Calculations finally match β-decay rates

When measuring β decay in medium and heavy nuclei, physicists have had to adjust, or quench, the calculated rate to match the observed rate by a factor of 0.75. Peter Gysbers (TRIUMF, Canada’s particle accelerator center), Gaute Hagen (Oak Ridge National Laboratory), and their colleagues have now developed an explanation for the mismatch between theory and observation that has puzzled nuclear physicists for the past half century. The researchers combined effective field theory and quantum many-body methods to calculate β-decay rates in different-sized nuclei. Comparing those calculations with measured rates led the researchers to conclude that the discrepancy arises from powerful particle interactions in the nucleus.

A clue about the mismatch was offered last year by researchers who derived the β-decay rates of light nuclei helium-6 and carbon-10. In particular, they calculated the Gamow–Teller transition, a type of β decay in which the spins of a β-neutrino pair align and change the total angular momentum. The calculation overestimated the rate relative to the observed rate but only by 10% at most. The modest discrepancy, the researchers concluded, may be due to correlations in the nuclear wave function.

Gysbers, Hagen, and their colleagues went further by calculating the Gamow–Teller transitions for heavier nuclei up to tin-100. That radioactive isotope of tin has the strongest Gamow–Teller transition yet measured. The figure shows the calculated and observed Gamow–Teller strengths for several medium-mass nuclei; the subscripts indicate the total angular momentum of each state. By using effective field theory to better adapt the model to the data, the researchers calculated a quenching factor range of 0.73 to 0.85 for tin-100, which agreed reassuringly with the observed factor.

The researchers found that two-body currents in the nucleus, which aren’t represented by the simpler nuclear shell model physicists have used for decades, brought the calculated factor into agreement with the measured factor. The methods used here may have broader utility as well by uncovering the nuclear processes that operate in stars to form the elements. (P. Gysbers et al., Nat. Phys., 2019, doi:10.1038/s41567-019-0450-7 .)