The standard electroweak model (SM) postulates that the properties of, and processes involving, hadrons result from their quark structure. A sensitive way to test the completeness of the SM or to search for new interactions is by checking how the quark states transform when they interact weakly. Precision measurements of nuclear masses (Georg Bollen, David Morrisey) along with measurements of other decay properties in specific transitions (Chris Wrede) provide the relevant inputs for such tests. In addition, theoretical calculations (Alex Brown) of the breaking of isospin symmetry in those nuclei are crucial for the tests. In some transitions, it also is necessary to measure so-called “correlation” parameters in the weak decays (Oscar Naviliat Cuncic) in order to derive additional information about the nuclear structure.
The violation of parity (P) (or mirror) symmetry, discovered in the decay of 60Co nuclei, is a specific feature of the weak interaction. For nuclear beta decay, the SM postulates that parity symmetry is maximally violated. Although there is a coherent description of all weak processes that violate parity, there is no explanation for it. New models incorporate mechanisms that restore mirror symmetry under conditions that could have prevailed at an early stage of the universe. Precision measurements of correlations that violate mirror symmetry (Oscar Naviliat Cuncic) offer a window to search for interactions that restore parity. Such measurements require the development of specific tools, like the thermalization of fast beams (David Morrisey), the manipulation of low energy beams (Georg Bollen), or the production of highly polarized nuclei by optical pumping (Paul Mantica).
The combined CP-symmetry under charge conjugation (C) and parity is equivalent to the symmetry under time reversal (T). CP-symmetry has also been found to be violated in weak decays. The dominance of matter over anti-matter in the universe provides another indication of the violation of CP-symmetry but this cannot be accounted for by the known mechanisms of CP-violation observed in weak decays. Correlation measurements in the decay of polarized nuclei (Paul Mantica, Oscar Naviliat Cuncic) offer several T-violating observables for this search. Another sensitive property is the electric dipole moments of atoms and nuclei. Many-body theories are used to study the features in the nuclear structure that can result in the most sensitive candidates for such measurements (Vladimir Zelevinsky). Another sensitive property is the electric dipole moments of atoms and nuclei, such as Radium-225 and Xe-129 (Singh).
Double Beta Decay
Double-beta (ββ) decay is a second-order weak process in which two neutrons inside a nucleus spontaneously transform into two protons. Whereas the lepton-number-conserving type (associated with the simultaneous emission of two neutrinos) of ββ-decay is well-known, the existence of the lepton-number violating variant (neutrinoless ββ-decay) is uncertain and motivates strong experimental and theoretical efforts because it would prove that neutrinos are Majorana rather than Dirac particles, and could be used to constrain the neutrino mass. At NSCL, experiments aimed at making precision mass measurement to determine Q-values of relevance for ββ-decay (Georg Bollen, David Morrisey) and constraining nuclear matrix elements critical for extracting the neutrino mass if neutrinoless ββ-decay is discovered (Remco Zegers), are performed. These efforts are supported by theoretical studies of ββ-decay, focusing in particular on nuclear matrix elements (Alex Brown).