Nuclear Physics Group

Superallowed β Decay


Figure 4 : Beta minus decay of a neutron to a proton through the conversion of a down quark to an up quark and the emission of a virtual W- particle that decays to an electron (e-) and an anti-electron neutrino.
Figure 5 : Ft-values for the 13 superallowed Fermi beta decays that have been measured with high precision, corrected for radiative and isospin symmetry breaking effects. The constancy of these data confirms the Conserved Vector Current (CVC) hypothesis at the level of 1.3 parts in 104, and their weighted average is used to establish the most precise value for the "up-down" element of the CKM quark-mixing matrix.

The process of nuclear beta decay takes place when the nucleus contains an excess of protons or neutrons relative to the combinations that form stable isotopes. Beta decay then converts a neutron into a proton, or a proton into a neutron, to achieve a more strongly bound, or stable, configuration of nucleons.


Considering the quark substructure of the nucleons, the neutron is comprised of two "down" type quarks and one "up" type quark, while the proton is comprised of two "up" type quarks and one "down" type. As shown in Fig. 4, the beta decay process thus corresponds to the conversion of a down quark to an up quark, converting the neutron into a proton, with the emission of a virtual W- particle that decays into an electron and an anti-electron neutrino. Conversely, the conversion of an up quark in a proton to a down quark results in a neutron and a virtual W+ particle that decays to a positron and electron neutrino. These processes are governed by the weak nuclear interaction, which is unified with electromagnetism in the Standard Model of particle physics to form the electroweak force.


For a particular subset of nuclear beta decays, known as superallowed beta decays, the quantum-mechanical wavefunction of the entire nucleus is left essentially unchanged by the beta decay process with the exception of one proton being converted into a neutron. These special decays are particularly amenable to theoretical analysis and precise experimental measurements for superallowed beta decays thus provide demanding tests of the Standard Model description of electroweak interactions. For example, while the half-lives associated with the full range of nuclear beta decays span some 20 orders of magnitude (from thousandths of a second to billions of years), Fig. 5 shows that all 13 of the superallowed Fermi beta decays that have been measured precisely to date have identical (within their error bars) intrinsic strengths, confirming the Conserved Vector Current (CVC) hypothesis of the Standard Model at the level of 0.01% precision. These superallowed beta decay data also currently provide the most precise determination of the "up-down" element of Cabibbo-Kobayashi-Maskawa (CKM) quark mixing matrix that describes the transformation between the mass eigenstates and the weak interaction eigenstates of the Standard Model quarks.


Our research group at the University of Guelph leads an active program of high-precision superallowed Fermi beta decay half-life and branching ratio measurements with the GRIFFIN Spectrometer and a 4π gas proportional beta counting facility at ISAC-I. Recent highlights from this research program can be found in our publication and theses lists.