αs determination from event shapes at DELPHI

Abstract

Recent studies of the strong coupling α s using the DELPHI detector at LEP [1,2] are reviewed. For the updated analysis of the data at √ s = M Z , a large number of shape observables and their dependence on the event orientation are measured and compared to theoretical calculations in O ( α s 2). The adjustment of the renormalization scale μ turns out to be of major importance. Applying the so-called physical scale μ 2 = Q 2 leads to a poor description of the data, whereas a combined fit of α s and the renormalization scale value yields an excellent description of the high statistics data. For these scale values perfect consistency of the fit values of α s ( M Z ) is obtained for all the observables considered within a range of Δα s = ±0.0025. Taking also account of heavy quark mass effects yields the preliminary result α s ( M Z 2) = 0.117 ± 0.003. Additional studies include an α s determination using theoretical predictions in the next to leading log approximation (NLLA), matched NLLA and O ( α s 2) predictions as well as theoretically motivated optimized scale setting methods. The running of α s is measured from the DELPHI data at √ s = 130 GeV − 183 GeV. Here an analytical power ansatz for the hadronization corrections is compared with hadronization corrections obtained by using Monte Carlo generators.