At the Large Hadron Collider, protons are smashed into lead nuclei. At the moment of impact, hundreds of new particles are born, and some of them fly out in narrow beams — jets. Inside such a jet, like in a dense stream, protons and neutrons can stick together to form larger nuclei, for example a deuteron — the nucleus of heavy hydrogen.

Physicists from the ALICE collaboration have for the first time measured how many deuterons and antideuterons are produced inside jets compared to regions away from them. To do this, they selected events with a leading particle that set the jet direction and looked at what happens in three zones: along the jet, in the opposite direction, and sideways. It turned out that deuterons appear noticeably more often in jets than in the background particle flow. This effect is stronger than in ordinary proton–proton collisions.

The result was compared with the PYTHIA computer model, which describes particle production in collisions. The model qualitatively reproduces the enhanced deuteron production in jets, though not in every detail. This means that the mechanism by which protons and neutrons stick together inside jets is similar to ordinary nuclear reactions, but under the extreme conditions of collisions it works differently.

Such measurements help us understand how nuclei, from the lightest to the heaviest, are assembled from quarks and gluons. This is also important for astrophysics: deuterons are also produced in cosmic rays, and their origin is still not fully understood.