Abstract
In this Letter, the first evidence of the Λ4He antihypernucleus is presented, along with the first measurement at the LHC of the production of (anti)hypernuclei with mass number A=4, specifically (anti)Λ4H and (anti)Λ4He. In addition, the antiparticle-to-particle ratios for both hypernuclei (Λ4H‾/Λ4H and Λ4He‾/Λ4He) 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 root s_NN = 5.02 TeV. The yields measured for the average of the charge-conjugated states are found to be [0.78±0.19 (stat)±0.17 (syst)]×10−6 for the (anti)Λ4H and [1.08±0.34 (stat)±0.20 (syst)]×10−6 for the (anti)Λ4He, and the measured antiparticle-to-particle ratios are in agreement with unity. The presence of (anti)Λ4H and (anti)Λ4He excited states is expected to strongly enhance the production yield of these hypernuclei. The yield values exhibit a combined deviation of 3.3 σ from the theoretical ground-state-only expectation, while the inclusion of the excited states in the calculations leads to an agreement within 0.6 σ with the present measurements. Additionally, the measured (anti)Λ4H and (anti)Λ4He masses are compatible with the world-average values within the uncertainties.
