(Anti)deuteron production in and out of jets in p$-$Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 5.02 TeV with the ALICE detector at the LHC

Heavy-ion collisions at ultra-relativistic energies are one of the fundamental tools to explore the extreme conditions of the nuclear matter. In fact, in these collisions, a deconfined state of quarks and gluons, namely the Quark-Gluon Plasma (QGP), is created. This state of matter is relevant not only in nuclear physics, but also in astrophysics. In fact, it is established that some microseconds after the Big Bang, our universe was in this state of matter, and also the QGP is a candidate to explain the core of neutron stars and other compact astrophysical objects.

 

In heavy-ion collisions a large number of particles are produced: from hadrons (among which pions, kaons and protons are the most abundantly produced) to composite objects such as light nuclei (d, t, $^3$He, $^4$He) and hypernuclei, along with their antiparticles. The production rate at the CERN Large Hadron Collider (LHC) for deuterons is approximately one every one thousand p$-$Pb  collisions, while for $^3$He is even lower, one every one million events. Hence, the study of the production of  light (anti)nuclei is particularly challenging. Moreover, the production mechanism of light (anti)nuclei, despite the abundance of experimental results, is still a highly debated topic in the scientific community. In fact, if we consider the energy scale in which the hadronization process happens, $\Lambda_{\mathrm{QCD}} \sim $ 200 MeV, the QCD perturbative approach can not be applied. For this reason, the production of light (anti)nuclei is described with some phenomenological models. In particular, it is possible to identify two different classes of models: the Statistical Hadronization Model and the baryon coalescence. 

 

With the current experimental results, both models qualitatively reproduce the data, even if some tensions are present in selected results. In particular, while for the deuteron production the models do not have enough discrimination power, for the $^3$He case the data are not precise enough.  Then, in order to discriminate between the two models, novel experimental observables are needed. One innovative approach is to study the production of light (anti)nuclei in and out of jets. In fact, as predicted by the coalescence model, an enhanced nucleus production is expected in jets, due to the nucleons proximity in the phase space.  

 

The production of nuclei in jets has been studied in pp collision by the ALICE experiment at CERN. In fact, even if the detector is specifically designed in order to study and characterize the QGP, its excellent particle identification capabilities allow the studies on light (anti)nucleus production. In particular, deuteron, triton, $^3$He, $^4$He and their respective antiparticles have been identified in pp, p$-$Pb, Xe$-$Xe and Pb$-$Pb collisions at different energies, exploiting the information from the Inner Tracking System (ITS), Time Projection Chamber (TPC) and Time-Of-Flight (TOF) detectors. 

 

In this work, the (anti)deuteron and antiproton production in and out of jets in p$-$Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 5.02 TeV is presented. With the obtained transverse momentum distributions, the coalescence parameter in and out of jets, which is related to the probability to form a deuteron via coalescence, is evaluated. The experimental results show an enhanced deuteron coalescence probability in jets, as predicted by the coalescence model. The experimental data are also compared with both the values obtained in pp collisions and with the expectations from the coalescence model. The results are then discussed in the knowledge of the current theoretical framework. To further understand the experimental results, the deuteron-over-proton ratio in both pp and p$-$Pb collisions in and out of jets is evaluated, and the obtained results are coherent with the coalescence picture. 

 

The presented results contribute in a deeper understanding on the production of light (anti)nuclei. This information is not only crucial in the field of QCD, in order to understand how this object are formed, but also in the astrophysical field. In fact, these measurements can be used as an input for studies in the search of dark matter via the measurement of light (anti)nuclei in space, carried out from the AMS-02 and GAPS experiments.