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