Fundamental symmetries  Fundamental Interactions  Fundamental Interactions

Introduction

The exploration of the fundamental constituents of matter, of their interactions and of the underlying basic symmetries is an essential part of physics. This exploration is generally considered as the task of Particle Physics. Part of this activity requires however the use of nuclei, the theoretical insight and the instrumentation of Nuclear Physics. It could be named "Nuclear Particle Physics". As a consequence, this field of research, duly considered as part of Nuclear Physics, enters quite naturally into the purview of NuPECC just as nuclear astrophysics, which derives its rationale from astrophysics, benefits from NuPECC co-ordination and care. Beyond what one may call "nuclear particle physics", we briefly discuss also in this report rare reactions or decay-modes of pions and muons. They are conceptually equally important approaches to the exploration of fundamental symmetries and share with nuclear particle physics some of its research environment. Like double beta-decay they even exploit sometimes coherent effects in heavy nuclei. It would be detrimental for physics if this intellectually so important "intermediate energy" activity would somehow "fall in the cracks" between Particle and Nuclear Physics. It is a common prejudice that the exploration of the fundamental constituents of matter, at smaller and smaller dimension scales, requires the ultimate energies of the highest energy particle accelerators. It is expected that novel particles observed at these accelerators will provide the clue to a unified description of Nature. We hope to illustrate in this brief report that nuclear particle physics on the "high precision" frontier provides results complementary to those we expect at the "high energy" one. This is so because of the multiple quantum-states provided by nuclei and the possibility to perform experiments of very high precision in a nuclear physics environment. Instead of observing directly the novel particles, one pins them down by observing their tiny influence on low-energy processes. Many recent review papers stress the complementarity of the two approaches. The common struggle of Particle Physics both at high energies and at low energies in the nuclear environment is to find new physics beyond the Standard Model. This model is surprisingly successful but is not believed to be the ultimate description of nature because, among other reasons, of the many ad-hoc parameters it introduces. Some indications of new physics are just on the horizon, as is discussed in the first part of this report. Some such earlier hints were discarded following closer scrutiny. What should be stressed, however, is that in the dynamics of this search, ingenuity always pushed the precision-frontiers to new unprecedented limits. To mention some examples, the dipole moments of atoms and that of the neutron now reach the 10^-25 e-cm limit and are expected to be improved even further by two orders of magnitude. This tiny dipole moment would correspond, on the scale of the Earth, to a deviation of a couple of microns from sphericity! Some decay-modes of muons forbidden by the Standard Model will be observed at the 10^-12-10^-13 level if some attractive scenarios of new physics beyond the Standard Model turn out to be correct. This will require years of experimentation with muon fluxes of a few 10⁸ per second provided by proton-accelerators of several hundred kW. Symmetry-tests and rare-decay searches explore energy-domains in the range 1-100 TeV. Particles of such a large mass may show up in processes which are strictly forbidden in the Standard Model (e.g. violating lepton number), or may signal one of the more natural generalisations of the Standard Model like the left/right symmetry.