Measurement of the energy dependence of hadronic jet rates and the strong coupling alpha_s from the four-jet rate with the DELPHI detector at LEP
The DELPHI Collaboration, J. Abdallah
TL;DR
The study uses DELPHI data from 89–209 GeV to measure the four-jet rate in electron-positron annihilation, testing perturbative QCD and extracting the strong coupling α_s at next-to-leading order. Jet rates are analyzed with JADE, DURHAM, and CAMBRIDGE clustering, with hadronisation corrections derived from MC models and a careful treatment of renormalisation-scale uncertainties. The final result, α_s(M_Z^2) = 0.1175 ± 0.0030, agrees with the world average and other LEP measurements, and the running of α_s is observed with a slope compatible with QCD expectations. Using experimentally optimised scales, the CAMBRIDGE algorithm yields a robust, precise determination and confirms the energy dependence of α_s across LEP1/LEP2 energies.
Abstract
Hadronic events from the data collected with the DELPHI detector at LEP within the energy range from 89 GeV to 209 GeV are selected, their jet rates are determined and compared to predictions of four different event generators. One of them is the recently developed APACIC++ generator which performs a massive matrix element calculation matched to a parton shower followed by string fragmentation. The four-jet rate is used to measure alpha_s in the next-to-leading-order approximation yielding alpha_s(M_Z^2) = 0.1175 +/- 0.0030. The running of alpha_s determined by using four-jet events has been tested. The logarithmic energy slope is measured to be dα_s^{-1} / d\log E_{cm} = 1.14 +/- 0.36. Since the analysis is based on four-jet final states it represents an alternative approach to previous DELPHI alpha_s measurements using event shape distributions.