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