First Measurement of A = 4 Hypernuclei and Antihypernuclei at the LHC

In this Letter, the first evidence of the (Formula presented) antihypernucleus is presented, along with the first measurement at the LHC of the production of (anti)hypernuclei with mass number (Formula presented), specifically (Formula presented) and (Formula presented). In addition, the antiparticle-to-particle ratios for both hypernuclei ((Formula presented) and (Formula presented)) are shown, which are sensitive to the baryochemical potential of the strongly interacting matter created in heavy-ion collisions. The results are obtained from a data sample of central Pb-Pb collisions, collected during the 2018 LHC data taking at a center-of-mass energy per nucleon pair of (Formula presented). The yields measured for the average of the charge-conjugated states are found to be (Formula presented) for the (Formula presented) and (Formula presented) for the (Formula presented), and the measured antiparticle-to-particle ratios are in agreement with unity. The presence of (Formula presented) and (Formula presented) excited states is expected to strongly enhance the production yield of these hypernuclei. The yield values exhibit a combined deviation of (Formula presented) from the theoretical ground-state-only expectation, while the inclusion of the excited states in the calculations leads to an agreement within (Formula presented) with the present measurements. Additionally, the measured (Formula presented) and (Formula presented) masses are compatible with the world-average values within the uncertainties.