Experiments at CERN and the Accelerator Laboratory in Jyväskylä, Finland, have revealed that the radius of an exotic nucleus, 26mAl, is much larger than previously thought.
The work sheds light on the effects of the exotic nucleus on quarks – the elementary particles that make up protons, neutrons and other composite particles.
The research, ‘Nuclear Charge Radius of 26mAl and Its Implication for Vud in the Quark Mixing Matrix,’ is published in Physical Review Letters.
The effect of forces on quarks
Among nature’s four known fundamental forces – 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 Standard Model of particle physics, which describes all particles and their interactions with one another, does not predict the value of this probability. However, for a given quark flavour, it does predict the sum of all possible probabilities to be exactly one.
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 one, it would imply new physics beyond the Standard Model.
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.
How does an exotic nucleus impact quarks?
The first of these probabilities manifests itself in the beta decay of an exotic 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 decay, an exact determination of this probability is generally not feasible.
Therefore, the researchers turned 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 decay is the (charge) radius of the decaying exotic nucleus.
The 26mAl nucleus is unique
This is where the new result for the radius of the 26mAl nucleus, which undergoes superallowed beta decay, comes in.
The result was obtained by measuring the response of the 26mAl exotic nucleus to laser light in experiments conducted at CERN’s ISOLDE 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.
“Charge radii of other nuclei that undergo superallowed beta decays have been measured previously at ISOLDE and other facilities, and efforts are underway to determine the radius of 54Co at IGISOL,” explained Peter Plattner, ISOLDE physicist and lead author of the paper.
“But 26mAl is a rather unique case, although it is the most precisely studied of exotic nuclei. Its radius has remained unknown until now, and, as it turns out, it is much larger than assumed in the calculation of the probability of the down quark transforming into the up quark.”
CERN theorist Andreas Juttner concluded: “Searches for new physics beyond the Standard Model, including those based on the probabilities of quarks changing flavour, are often a high-precision game.
“This result underlines the importance of scrutinising all relevant experimental and theoretical results in every possible way.”
