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Longstanding puzzle about beta decay solved

Image: Andy Sproles, Oak Ridge National Laboratory, U.S. Department of Energy

Beta decay



An international collaboration including contributions from TU Darmstadt and the ExtreMe Matter Institute (EMMI) at GSI solved a 50-year-old puzzle that explains why beta decays of atomic nuclei are slower than what is expected based on the beta decay of free neutrons. The findings, published in the scientific journal Nature Physics, fill a long-standing gap in our understanding of beta decay, an important process in nuclear physics applications and in the synthesis of heavy elements in stars.

Beta decay is the main decay channel of atomic nuclei: a conversion of a neutron inside the nucleus into a proton (or vice-versa), which produces a different element with proton number plus (or minus) one. In this way beta decay plays a central role in the synthesis of new elements in our universe. As an interplay of the strong force that binds neutrons and protons inside the nucleus and the weak interaction, beta decay also holds important clues for physics beyond the Standard Model and has been the focus of concentrated efforts in physics since the early 1900s.

However, a puzzle has withstood a first-principle understanding: the beta decay of neutrons bound within nuclei are significantly slower than what would be expected on the basis of decay times of free neutrons. In the past, this systematic discrepancy was taken care of by implementing a constant called ‘quenching’. This workaround was able to reconcile observed beta-decay rates of neutrons inside and outside the nucleus and realigned theoretical models with experimental measurements.

“For a long time, we have lacked a fundamental understanding of nuclear beta decay,” said EMMI professor Achim Schwenk from TU Darmstadt, who is part of the collaboration. “In complex microscopic computations we now demonstrated for the first time that strong correlations in the nucleus as well as the strong interaction with another neutron or proton slow down beta decay inside the nucleus. Such interaction effects are predicted in effective field theories of the strong and weak interactions.”

To demonstrate this, the theoretical physicists systematically calculated beta decays for a variety of light and medium-mass nuclei, starting from a nucleus with only three nucleons up to tin-100 with 50 protons and 50 neutrons. The beta decay of tin-100 was first observed at GSI in the year 2012. The results of the collaboration were in very good agreement with experimental data and demonstrate that the quenching factor is not needed when both the strong and weak interaction effects are considered consistently.

The advances in taking the weak interaction with single neutrons and protons to large atomic nuclei have been made possible by theoretical developments of effective field theory, as well as by great progress in many-body theory and powerful supercomputing capabilities.

In addition to a better understanding of beta decays for the synthesis of heavy elements in supernovae and neutron star mergers, the researchers also hope to gain new insights into double-beta decays, in particular neutrino-less double-beta decay, where an analogous quenching puzzle exists. (cp)

Further information:

Beta decay
Graphical representation of the beta decay of a neutron (red) into a proton (blue), which can interact with another proton (blue next to it) in the nucleus. These two-particle effects as well as strong correlations in the atomic nucleus lead to slower beta decays than would be expected from the decay of a single (free) neutron.
Image: Andy Sproles, Oak Ridge National Laboratory, U.S. Department of Energy