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.
as written by the authors arXiv:2602.22880
This article presents the first measurement of the (anti)deuteron and antiproton transverse-momentum distributions, the deuteron coalescence parameter, and the ratio between the yields of deuteron and proton in and out of jets in p-Pb collisions at $$\sqrt{s_{\rm NN}} = 5.02$$ TeV. Three regions are used to study the jet-correlated production: Toward, Away, and Transverse to the jet axis, which direction is approximated by the leading particle in the event with $$p_{\rm T}^{\rm lead} > 5$$ GeV/$$c$$. The jet contribution is obtained from the Toward region by subtracting the underlying event, which dominates the Transverse region. In p-Pb collisions, an enhancement of the coalescence parameter $$B_{\rm 2}$$ is observed in jets compared to the underlying event. The enhancement is larger than the one observed in a similar analysis already in pp collisions. The results are compared with predictions from PYTHIA 8.314 using the Angantyr model with a deuteron production model based on ordinary nuclear reactions. The model is able to qualitatively reproduce the large enhancement of the coalescence parameter in jets with respect to the out-of-jets one. The results are further compared in detail with similar ones from previous studies for pp collisions at $$\sqrt{s} = 13$$ TeV.
The deuteron is the only stable nucleus consisting of two different particles: a proton and a neutron. All other two-particle nuclei (diproton, dineutron) fall apart instantly.