Studies of QCD at LEPII

A combination of measurements of alpha_s in e+e- annihilation at LEP is presented. Distributions of various event-shape variables measured by ALEPH, DELPHI, L3 and OPAL at centre-of-mass energies from 41 GeV to 206 GeV are analysed in a common theoretical framework using O(a_s**2)+NLLA predictions. The dominant theoretical uncertainties associated to missing higher orders are studied in detail.Comment: 3 pages, 3 figure

Das Higgs-Teilchen und der Rest der Welt : Langjährige Suche ist endlich von Erfolg gekrönt

Physiker am CERN in Genf haben in diesem Sommer die Beobachtung eines neuen Teilchens bekannt gegeben. Dabei handelt es sich um das schwerste je beobachtete elementare Teilchen, das 134mal so schwer wie das Proton ist. Vieles spricht dafür, dass es sich bei der Beobachtung um das lange gesuchte Higgs-Teilchen handelt, den letzten fehlenden Baustein im Standardmodell der Teilchenphysik. Die Arbeitsgruppe von Prof. Düren und Dr. Stenzel am II. Physikalischen Institut der Justus-Liebig-Universität ist als Mitglied der ATLAS-Kollaboration an den Messungen beteiligt

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

Search for dark matter produced in association with bottom or top quarks in √s = 13 TeV pp collisions with the ATLAS detector

A search for weakly interacting massive particle dark matter produced in association with bottom or top quarks is presented. Final states containing third-generation quarks and miss- ing transverse momentum are considered. The analysis uses 36.1 fb−1 of proton–proton collision data recorded by the ATLAS experiment at √s = 13 TeV in 2015 and 2016. No significant excess of events above the estimated backgrounds is observed. The results are in- terpreted in the framework of simplified models of spin-0 dark-matter mediators. For colour- neutral spin-0 mediators produced in association with top quarks and decaying into a pair of dark-matter particles, mediator masses below 50 GeV are excluded assuming a dark-matter candidate mass of 1 GeV and unitary couplings. For scalar and pseudoscalar mediators produced in association with bottom quarks, the search sets limits on the production cross- section of 300 times the predicted rate for mediators with masses between 10 and 50 GeV and assuming a dark-matter mass of 1 GeV and unitary coupling. Constraints on colour- charged scalar simplified models are also presented. Assuming a dark-matter particle mass of 35 GeV, mediator particles with mass below 1.1 TeV are excluded for couplings yielding a dark-matter relic density consistent with measurements