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

Abstract

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 ) H Λ 4 and ( anti ) He Λ 4 . In addition, the antiparticle-to-particle ratios for both hypernuclei ( H ¯ Λ ¯ 4 / H Λ 4 and He ¯ Λ ¯ 4 / He Λ 4 ) 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 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 ) H Λ 4 and [ 1.08 ± 0.34 ( stat ) ± 0.20 ( syst ) ] × 10 − 6 for the ( anti ) He Λ 4 , and the measured antiparticle-to-particle ratios are in agreement with unity. The presence of ( anti ) H Λ 4 and ( anti ) He Λ 4 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 ) H Λ 4 and ( anti ) He Λ 4 masses are compatible with the world-average values within the uncertainties.