In this Letter, the first evidence of the ${}^4_{\bar{\Lambda}}\overline{\mathrm{He}}$ antihypernucleus is presented, along with the first measurement at the LHC of the production of (anti)hypernuclei with mass number $A=4$, specifically (anti)${}^4_{\Lambda}\mathrm{H}$ and (anti)${}^4_{\Lambda}\mathrm{He}$. In addition, the antiparticle-to-particle ratios for both hypernuclei (${}^4_{\bar{\Lambda}}\overline{\mathrm{H}}$ / ${}^4_{\Lambda}\mathrm{H}$ and ${}^4_{\bar{\Lambda}}\overline{\mathrm{He}}$ / ${}^4_{\Lambda}\mathrm{He}$) 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.
Submitted to: PRL
e-Print: arXiv:2410.17769 | PDF | inSPIRE
CERN-EP-2024-265
Figure group
Figure 1
Invariant-mass spectra of $^{4}_{\overline{\Lambda}}\overline{\rm He}$ candidates on the left and $^{4}_{\overline{\Lambda}}\overline{\rm H}$ candidates on the right. These spectra are fitted with a KDE template for the signal peak (blue curve) and an exponential function to model the background (orange curve). The combined invariant-mass spectra for the sum of particle and antiparticle states of each (anti)hypernucleus are shown in the End Matter. |   |
Figure 2
Measured integrated yields of the (anti)$^{4}_{\Lambda}{\rm He}$ on the left and the (anti)$^{4}_{\Lambda}{\rm H}$ on the right (average of particle and antiparticle state for each hypernucleus). The x-axis reports the measured mass, which is compared to the world-average values obtained from the Hypernuclei Database . The horizontal width of the blue and green lines corresponds to the uncertainty of the world-average values. The statistical uncertainties on the measured values are given by the bars and the systematic uncertainties by the boxes around the central values. A comparison is shown between the dN/dy values predicted by the SHM at $T_{\rm ch} = 155$ MeV, considering only the ground state (green line), and those including feed-down contributions from excited states (blue line) for each (anti)hypernucleus . The shaded areas around the predictions of the SHM correspond to the deviation of the expected yield by a variation of T$_{\mathrm{ch}}$ of 1.5 MeV. The arrows connecting the experimental results and the dN/dy values predicted with the SHM (for $T_{\rm ch} = 155$ MeV) indicate their difference expressed as number of standard deviations using only the experimental uncertainties. |  |
Figure 3
Antiparticle-to-particle ratio for both investigated hypernuclei. The uncertainties are given by the quadratic sum of the statistical uncertainties and the systematic uncertainty on the absorption of the (anti)hypernuclei decay products (4.3% for the $^{4}_{\overline{\Lambda}}\overline{\rm He}$ / $^{4}_{\Lambda}{\rm He}$ , 4.5% for the $^{4}_{\overline{\Lambda}}\overline{\rm H}$ / $^{4}_{\Lambda}{\rm H}$ ) and the absorption of the (anti)hypernuclei themselves (5.5% for the $^{4}_{\overline{\Lambda}}\overline{\rm He}$ / $^{4}_{\Lambda}{\rm He}$, 6.3% for the $^{4}_{\overline{\Lambda}}\overline{\rm H}$ / $^{4}_{\Lambda}{\rm H}$). |  |
Figure A.1
Invariant-mass spectra of (anti)$^{4}_{\Lambda}{\rm He}$ candidates on the left and of (anti)$^{4}_{\Lambda}{\rm H}$ candidates on the right. Each spectrum shows the sum of the particle and antiparticle states and is fitted with a KDE template for the signal peak (blue curve) and an exponential function to model the background (orange curve). |   |