Antimatter ${}_{\mathrm{\Lambda }}{}^4\mathrm{H}$ hypernucleus production and the ${}_{\mathrm{\Lambda }}{}^3\mathrm{H}/{}^3\mathrm{He}$ puzzle in relativistic heavy-ion collisions

We show that the measured yield ratio ${}_{\mathrm{\Lambda }}{}^3\mathrm{H}/{}^3\mathrm{He}({}_{\overline{\mathrm{\Lambda }}}{}^3\overline{\mathrm{H}}/{}^3\overline{\mathrm{He}})$ in $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{NN}}=200$ GeV and in $\mathrm{Pb}+\mathrm{Pb}$ collisions at $\sqrt{s_{NN}}=2.76$ TeV can be understood within a covariant coalescence model if (anti-)$\mathrm{\Lambda }$ particles freeze out earlier than (anti-)nucleons but their relative freeze-out time is closer at $\sqrt{s_{NN}}=2.76$ TeV than at $\sqrt{s_{NN}}=200$ GeV. The earlier (anti-)$\mathrm{\Lambda }$ freeze-out can significantly enhance the yield of (anti)hypernucleus ${}_{\mathrm{\Lambda }}{}^4\mathrm{H}\left({}_{\overline{\mathrm{\Lambda }}}{}^4\overline{\mathrm{H}}\right)$, leading to that ${}_{\overline{\mathrm{\Lambda }}}{}^4\overline{\mathrm{H}}$ has a comparable abundance with ${}^4\overline{\mathrm{He}}$ and thus provides an easily measured antimatter candidate heavier than ${}^4\overline{\mathrm{He}}$. The future measurement on ${}_{\mathrm{\Lambda }}{}^4\mathrm{H}\left({}_{\overline{\mathrm{\Lambda }}}{}^4\overline{\mathrm{H}}\right)$ would be very useful to understand the (anti-)$\mathrm{\Lambda }$ freeze-out dynamics and the production mechanism of (anti)hypernuclei in relativistic heavy-ion collisions.

Figure 1

Transverse momentum distributions of $p$, $d$, ${}^3\mathrm{He}$, ${}_{\mathrm{\Lambda }}{}^3\mathrm{H}$, and ${}_{\mathrm{\Lambda }}{}^4\mathrm{H}$ at midrapidity in central collisions of $\mathrm{Au}+\mathrm{Au}$ at $\sqrt{s_{NN}}=200$ GeV (a) and $\mathrm{Pb}+\mathrm{Pb}$ at $\sqrt{s_{NN}}=2.76$ TeV (b) predicted by a coalescence model with various freeze-out configurations. For $\mathrm{Au}+\mathrm{Au}$ collisions, the data of protons are taken from the PHENIX [41] and those of $d$ and ${}^3\mathrm{He}$ are taken from STAR [1]. The data of $p$, $d$, ${}^3\mathrm{He}$, and ${}_{\mathrm{\Lambda }}{}^3\mathrm{H}$ for $\mathrm{Pb}+\mathrm{Pb}$ collisions are taken from ALICE [4, 14, 42].

Figure 2

Transverse momentum distribution of $\mathrm{\Lambda }$'s in central collisions of $\mathrm{Au}+\mathrm{Au}$ at $\sqrt{s_{NN}}=200$ GeV (a) and $\mathrm{Pb}+\mathrm{Pb}$ at $\sqrt{s_{NN}}=2.76$ TeV (b) from coalescence model calculations with various freeze-out configurations. The experimental data are taken from STAR [43] for the $\mathrm{Au}+\mathrm{Au}$ collisions and from ALICE [44] for the $\mathrm{Pb}+\mathrm{Pb}$ collisions.

Figure 3

The predicted $dN/dy$ of ${}_{\mathrm{\Lambda }}{}^3\mathrm{H}$ and ${}^3\mathrm{He}$ at midrapidity as a function of their root-mean-square radii $r_{\mathrm{rms}}$ in central collisions of $\mathrm{Au}+\mathrm{Au}$ at $\sqrt{s_{NN}}=200$ GeV (a) and $\mathrm{Pb}+\mathrm{Pb}$ at $\sqrt{s_{NN}}=2.76$ TeV (b) from the coalescence model with various freeze-out configurations. The stars (squares) indicate the empirical size values $r_{\mathrm{rms}}=4.9\left(1.76\right)$ fm for ${}_{\mathrm{\Lambda }}{}^3\mathrm{H}\left({}^3\mathrm{He}\right)$.

Figure 4

The predicted $dN/dy$ of light (anti)(hyper)nuclei at midrapidity as a function of $\frac{B}{\left|B\right|}m$ in central collisions of $\mathrm{Au}+\mathrm{Au}$ at $\sqrt{s_{NN}}=200$ GeV (a) and $\mathrm{Pb}+\mathrm{Pb}$ at $\sqrt{s_{NN}}=2.76$ TeV (b) from the coalescence model with various freeze-out configurations. The squares in (a) represent the results for ${}^4\mathrm{He}$ and ${}^4\overline{\mathrm{He}}$ in the $\mathrm{Au}+\mathrm{Au}$ collisions by considering the binding energy effects to fit STAR data [2, 10] while the square in (b) is the preliminary result for ${}^4\mathrm{He}$ in $0\%–20\%$ centrality $\mathrm{Pb}+\mathrm{Pb}$ collisions from ALICE measurement [58].