Large Hadron Collider Reveals an Atomic Nucleus Shaped Like a Bowling Pin

Every atom of matter in the Universe is built around a nucleus.

It's a tightly packed bundle of protons and neutrons that makes up the atomic core, defines its elemental identity, and contains most of its mass.

Textbooks often depict this blob of particles as a sort of spherical raspberry, around which electrons whirl in a series of orbits – but that isn't always the most accurate picture.

Now, in the Large Hadron Collider at CERN, physicists have found evidence of one of those shapes: a nucleus structured like a bowling pin lurking inside every atom of neon.

It's not a mere curiosity, either. Understanding this shape has implications for nuclear physics – and for understanding the strange collective behavior of matter produced when atomic nuclei collide at tremendous energies.

Not all nuclei are built alike. Some can have shapes that diverge from a sphere; radium-224, for example, has a distinctly lopsided pear-shaped atomic nucleus. Understanding those shapes can help physicists predict how the nuclei might misbehave under extreme conditions.

But actually seeing those shapes isn't quite so simple, and not just because atomic nuclei are so small.

In ordinary measurements, a nucleus doesn't sit still. Its quantum state effectively averages over all possible orientations, so a non-spherical nucleus with zero angular momentum will still appear spherical when viewed in the laboratory.

This is where it comes in handy to have a particle collider at your disposal. When atomic nuclei smash together, their intrinsic, underlying structure can influence the flow of the matter produced in the collision.

Imagine placing two differently shaped objects in a stream of water, hidden from view. By studying how the water flows around each object downstream, you could work backward to learn something about the shape of the object that disturbed it.

The principle here is similar, although instead of disturbing an existing flow, the nuclear collision creates the flowing matter in the first place.

For their experiment, the CMS Collaboration compared collisions between two different atomic nuclei – oxygen-16 and neon-20.

They weren't smashing oxygen and neon together, but studied the outcomes of different collider runs involving oxygen-oxygen collisions and neon-neon collisions.

The reason for this is that oxygen-16 and neon-20 make useful comparison partners. The two nuclei are relatively close in mass, but likely have very different intrinsic structures.

Calculations predict (and several previous works have found evidence) that oxygen-16 has a tetrahedral nuclear structure, while the predicted structure for neon is more like a bowling pin.

The researchers compared how particles flowed out of each kind of collision, looking for patterns in the flow that could reveal the starting geometry of the nuclei involved.

They were looking to see whether particles were more likely to emerge in certain directions – patterns known as elliptic and triangular flow. If the underlying nuclear structures really were different, subtle differences would be apparent in both of these flow patterns.

In particular, the difference in structure should show up in the relative strength of the elliptic flow, especially in the most head-on collisions.

This is what the researchers found – with a caveat. Further investigation is needed because the measured changes in triangular flow did not match the model predictions quantitatively.

This is still a really cool result, though, because what it does show is that the collective flow in these collisions is genuinely sensitive to nuclear geometry. We may not have a photograph of a teeny tiny bowling pin, but the evidence is still consistent with nuclear deformation.

This means that a source as subtle as the spatial arrangement of protons and neutrons in an atomic nucleus can leave a detectable signature in the particulate spray that spews forth when nuclei collide.

Related: After 50 Years of Searching, Physicists May Have Finally Found 'Glueball' Particles

The findings therefore suggest that particle colliders could give physicists a new way to probe the structures of atomic nuclei – by smashing them together and reading their shape in the ejected guts, like lab-coated haruspices.

"These results establish the sensitivity of collective flow in light ion collisions to both the initial geometry and the hydrodynamic medium response," the researchers write.

"They provide stringent constraints on models of small-system collectivity and offer valuable implications for nuclear structure studies."

The findings have been published in Physical Review Letters.