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


Features

More Features »

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: Nuclear Spectroscopy Reveals New Shapes of Excited Nuclei
Nuclear Physics

Synopsis: Nuclear Spectroscopy Reveals New Shapes of Excited Nuclei

Cadmium nuclei take on multiple shapes at low excitation energies, a discovery that overturns a long-accepted tenet of nuclear structure. Read More »

Synopsis: Seeking Stardust in the Snow  
Astrophysics

Synopsis: Seeking Stardust in the Snow  

Iron-60 found in fresh Antarctic snow was forged in nearby supernovae and could help deduce the structure and origin of interstellar dust clouds.   Read More »

Synopsis: Tin Gets Kinky
Nuclear Physics

Synopsis: Tin Gets Kinky

The observation that tin nuclei suddenly increase in size when the number of neutrons they contain reaches a “magic” number helps test models of nucleon interactions. Read More »

More Articles