First measurement of A = 4 (anti)hypernuclei at the LHC

In this Letter, the first evidence of the $^4_{\bar{\Lambda}}\overline{\rm He}$ antihypernucleus is presented, along with the first measurement at the LHC of the production of (anti)hypernuclei with mass number $A=4$, specifically (anti)$^4_{\Lambda}\mathrm{H}$ and (anti)$^4_{\Lambda}\mathrm{He}$. In addition, the antiparticle-to-particle ratios for both hypernuclei ($^4_{\bar{\Lambda}}\overline{\rm H}$ / $^4_{\Lambda}\mathrm{H}$ and $^4_{\bar{\Lambda}}\overline{\rm He}$ / $^4_{\Lambda}\mathrm{He}$) 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 $\sqrt{s_{\mathrm{NN}}} =$ 5.02 TeV. The yields measured for the average of the charge-conjugated states are found to be $[0.78 \pm 0.19 \; \mathrm{(stat.)} \pm 0.17 \; \mathrm{(syst.)}] \times 10^{-6}$ for the (anti)$^4_{\Lambda}\mathrm{H}$ and $[1.08 \pm 0.34 \; \mathrm{(stat.)} \; \pm 0.20 \; \mathrm{(syst.)}] \times 10^{-6}$ for the (anti)$^4_{\Lambda}\mathrm{He}$, and the measured antiparticle-to-particle ratios are in agreement with unity. The presence of (anti)$^4_{\Lambda}\mathrm{H}$ and (anti)$^4_{\Lambda}\mathrm{He}$ excited states is expected to strongly enhance the production yield of these hypernuclei. The yield values exhibit a combined deviation of 3.3$\sigma$ from the theoretical ground-state-only expectation, while the inclusion of the excited states in the calculations leads to an agreement within 0.6$\sigma$ with the present measurements. Additionally, the measured (anti)$^4_{\Lambda}\mathrm{H}$ and (anti)$^4_{\Lambda}\mathrm{He}$ masses are compatible with the world-average values within the uncertainties.