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Experiments at CERN and the Accelerator Laboratory in Jyväskylä, Finland, have revealed that the radius of an exotic nucleus of aluminium, ^26mAl,  is much larger than previously thought. The result, described in a just published in Physical Review Letters, sheds light on the effects of the weak force on quarks – the elementary particles that make up protons, neutrons and other composite particles.

Strengthens the Standard Model of particlephysics 

Among the four known fundamental forces of nature – the electromagnetic force, the strong force, the weak force and gravity – the weak force can, with a certain probability, change the “flavour” of a quark. The of particle physics, which describes all particles and their interactions with one another, does not predict the value of this probability, but, for a given quark flavour, does predict the sum of all possible probabilities to be exactly 1. Therefore, the probability sum offers a way to test the Standard Model and search for new physics: if the probability sum is found to be different from 1, it would imply new physics beyond the Standard Model.

Weakinteraction is shownby the decay of particles 

Interestingly, the probability sum involving the up quark is presently in apparent tension with the expected unity, although the strength of the tension depends on the underlying theoretical calculations. This sum includes the respective probabilities of the down quark, the strange quark and the bottom quark transforming into the up quark.

The first of these probabilities manifests itself in the beta decay of an atomic nucleus, in which a neutron (made of one up quark and two down quarks) changes into a proton (composed of two up quarks and one down quark) or vice versa. However, due to the complex structure of the atomic nuclei that undergo beta decays, an exact determination of this probability is generally not feasible. Researchers thus turn to a subset of beta decays that are less sensitive to the effects of nuclear structure to determine the probability. Among the several quantities that are needed to characterise such “superallowed” beta decays is the (charge) radius of the decaying nucleus.

This is where the new result for the radius of the ^26mAl nucleus, which undergoes a superallowed beta decay, comes in. The result was obtained by measuring the response of the ^26mAl nucleus to laser light in experiments conducted at CERN’s facility and the Accelerator Laboratory’s IGISOL facility. The new radius, a weighted average of the ISOLDE and IGISOL datasets, is much larger than predicted, and the upshot is a weakening of the current apparent tension in the probability sum involving the up quark.

The bestway to determine the probability 

Past and present particle physics experiments worldwide, including the experiment at the , have contributed, and are continuing to contribute, significantly to our knowledge of the effects of the weak force on quarks through the determination of various probabilities of a quark flavour change. However, nuclear physics experiments on superallowed beta decays currently offer the best way to determine the probability of the down quark transforming into the up quark, and this may well remain the case for the foreseeable future.

Measurementscontinue in the AcceleratorLaboratory 

In the future, the IGISOL facility at the Accelerator Laboratory will be used to measure beams also for other nuclei such as the cobalt-54 nucleus.  

- It is great that we have a unique framework for this kind of research, which is at the cutting edge of international research. It also attracts a lot of researchers and research collaboration to Jyväskylä," says Professor Anu Kankainen from Department of Physics.  

The article was published in Physical Review Letters on 27 November 2023 and was selected as an Editor's Suggestion publication, with a separate Viewpoint article. A press release on the publication is also available on the CERN website. 

Further information: 

• IGISOL Research Group: /en/research-groups/exotic-nuclei-and-beams-igisol  

• Press release of the CERN:   

• Viewpoint article:  

 Article information:

• P. Plattner et al., Nuclear Charge Radius of 26mAl and Its Implication for Vud in the Quark Mixing Matrix, Phys. Rev. Lett. 131, 222502 

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