The Large Hadron Collider, besides being the highest-energy heavy-ion collider, is also a factory of antimatter. Light nuclei and antinuclei, such as deuterons 2H (or d), tritons 3H, 3He, and 4He, are abundantly produced in both proton-proton and lead-lead collisions. Given the large collision energy, nuclei and antinuclei are formed in almost exactly equal amounts. The production mechanism of light (anti)nuclei in these collisions is not yet fully understood, with competing theoretical models based on the “coalescence” of protons and neutrons or on statistical hadronization from a system of quarks. ALICE can identify light (anti)nuclei using their large energy loss in the detectors and their low velocity. Production measurements have been carried out in all collision systems, showing that the ratios of light nuclei to protons follow of smooth trend as a function of the multiplicity of the collision and are independent of the collision system and energy (see figure). Both production models qualitatively describe the measurements, with coalescence providing also a quantitative agreement. Understanding the (anti)nuclei production is also relevant for dark matter searches in Space.
While the nature of dark matter remains unknown, the search for anti-nuclei in the outer atmosphere is a very promising channel for its possible discovery. Theory models predict that low-energy antinuclei, such as anti-3He, can originate from galactic dark-matter annihilation. However, also ordinary cosmic-ray collisions in the atmosphere can produce light nuclei and antinuclei, with a similar process as those that occur in LHC collisions. For this reason, the nuclei production measurements by ALICE are important for these searches. In addition, antinuclei can annihilate in the interstellar medium before reaching the atmosphere. The inelastic interaction probability of anti-3He in the interstellar medium is an essential ingredient for calculations of the anti-3He flux near Earth.
The anti-3He inelastic interaction cross section was measured for the first time, using the ALICE detector material as an effective target. The measurement was used to estimate the transparency of our galaxy to the propagation of anti-3He. It has been found that anti-3He nuclei can travel long distances of several kiloparsecs, with about 50% of nuclei produced in dark-matter annihilation reaching the near-Earth environment (see figure). The uncertainties associated with the anti-3He absorption are about 10-15%, representing a large improvement in the knowledge of the anti-3He inelastic interactions - previously it was not possible to quantify the uncertainty due to the lack of measurements.