Light nuclei, surviving the fireball

There is a question in particle physics that sounds simple and is anything but: how do atomic nuclei survive a collision that melts everything? When protons smash together at the Large Hadron Collider, the energy is so concentrated that the protons dissolve into their constituents, and for an instant the collision zone resembles the state of the universe a microsecond after the Big Bang, a soup of quarks and gluons with no structure at all. And yet, out of this fireball, particles as structured as deuterons, the heavy hydrogen nucleus made of a proton and a neutron bound together, appear to emerge intact, and have done so in collisions for decades. A paper in the 10 December issue of Nature reports how the ALICE Collaboration has approached an explanation.

The puzzle is a matter of odds. A deuteron is a loose pairing, a proton and a neutron held by a bond so delicate that it can be broken by a modest push. Inside the collision fireball, where the energy density is enough to dissolve the strongest nuclei, the survival of such a fragile object seems almost absurd. The standard accounts have offered two broad families of explanation: either the deuterons are formed late, after the fireball has cooled and expanded, from the chance reunion of a nearby proton and neutron, or they are produced early, in the dense phase, by processes that the extreme conditions make possible. The two pictures make different predictions, and the difference has been hard to measure.

The ALICE result discriminates between the pictures by studying the collisions in unusual detail. By examining the patterns of particles surrounding the deuterons, and the correlations between the nucleus and its neighbours, the team could test whether the deuterons behaved as products of a random late reunion or of an earlier, more structured process. The evidence, the collaboration reports, points toward a specific balance of the mechanisms, with the survival of the light nuclei explained by the statistical assembly of particles late in the expansion, mediated by the attractive interactions between nearby nucleons.

The reason the question matters beyond the LHC is cosmology. The universe’s first minutes produced light nuclei, and the observed abundances of deuterium and helium in the cosmos are among the primary tests of the Big Bang model. Understanding how nuclei form in extreme conditions, whether in the early universe or in laboratory fireballs, is therefore not an esoteric corner of physics but a link between the laboratory and the origin of matter. The ALICE results put that link on a firmer experimental footing.

There is a pleasing humility in the subject. The deuteron is one of the oldest objects in nuclear physics, known and measured for nearly a century, and it remains a source of fundamental questions: how the strong force behaves in dense matter, how structure emerges from chaos, how the fragile survives the violent. The experiments at the LHC are, in a sense, asking the universe’s oldest question, how did the pieces of matter come to be, in the most extreme conditions they can arrange, and the deuteron, tiny and delicate, turns out to be one of the most instructive witnesses they have.