Observation of the antimatter hypernucleus H ¯ Λ ¯ 4

Abstract

At the origin of the Universe, an asymmetry between the amount of created matter and antimatter led to the matter-dominated Universe as we know it today. The origins of this asymmetry remain unknown so far. High-energy nuclear collisions create conditions similar to the Universe microseconds after the Big Bang, with comparable amounts of matter and antimatter^1-6. Much of the created antimatter escapes the rapidly expanding fireball without annihilating, making such collisions an effective experimental tool to create heavy antimatter nuclear objects and to study their properties^7-14, hoping to shed some light on the existing questions on the asymmetry between matter and antimatter. Here we report the observation of the antimatter hypernucleus ${}_{\overline{\Lambda }}{}^4\overline{H}$ , composed of a $\overline{\Lambda }$ , an antiproton and two antineutrons. The discovery was made through its two-body decay after production in ultrarelativistic heavy-ion collisions by the STAR experiment at the Relativistic Heavy Ion Collider^15,16. In total, 15.6 candidate ${}_{\overline{\Lambda }}{}^4\overline{H}$ antimatter hypernuclei are obtained with an estimated background count of 6.4. The lifetimes of the antihypernuclei ${}_{\overline{\Lambda }}{}^3\overline{H}$ and ${}_{\overline{\Lambda }}{}^4\overline{H}$ are measured and compared with the lifetimes of their corresponding hypernuclei, testing the symmetry between matter and antimatter. Various production yield ratios among (anti)hypernuclei (hypernuclei and/or antihypernuclei) and (anti)nuclei (nuclei and/or antinuclei) are also measured and compared with theoretical model predictions, shedding light on their production mechanisms.