Synopsis: Benchmarking the Standard Model

Measurements of isospin symmetry-breaking corrections to beta-decay transitions in chlorine nuclei provide a stringent test of the standard model.

Isospin is a quantum number originally developed in the 1930s to describe the symmetry between the newly discovered neutron and the more thoroughly studied proton. Today, researchers study isospin in the context of quark physics and specifically the symmetry properties of up and down quarks. Precision experiments that look for isospin-symmetry-breaking corrections to nuclear transitions can help determine quantities like the Cabibbo-Kobayashi-Maskawa (CKM) matrix elements, which describe how the family tree of quarks is organized. They also help test the conserved vector current hypothesis, which is a fundamental tenet of the standard model and is analogous to the conservation of electric current in electromagnetic interactions. In a paper in Physical Review Letters, Dan Melconian at Texas A&M, College Station, and his colleagues report their measurement of isospin symmetry breaking in a positron-decay transition from chlorine-32. Their results yield convincing support for one of the most demanding tests of the standard model: verification of the unitarity of the CKM matrix.

In their experiments, Melconian et al. measured the gamma-ray yields following positron-decay transitions from the 290-millisecond decay of chlorine-32 nuclei. Collisions of sulfur-32 nuclei with a hydrogen gas target generated reaction products from which the chlorine nuclei were selected. These nuclei were collected on an aluminum-Mylar tape that was rapidly shuttled to a detection station nearby. Observed gamma-ray emission from the so-called superallowed transition in the decay revealed isospin symmetry breaking that is the largest measured to date. This result was then demonstrated to be in good agreement with nuclear shell-model calculations performed by the authors, who used the same theoretical methods as those employed in the test of CKM matrix unitarity. Agreement between theory and experiment for a transition with large broken symmetry is an important validation of previous unitarity tests done with transitions having much smaller symmetry breaking. – David Voss


Announcements

More Announcements »

Subject Areas

Nuclear Physics

Previous Synopsis

Chemical Physics

Diffused by Symmetry

Read More »

Next Synopsis

Mesoscopics

Quantum Hall Anomaly in 3D

Read More »

Related Articles

Synopsis: Fission Takes Its Time
Nuclear Physics

Synopsis: Fission Takes Its Time

Nuclear fission simulations show that the evolution of a splitting plutonium nucleus may be slower than previously thought. Read More »

Synopsis: Nucleus is Surprisingly Pear Shaped
Nuclear Physics

Synopsis: Nucleus is Surprisingly Pear Shaped

Experiments confirm that the barium-144 nucleus is pear shaped and hint that this asymmetry is more pronounced than previously thought. Read More »

Viewpoint: Can Four Neutrons Tango?
Nuclear Physics

Viewpoint: Can Four Neutrons Tango?

Evidence that the four-neutron system known as the tetraneutron exists as a resonance has been uncovered in an experiment at the RIKEN Radioactive Ion Beam Factory. Read More »

More Articles