Precision measurements probing nature's fundamental interactions help physicists to address the great challenges they face now, such as finding and verifying a theory beyond the Standard Model of particle physics that might, ultimately, unify gravity and quantum mechanics. While the energy scale of such theories might be out of reach of all but the largest colliders, or even beyond the reach of any experiment, it is possible to probe for suppressed effects at low energy scales in experiments of outstanding precision. Such experiments, e.g., probe the value of fundamental constants and set limits on their time dependence, try to understand the dominance of matter over antimatter, or search for new types of interactions. Precision measurements have been playing a central role in the shifts of paradigms in physics in the twentieth century and will very likely continue to do so.
In two volumes of Annalen der Physik a series of articles will address current precision efforts investigating fundamental interactions and their properties at lowest energies. Both volumes will contain review articles with broad or more focused topical overviews, as well as a series of original papers reporting on new experimental or technical progress from recent and ongoing experiments.
Review articles in this volume focus mainly on fundamental constants and symmetry tests. The last decade has witnessed significant progress in the precision of the unitarity test of the Cabibbo-Kobayashi-Maskawa quark-mixing matrix. The precision of both the Vud and Vus matrix elements has improved due to new and more precise measurements as well as important theoretical progress in nuclear and kaon decays, respectively. Further improvements, leading to a gain of another factor of about two in the unitarity test might be possible in the foreseeable future, thereby further improving the sensitivity to new physics, not included in the Standard Model (J. Hardy, I. S. Towner, pp. 443–451). Penning traps will continue to play an important role in this respect as is illustrated by recent results with the TITAN Penning trap mass spectrometer (A. A. Kwiatkowski et al., pp. 529–537).
Among the fundamental constants of Nature the fine structure constant, α, has a particular status, being the corner stone of quantum electrodynamics but also as a keystone for the determination of other fundamental physical constants. State of the art atomic physics methods to determine fundamental constants are reviewed and discussed (S. G. Karshenboim, pp. 472–483), together with the opportunity to provide a precise value of the ratio h/mu between the Planck constant and the atomic mass constant (R. Bouchendira et al., pp. 484–492). New quantum-mechanical calculations show that microwave and sub-millimeter molecular transition frequencies for a number of molecules which are usually observed in astronomical sources exhibit a very high sensitivity to the value of the fine structure constant, but also to the electron-to-proton mass ratio (M. G. Kozlov and S. A. Levshakov, pp. 452–471). This offers unique possibilities to test space- and time-invariance of fundamental constants by comparing precise laboratory measurements of the molecular rest frequencies and their astronomical counterparts, which could lead to astrophysical tests of Einstein's Equivalence Principle at an unprecedented level of sensitivity. Another unique laboratory to investigate the fine structure constant, as well as other fundamental quantities such as, e.g., the atomic mass of the electron are highly charged ions, and in particular the simple systems formed by hydrogen-like ions. In these, the bound electrons are subjected to extreme fields which generate a number of interesting features. High precision is achieved by confinement of the ions in Penning traps allowing specific manipulation and measurement techniques to be applied (M. Vogel and W. Quint, pp. 505–513). Finally, an improved determination of two other fundamental constants, i.e. the Rydberg constant and the 1S Lamb shift, is now possible through the precise determination of the 1S-3S and 1S-3D two-photon transitions in atomic hydrogen, which is reported using direct frequency-comb spectroscopy in a Doppler-free arrangement (E. Peters et al., pp. L29-L34). In addition, this could shed light on the current discrepancy in the determination of the proton charge radius.
Over the last few years experiments with antimatter at the AD facility at CERN have provided important new physics results. These allow for different new tests of the symmetry of the Standard Model under the combined CPT operation (Charge conjugation, Parity and Time reversal) through comparisons of properties of particles and corresponding antiparticles, such as the magnetic moment of the proton and the antiproton, or the hydrogen and anti-hydrogen ground state hyperfine splitting (Y. Yamazaki and S. Ulmer, pp. 493–504). In the context of tests of very general and basic principles, experimental sensitivity, systematic effects and initial data searching for a long-range coupling between rubidium nuclear spins and the mass of the Earth are finally reported as well (D. F. Jackson Kimball et al., pp. 514–528).
In the second volume additional reviews and results from dedicated efforts to study fundamental constants, to test basic symmetries of the Standard Model and search for new physics, will be reported: e.g. nuclear beta decay and neutron decay provide unique opportunities to search for new weak interactions providing results that are complementary to and compete in sensitivity with direct searches for new bosons at the LHC. Also neutrinos constitute unique probes to study different fundamental physics aspects via either laboratory or oscillation experiments or even via neutrino astronomy. Finally, many experiments are focusing lately on testing either the parity or CP symmetry of the weak interaction in atomic systems or via electric dipole moment searches, as well as CPT symmetry or Lorentz invariance.