CERN Accelerating science

ALICE (A Large Ion Collider Experiment) collected over two billion proton-proton collision events at a center-of-mass energy of 13 TeV during the second running period (from 2016 to 2018) of the Large Hadron Collider (LHC). This rich data sample allows for studies of at this collision energy rarely produced objects, such as light nuclei. In this work, the first multiplicity and transverse-momentum ($p$$_{T}$) differential measurement of helium ($^{3}$He) nuclei and triton ($^{3}$H) as well as their corresponding antinuclei in proton-proton collisions at the LHC is presented. The interaction of quarks and gluons, the constituents of all hadrons, is described by quantum chromodynamics (QCD), the theory of the strong interaction. Thus, QCD is the underlying theory of the formation process of light nuclei. In practice, the light nuclei formation in collisions at relativistic energies is modeled using two phenomenological approaches: the statistical hadronization model and the coalescence model. The first expresses the production of all hadrons according to the laws of statistical physics, assuming emission from a medium in local thermal equilibrium. In this approach, the hadron yields are determined by the hadron mass, the chemical freeze-out temperature, the baryon chemical potential, and the system volume. The statistical hadronization model successfully describes the hadron yields over a wide mass range, going from the lowest (a few GeV) to the highest (a few TeV) center-of-mass collision energies. It is effectively applied to small and large systems ranging from $e$+$e$- to central uranium-uranium collision. The coalescence model, on the other hand, describes the production mechanism of nuclei on the microscopic level. In the simplest version, a nucleus is formed when its constituent nucleons are close in phase space (momentum and spatial distance are small). More sophisticated versions of the coalescence model take the emission source size and the nuclear radius into account. The key parameter of the model is the coalescence parameter BA , with A being the mass number of the nucleus. Experimentally $B$$_{A}$ is accessed as the ratio of the nucleus yield to the product of its constituent nucleon yields. The coalescence model is successfully used to describe and predict the formation of nuclei in relativistic collisions. Additionally, it is systematically used by the astrophysics community to predict antinuclei fluxes originating from collisions of cosmic rays with the interstellar medium. The ALICE detector at the LHC is perfectly suited to track and identify light nuclei. The identification capability of the ALICE Time Projection Chamber (TPC) allows for excellent separation power in the low-$p$$_{T}$ region via the measurement of the specific energy loss and it is complemented by a Time-of-Flight (TOF) detector extending the $p$$_{T}$ reach of the particle identification. Due to being doubly charged the helium nucleus is very well separated via its specific energy loss in the TPC and the measurement can be performed in the 1 ≤ $p$$_{T}$ ≤ 6 GeV/c interval. The 3H nucleus on other hand is identified using the combined information from TPC and TOF detectors limiting the transverse-momentum reach of the measurement to 1 ≤ $p$$_{T}$ ≤ 2.5 GeV/c. Additionally, the multiplicity and $p$$_{T}$ differential (anti)proton spectra serving as important references for light nuclei are measured in a transverse-momentum interval of 0.6 ≤ $p$$_{T}$ ≤ 5 GeV/c. In this work, two new data-driven correction methods have been developed, exploiting recent ALICE measurements of the inelastic hadronic interaction cross section. In the first method, the inelastic hadronic interaction cross section implemented in the Monte Carlo simulations, which i are needed for the efficiency and acceptance correction, are reweighted, reducing the systematic uncertainty of this contribution by a factor of three compared to previous analyses. The second method uses the well-known proton inelastic cross section to evaluate the effect of the material budget on the particle spectra when using the TOF detector. The $p$$_{T}$-spectra and the resulting integrated yields obtained in this work are used to study the formation process of light nuclei. Additionally, the $p$$_{T}$-spectra of deuterons in pp collisions at a center-of-mass energy of 13 TeV are presented and compared to the proton measurements. To study the light nuclei formation process as a function of the emission source radius, the results obtained in this work are discussed in the context of previously published ALICE results of light nuclei. The integrated deuteron-to-proton and helium-to-proton yield ratios show a smoothly increasing evolution with the event averaged charged-particle multiplicity density (dNch/dη), going from pp to central lead-lead collisions. The trend is in both cases described by the statistical hadronization and the coalescence model. However, in the intermediate multiplicity region (dNch/dη ≈ 30), a tension between the models and the measured helium-to-proton yield ratio is observed. The coalescence parameters $B$$_{2}$ and $B$$_{3}$ follow a decreasing trend with the average charged-particle multiplicity, which is, to first order, described by the latest coalescence models. The coalescence parameters are determined as a function of $p$$_{T}$ , showing a clear dependence on $p$$_{T}$. The $p$$_{T}$ dependent coalescence parameter is compared to coalescence-model predictions using different nuclear wave functions. Surprisingly, $B$$_{2}$ agrees best with the prediction using the solution of a harmonic oscillator as the wave function, while more sophisticated wave functions (e.g. van Hulthen) do not describe the data. $B$$_{3}$ on the other hand is not predicted using a Gaussian nuclear wave function, which is the only one available at the moment. In the future, this approach can be used to study the nuclear wave function of exotic hyperons. Additionally, the measurement of the coalescence parameter will give a fundamental baseline for space-bound experiments measuring cosmic antinuclei fluxes. The experiments try to discover physics beyond the standard model via indirect dark matter searches. Antinuclei are one of the most potent probes due to their low background component originating from hadronic interactions in the galaxy. The proper estimate of this component is nevertheless crucial and $B$$_{A}$ measurements as presented in this work are therefore essential. With the third running period of the LHC starting in 2022 a new precision era for light- (anti)nuclei measurements will start. Among many new and exciting opportunities, the measurement of the triton-to-helium yield ratio, which at the moment does not have the necessary precision, will give a defined answer on the underlying nuclei formation process.