The Virtue of Precision Particle Physics

Abstract

Precision studies and tests of fundamental interactions and symmetries are a focus point of modern particle physics. With the success of the present Standard Model (SM) of particle physics and simultaneously with the need for new physics to fix known deficiencies a broad search program is under way to probe at all energy scales for the unknown. Precision measurements of quantities which can be accurately predicted in the SM and also especially those which are predicted to be null or negligibly small turn out to provide some of the most stringent constraints for many new physics scenarios. Besides some detours, this paper concentrates on precision physics using neutrons, pions and muons, the lightest unstable particles of their kind, produced at the most powerful proton accelerators.KEYWORDS: precision particle physics, symmetry, lepton flavor, charge parity, dipole moment, weak interaction, rare decay, exotic atom, spectroscopy Setting the sceneComplementarity is a buzz word in today's research. Here it is used to describe the relation between the direct production of new particles and observations of virtual contributions from such particles. Direct production of new heavy particles and resonances and their detection through their decays is the realm of high energy collider particle physics. The recent discovery of a Higgs boson [1,2] displaying so far all features expected for the Standard Model (SM) Higgs is the latest discovery and triumph of this successful line of research. Indeed, the careful investigation of this Higg's particle properties (see, e.g. [3]) and the continued search for possible new particles with larger masses is the ongoing high priority program at CERN's LHC which will resume operation at higher energy soon.The broader look at 'fundamental physics in the era of LHC' has identified many complementary efforts and established the case and the need for a program at the high intensity, low energy, precision physics frontier, e.g. [4][5][6]. The observation of indirect, short distance effects of virtual particles becomes possible in the comparison of precision measurements and precision theoretical predictions. As it turns out, many of these experiments at low energy probe physics at considerably higher energy scale than direct collider searches, often multi-TeV or even 1000 TeV.Some finite observables allow, both, very precise and accurate experimental determinations and very precise and accurate, often multi-loop, calculations within the SM or some of its extensions. Prime examples for these observables are the anomalous magnetic moments a e/µ = (g − 2)/2 of the electron [7] and the muon [8,9]. For instance, the best determination today of the fine structure constant α relies on the a e measurement and a high precision QED calculation [10]. Likewise, the electron mass [11] is obtained by comparing a high precision quantum jump spectroscopy measurement in a Penning trap with a high precision calculation of the bound electron g-factor. More examples can be easily found, as the extraction of fu...