Smashing particles reveals predicted pear-shaped nuclei

When it comes to sexy fruit, the apple (think Eve), the banana (enough said) and even the tart lemon may come to mind. But except in those with a penchant for middle-aged bodies, the first choice is probably not the pear.

For physicists at the University of Liverpool, there’s nothing better. They have for the first time discovered some atomic nuclei can take the shape of a pear. And that, they reckon, can contribute to understanding of nuclear structure and other fundamental interactions.

Ture, most atomic nuclei aren’t really spherical. More like a rugby ball. There are theories aplenty to predict when and how a nuclei looks the way it does. And for a while there have been math predictions that some should look, well, pear-like, with more mass at one end than the other.

With an honest to god experimental observation of one such atom, those theories have been confirmed, and in the process may have helped in experimental searches for electric dipole moments.

The Standard Model of particle physics predicts that the value of the EDM is so small that it lies well below the current observational limit. However, many theories that try to refine this model predict EDMs that should be measurable. In order to test these theories the EDM searches have to be improved and the most sensitive method is to use exotic atoms whose nucleus is pear-shaped. Quantifying this shape will therefore help with experimental programmes searching for atomic EDMs.

Professor Peter Butler, who carried out the measurements, said: “Our findings contradict some nuclear theories and will help refine others. The measurements will also help direct the searches for atomic EDMs currently being carried out in North America and in Europe, where new techniques are being developed to exploit the special properties of radon and radium isotopes.

pear“Our expectation is that the data from our nuclear physics experiments can be combined with the results from atomic trapping experiments measuring EDMs to make the most stringent tests of the Standard Model, the best theory we have for understanding the nature of the building blocks of the universe.”

The problem has been that nuclear isotopes thought to be peary (a word?) have been out of reach of measurement.

But at the ISOLDE facility at CERN, beams of very heavy, radioactive nuclei are being produced in proton collisions with a uranium carbide. They are then selectively extracted using their chemical and physical properties before being accelerated to 8% of the speed of light and allowed to crash into a target foil of isotopically pure nickel, cadmium or tin.

Apparently when this happens, all sorts of wonders occur. The relative motion of the heavy, speeded-up nucleus and the hapless target nucleus makes an electromagnetic impulse that excites the nuclei. Looking at the details of this lets wonks understand the nuclear shape.

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