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

In this Letter, the first evidence of the He¯Λ¯4 antihypernucleus is presented, along with the first measurement at the LHC of the production of (anti)hypernuclei with mass number A=4, specifically (anti)HΛ4 and (anti)HeΛ4. In addition, the antiparticle-to-particle ratios for both hypernuclei (H¯Λ¯4/HΛ4 and He¯Λ¯4/HeΛ4) 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 sNN=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)HΛ4 and [1.08±0.34(stat)±0.20(syst)]×10−6 for the (anti)HeΛ4, and the measured antiparticle-to-particle ratios are in agreement with unity. The presence of (anti)HΛ4 and (anti)HeΛ4 excited states is expected to strongly enhance the production yield of these hypernuclei. The yield values exhibit a combined deviation of 3.3σ from the theoretical ground-state-only expectation, while the inclusion of the excited states in the calculations leads to an agreement within 0.6σ with the present measurements. Additionally, the measured (anti)HΛ4 and (anti)HeΛ4 masses are compatible with the world-average values within the uncertainties.