CERN Accelerating science

Ultra-relativistic Heavy Ion Collisions (HICs) allow to reproduce the extreme conditions of temperature and energy density where the Quark-Gluon Plasma (QGP) is formed. Based on the current cosmological picture, QGP was the state of our universe few $\mu$s after the Big Bang. Moreover, there is evidence that a degenerate state of matter with similar properties to the QGP exists in the inner core of neutron stars and other compact astrophysical objects. Heavy ion collisions produce not only hadrons (among which pions, kaons and protons are the most abundantly produced) but also composite and even fragile objects such as light nuclei (d, t, $^3$He, $^4$He) and light $\Lambda$-hypernuclei, along with their antiparticles. Their measured yields decrease strongly with increasing (anti)baryon number -- the penalty factor for each additional (anti)baryon is about 300 -- hence (anti)$^4$He production is a very rare process. The production rate at the CERN Large Hadron Collider (LHC) for deuterons is approximately one every one thousand p--Pb collisions corresponding to a charged-particle density of about 20. The production rate of heavier nuclei, such as $^3$He, is even lower (one every one million events). Hence, the study of light (anti)nuclei production is particularly challenging. At the same time, light nuclei and their antimatter counter parts are particularly interesting since the production mechanism of such loosely bound states is still under debate. (Anti)nuclei have a binding energy of the order of 1 MeV per nucleon. This value is extremely low if compared with the chemical freeze-out temperature of a nucleus-nucleus collision ($T_{\rm{chem}} \sim$ 150 MeV). Hence, it is surprising to see how these loosely-bound objects like light (anti)nuclei can be produced and survive in such extreme conditions. The production of light (anti)nuclei is usually described using two classes of phenomenological models: the Statistical Hadronisation Models (SHMs) and the Coalescence models. Both classes of models describe different aspects of the production of light (anti)nuclei. The comparison between the experimental results of (anti)nuclei production and the hadronization models currently available is crucial to shed light on the production mechanism. A Large Ion Collider Experiment (ALICE) has been designed to deal with the harsh environment of the ultra-relativistic hadronic collisions and to study in details the characteristics of the QGP. ALICE has excellent tracking and Particle IDentification (PID) capabilities over a broad momentum range. This makes ALICE the most suited detector at the LHC to study light (anti)nuclei produced in such high-energy hadronic collisions. (Anti)nuclei with mass numbers up to A = 4, such as (anti)deuterons, (anti)tritons, (anti)$^3$He and (anti)$^4$He have been successfully identified in ALICE in the pseudorapidity region $|\eta|$ < 0.9 using different experimental techniques and different detectors, namely the Inner Tracking System (ITS), the Time Projection Chamber (TPC) and the Time Of Flight detector (TOF). In this work, measurements of (anti)deuteron and (anti)$^3$He production as a function of the transverse momentum (\pt) and event multiplicity in p--Pb collisions at a centre-of-mass energy per nucleon--nucleon pair \snn = 8.16 TeV are presented. In order to compare the results from different collision systems and energies with the expectations of the models, it is useful to study the production yields of nuclei for different multiplicity classes. Consequently, it is possible to investigate whether the production mechanism of light (anti)nuclei is similar in small and medium-large collision systems. In this context, the p--Pb system is particularly interesting as it links existing results in pp and Pb--Pb collisions, corresponding to small and large system sizes, respectively. One of the most interesting results obtained from the large variety of experimental data analyzed from ALICE is that the dominant production mechanism of light (anti)nuclei seems to depend solely on the event charged-particle multiplicity. Evidence for this comes from the continuous evolution of the nucleus-to-proton and nucleus-to-pion yield ratios with the event multiplicity across different collision systems and energies. The comparison with the predictions of the statistical hadronization and coalescence models favors the coalescence description for the deuteron-to-proton yield ratio with respect to the Canonical Statistical Model (CSM), which fails to simultaneously reproduce the deuteron-to-pion yield ratio. Additionally, the coalescence parameters $B_2$ and $B_3$, which are related to the probability to form a deuteron or a $^3$He via coalescence of their nucleon constituents, are measured as a function of the transverse momentum per nucleon and of the mean \mbox{charged-particle} multiplicity density. Such coalescence parameters confirm a smooth evolution from low to high multiplicity across different collision systems and energies. The results of $B_2$ as a function of the mean charged-particle multiplicity density show a good agreement with the coalescence model that uses the parameterization of the source radii based on femtoscopic techniques. The light (anti)nuclei results are also relevant for background studies in the search for dark matter via the measurement of (anti)nuclei in space and as input for the understanding of the formation of QCD bound states in high energy hadron physics.