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

Abstract

In this Letter, the first evidence of the ${}_{\bar{\Lambda }}^4\overline{\mathrm{He}}$ antihypernucleus is presented, along with the first measurement at the LHC of the production of (anti)hypernuclei with mass number $A=4$, specifically (anti)${}_{\Lambda }^4\mathrm{H}$ and (anti)${}_{\Lambda }^4\mathrm{He}$. In addition, the antiparticle-to-particle ratios for both hypernuclei (${}_{\bar{\Lambda }}^4\overline{\mathrm{H}}$ / ${}_{\Lambda }^4\mathrm{H}$ and ${}_{\bar{\Lambda }}^4\overline{\mathrm{He}}$ / ${}_{\Lambda }^4\mathrm{He}$) 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 $[0.78\pm 0.19(stat.)\pm 0.17(syst.)]\times 10^{-6}$ for the (anti)${}_{\Lambda }^4\mathrm{H}$ and $[1.08\pm 0.34(stat.)\pm 0.20(syst.)]\times 10^{-6}$ for the (anti)${}_{\Lambda }^4\mathrm{He}$, and the measured antiparticle-to-particle ratios are in agreement with unity. The presence of (anti)${}_{\Lambda }^4\mathrm{H}$ and (anti)${}_{\Lambda }^4\mathrm{He}$ 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)${}_{\Lambda }^4\mathrm{H}$ and (anti)${}_{\Lambda }^4\mathrm{He}$ masses are compatible with the world-average values within the uncertainties.

arXiv comment22 pages, 4 captioned figures, 1 table, authors from page 16, figures at http://alice-publications.web.cern.ch/node/11668

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