The nuclear physics of muon capture

We review the topic of muon capture in nuclei and show that significant nuclear physics is being learnt from recent experiments. The focus of many earlier experiments had been the particle physics and weak interactions aspects of the subject. Although these were useful confirmation of basic ideas, defining experiments mostly came from other areas. Thus, we focus on what can be learnt about nuclei especially 1+, 1− and 2− magnetic transitions. Valuable comparisons can be made to other charge exchange reactions such as (π−,γ), (π−,π0), (n,p), (d, 2He) and (t, 3He). However, all those experiments have resolutions of 300 keV to 1 MeV, or even worse, so γ-ray studies of muon capture provide resolutions 100–1000 times better, and thus define the levels unambiguously. For even–even nuclei one can also compare to (e,e′) at 180°, or (p,p′) at forward angles, for which the resolution is reasonable, typically 50–100 keV, but isospin mixing complicates the comparison. With the recent substantial progress in Shell Model calculations, we anticipate significant developments in our understanding of nuclear structure in the next few years.