arXiv subjects
Publications and source records attributed to Siegfried Bethke.
Quantum Chromodynamics predicts that the strong coupling strength $α_s$ decreases with increasing energy or momentum transfer, and vanishes at asymptotically high energies. The history and the status of experimental tests of asymptotic freedom are summarised in this review.
As a part of the homage on Guido Altarelli, summarised in the book "From my vast repertoire - the legacy of Guido Altarelli" edited by S. Forte, A. Levy and G. Ridolfi, this contribution collects some of the technological and scientific highlights of precision physics at LEP, the Large Electron-Positron collider operated, from 1989 to 2000, at the European Laboratory for Particle Physics, CERN.
We present a determination of the strong coupling constant $α_s(m_Z)$ using inclusive top-quark pair production cross section measurements performed at the LHC and at the Tevatron. Following a procedure first applied by the CMS collaboration, we extract individual values of $α_s(m_Z)$ from measurements by different experiments at several centre-of-mass energies, using QCD predictions complete in NNLO perturbation theory, supplemented with NNLL approximations to all orders, and suitable sets of parton distribution functions. The determinations are then combined using a likelihood-based approach, where special emphasis is put on a consistent treatment of theoretical uncertainties and of correlations between various sources of systematic uncertainties. Our final combined result is $α_s(m_Z) = 0.1177^{+0.0034}_{-0.0036}$.
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.
Determinations of the strong coupling strength, $α_s$, are summarised and a new world average value of $α_s (M_Z)$ is determined, using a new method of pre-averaging results within classes of measurements like hadronic $τ$ decays, deep inelastic scattering processes, lattice calculations, electron-positron annihilation processes and electro-weak precision fits. The overall result is $$α_s (M_Z) = 0.1184 \pm 0.0007,$$ unchanged from the value obtained in 2009. This presentation is an excerpt from the QCD review section of the 2012 Review of Particle Physics (RPP) of the Particle Data Group.
Data from high-energy physics (HEP) experiments are collected with significant financial and human effort and are mostly unique. An inter-experimental study group on HEP data preservation and long-term analysis was convened as a panel of the International Committee for Future Accelerators (ICFA). The group was formed by large collider-based experiments and investigated the technical and organisational aspects of HEP data preservation. An intermediate report was released in November 2009 addressing the general issues of data preservation in HEP. This paper includes and extends the intermediate report. It provides an analysis of the research case for data preservation and a detailed description of the various projects at experiment, laboratory and international levels. In addition, the paper provides a concrete proposal for an international organisation in charge of the data management and policies in high-energy physics.
Long-term preservation of data and software of large experiments and detectors in high energy physics is of utmost importance to secure the heritage of (mostly unique) data and to allow advanced physics (re-)analyses at later times. Summarising the work of an international study group, motivation, use cases and technical details are given for an organised effort to secure and enable future use of past, present and future experimental data. As a practical use case and motivation, the revival of JADE data and the corresponding latest results on measuring $α_s$ in NNLO QCD are reviewed.
Measurements of $α_s$, the coupling strength of the Strong Interaction between quarks and gluons, are summarised and an updated value of the world average of $α_s (M_Z)$ is derived. Building up on previous reviews, special emphasis is laid on the most recent determinations of $α_s$. These are obtained from $τ$-decays, from global fits of electroweak precision data and from measurements of the proton structure function $\F_2$, which are based on perturbative QCD calculations up to $O(α_s^4)$; from hadronic event shapes and jet production in $\epem$ annihilation, based on $O(α_s^3) $ QCD; from jet production in deep inelastic scattering and from $Υ$ decays, based on $O(α_s^2) $ QCD; and from heavy quarkonia based on unquenched QCD lattice calculations. Applying pragmatic methods to deal with possibly underestimated errors and/or unknown correlations, the world average value of $α_s (M_Z)$ results in $α_s (M_Z) = 0.1184 \pm 0.0007$. The measured values of $α_s (Q)$, covering energy scales from $Q \equiv \mtau = 1.78$ GeV to 209 GeV, exactly follow the energy dependence predicted by QCD and therefore significantly test the concept af Asymptotic Freedom.
