figure
Theoretical uncertainties for measurements of alpha_s from electroweak observables
One of the most precise measurements of the strong coupling constant
alpha_s(MZ) is obtained in the context of global analyses of precision
electroweak data. This article reviews the sensitivity of different electroweak
observables to alpha_s and describes the perturbative uncertainties related to
missing higher orders. The complete renormalisation scale dependence for the
relevant observables is calculated at next-to-next-to-leading order and a new
method is presented to determine the corresponding perturbative uncertainty for
measurements of alpha_s based on these observables.Comment: v4: Revised version with new tables and figure
Non-Singlet QCD Analysis of the Structure Function F_2 in 3-Loops
First results of a non--singlet QCD analysis of the structure function
in 3--loop order based on the non--singlet world data are
presented. Correlated errors are determined and their propagation through the
evolution equations is performed analytically. The value for is
determined to be compatible with results from other
QCD analyses. Low moments for , and with
correlated errors are calculated which may be compared with results from
lattice simulations.Comment: 1 Latex file, 1 style file, 2 figures Current World average on
alpha_s adde
Sediment resuspension and erosion by vortex rings
Particle resuspension and erosion induced by a vortex ringinteracting with a sediment layer was investigated experimentally using flow visualization (particle image velocimetry), high-speed video, and a recently developed light attenuation method for measuring displacements in bed level. Near-spherical sediment particles were used throughout with relative densities of 1.2–7 and diameters (d)(d) ranging between 90 and 1600 μm1600 μm. Attention was focused on initially smooth, horizontal bedforms with the vortex ring aligned to approach the bed vertically. Interaction characteristics were investigated in terms of the dimensionless Shields parameter, defined using the vortex-ring propagation speed. The critical conditions for resuspension (whereby particles are only just resuspended) were determined as a function of particle Reynolds number (based on the particle settling velocity and dd). The effects of viscous damping were found to be significant for d/δ<15d/δ<15, where δδ denotes the viscous sublayer thickness. Measurements of bed deformation were obtained during the interaction period, for a range of impact conditions. The (azimuthal) mean crater profile is shown to be generally self-similar during the interaction period, except for the most energetic impacts and larger sediment types. Loss of similarity occurs when the local bed slope approaches the repose limit, leading to collapse. Erosion, deposition, and resuspension volumes are analyzed as a function interaction time, impact condition, and sediment size
Theoretical Uncertainties in the QCD Evolution of Structure Functions and their Impact on
The differences are discussed between various next-to-leading order
prescriptions for the QCD evolution of parton densities and structure
functions. Their quantitative impact is understood to an accuracy of 0.02\%.
The uncertainties due to the freedom to choose the renormalization and
factorization scales are studied. The quantitative consequences of the
different uncertainties on the extraction of the strong coupling constant
from scaling violations in deep--inelastic scattering are estimated
for the kinematic regime accessible at HERA.Comment: 10 pages Latex, including 3 eps-figures, and a style file. To appear
in: Proc. of the International Workshop: QCD and QED in Higher Orders,
Rheinsberg, April, 1996, Nucl. Phys. {\bf B} (Proc. Suppl); The lay-out of
the paper has been changed, one figure sent separately before has been bound
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