Debris From Neon Collisions Carries the Bowling Pin Shape of the Nucleus

In July 2025 the Large Hadron Collider fed its ring something it had never collided before. Beams of oxygen nuclei went in, and then beams of neon. Over a few days the ALICE detector logged roughly 3 billion oxygen collisions and 400 million neon ones. The two nuclei are close in size, sixteen protons and neutrons against twenty, and that near match was the point of running both.

Physicists have spent twenty five years recreating the first microseconds of the universe by slamming heavy nuclei together. Gold and lead carry enough matter that the wreckage briefly becomes quark gluon plasma, a state in which quarks and gluons move freely instead of staying locked inside protons and neutrons. When Brookhaven’s collider produced it, the surprise was that the stuff flowed like a liquid with almost no internal friction rather than spraying out like a gas. The open question since has been how small a collision can get and still make some.

Flow is what gives it away. Two nuclei rarely meet dead center, so the region where they overlap is not a circle but a lens, shorter across one way than the other. Hot matter trapped in that lens pushes outward hardest where the wall is nearest, and particles stream out preferentially along the short axis. Detectors tally the particles arriving at every angle around the beam and split the tally into components: an elliptical one, a three lobed one, and so on. The size of each component reports on the shape the collision started with. In lead those components have been measured for years. In oxygen and neon, nobody had measured them at all.

Neon Pushed Harder Than Oxygen

In the most head-on collisions, the elliptical component came out about 8 percent larger for neon than for oxygen. The gap shrank to roughly 5 percent as the collisions grew more glancing, then held steady. The three lobed component behaved differently: level between the two systems in the most head-on events, then tilting toward neon as the collisions moved off center.

Those readings track what nuclear theory says the insides of the two nuclei look like. Oxygen-16 is thought to hold its sixteen particles as four helium cores parked at the corners of a tetrahedron, an arrangement that lattice calculations have reproduced from first principles. Neon-20 hangs one more helium core off an oxygen core, which leaves it stretched along one direction, a shape physicists have taken to calling a bowling pin. A longer nucleus makes a longer overlap, and a longer overlap pushes harder one way than the other.

“It is a bit like shining light on an object and seeing its shadow. You cannot see the object directly, but its shadow reveals its shape,” says Emil Gorm Dahlbæk Nielsen, a postdoctoral researcher at the Niels Bohr Institute and a co-author of the study.

Both Nuclei Are Round in the Laboratory

“But instead of carefully investigating nuclei at low energies, we smash them together at the highest energies we can create and can now read their shape from the imprint they leave behind,” says You Zhou, the associate professor who led the experiment. The paper is more guarded.

Neither nucleus is bowling pin shaped or tetrahedral in the everyday sense as it flies down the beam pipe. Both have zero spin, which in quantum terms means they look the same from every direction, and the shapes describe how the particles inside sit relative to one another, averaged over every orientation the nucleus could take. Turning a measured flow value back into a ground state shape therefore runs through a model, and the authors say as much, calling the approach complementary to the low energy experiments that have mapped nuclear shapes for decades rather than a replacement for them.

ALICE was not alone at the collider. ATLAS made similar measurements at about the same time, and CMS published its own in the same journal issue. Across the three detectors the ordering in head-on collisions came out the same, with neon flowing most elliptically and nearly spherical lead the least. CMS separately watched fast particles losing energy while crossing these small fireballs, a different symptom of the same medium.

The Models Missed the Comparison

The measurement got most interesting where it stopped matching. Trajectum, a simulation that treats the plasma as a fluid, was fed two independent state of the art descriptions of what oxygen and neon look like inside, and it tracked the individual flow measurements closely across most of the range.

Then the team divided neon by oxygen. Because the two systems are so alike, that division cancels most of what happens after the collision and leaves the starting geometry exposed. Both versions of the simulation predicted a wider gap between the two systems than the detector recorded. A rival model landed closer, one in which the quarks and gluons inside each proton and neutron occupy a far tighter clump than the value inferred from lead collisions. The mismatch amounts to a few percent, and it sits on the least settled ingredient in the whole calculation.

The question underneath is old and local to this group. The Niels Bohr Institute, where several of the authors work, is where Aage Bohr established in the early 1950s that nuclei need not be spherical; he shared the 1975 Nobel Prize in Physics with Ben Mottelson and James Rainwater for connecting the motion of individual particles inside a nucleus to the collective behavior of the whole. Shape is one of the few direct readouts of the strong force, which holds those particles together and which physicists are still working to describe fully.

Nothing in the experiment ever sees the plasma. The droplet forms and vanishes long before any instrument could resolve it, and what reaches the detector is a list of angles. Out of that list came a neon to oxygen ratio that the best available pairing of nuclear structure and fluid dynamics still overshoots. Closing that gap will say something about how quarks and gluons are packed inside a single proton, which is not the question this run set out to ask.

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