Data from \epem annihilation into hadrons, collected by the \Jade experiment at centre-of-mass energies between 14 GeV and 44 GeV, are used to study moments of event shape distributions. Models with hadronisation parameters tuned to the LEP 1 precision data provide an adequate description of the low energy data studied here. The NLO QCD calculations, however, show systematic deficiencies for some of the moments. The strong coupling measured from the moments which are reasonably described by NLO QCD, \asmz=0.1287\pm0.0007\stat\pm0.0011\expt\pm0.0022\had\pm0.0075\theo is consistent with the world average.
Quantum Chromodynamics (QCD), the gauge field theory of the Strong Interaction, has specific features, asymptotic freedom and confinement, which determine the behaviour of quarks and gluons in particle reactions at high and at low energy scales. QCD predicts that the strong coupling strength $\as$ decreases with increasing energy or momentum transfer, and vanishes at asymptotically high energies. In this review, the history and the status of experimental tests of asymptotic freedom are summarised. The world summary of measurements of $\as$ is updated, leading to an unambiguous verification of the running of $\as$ and of asymptotic freedom, in excellent agreement with the predictions of QCD. Averaging a set of measurements balanced between different particle processes and the available energy range, results in a new and improved world average of $\amz = 0.1189 \pm 0.0010 .$
A review of measurements of $α_s$ is given, representing the status of April 2004. The results prove the energy dependence of $α_s$ and are in excellent agreement with the expectations of Quantum Chromodynamics, QCD. Evolving all results to the rest energy of the $Z^0$ boson, the world average of $α_s(M_Z)$ is determined from measurements which are based on QCD calculations in complete NNLO perturbation theory, giving $α_s(M_Z) = 0.1182 \pm 0.0027 .$
Studies of hadronic final states of $e^+e^-$ annihilations at LEP are reviewed. The topics included cover hadronic event shapes, measurements of $α_s$, determinations of QCD colour factors and tests of the non-Abelian gauge structure of QCD, differences between quark and gluon jets, QCD with heavy quarks and selected results of two-photon scattering processes.
An update of measurements of the strong coupling constant $α_s$ is given, representing the status of September 2002. The results convincingly prove the energy dependence of $α_s$ and are in excellent agreement with the expectations of Quantum Chromodynamics, QCD. Evolving all results to the rest energy of the $Z^0$ boson, the new world average of $α_s(M_Z)$ is determined from measurements which are based on QCD calculations in complete NNLO perturbation theory, giving $$ α_s(M_Z) = 0.1183 \pm 0.0027 . $$
Studies of hadronic final states of $e^+e^-$ annihilations, observed at the Large Electron Positron Collider LEP at CERN, are reviewed. The topics included cover measurements of $α_s$, hadronic event shapes and hadronisation studies, tests of asymptotic freedom and of the non-Abelian gauge structure of QCD, differences between quark and gluon jets, tests of power corrections and selected results of two-photon scattering processes. The improvements obtained at LEP are demonstrated by comparing to results from the pre-LEP era. This article consists of a reproduction of slides presented at the LEPFest in October 2000, supplemented by a short descriptive text and a list of relevant references.
A short review of the history and a 'slide-show' of QCD tests in $e^+e^-$ annihilation is given. The world summary of measurements of $α_s$ is updated.
Measurements which probe the energy dependence of $α_s$, the coupling strength of the strong interaction, are reviewed. Jet counting in $e^+ e^-$ annihilation, combining results obtained in the centre of mass energy range from 22 to 133 GeV, provides direct evidence for an asymptotically free coupling, without the need to determine explicit values of $α_s$. Recent results from jet production in $e p$ and in $p \overline{p}$ collisions, obtained in single experiments spanning large ranges of momentum transfer, $Q^2$, are in good agreement with the running of $α_s$ as predicted by QCD. Mass spectra of hadronic decays of $τ$-leptons are analysed to probe the running $α_s$ in the very low energy domain, $0.7 GeV^2 < Q^2 < M_τ^2$. An update of the world summary of measurements of $α_s(Q^2)$ consistently proves the energy dependence of $α_s$ and results in a combined average of $α_s(M_Z) = 0.118 \pm 0.006$.