Observation of an Antimatter Hypernucleus

Abstract

The STAR CollaborationNuclear collisions recreate conditions in the universe microseconds after the Big Bang. Only a very small fraction of the emitted fragments are light nuclei, but these states are of fundamental interest. We report the observation of antihypertritons -composed of an antiproton, antineutron, and antilambda hyperon -produced by colliding gold nuclei at high energy. Our analysis yields 70 ± 17 antihypertritons ( Nuclei are abundant in the universe, but antinuclei that are heavier than the antiproton have been observed only as products of interactions at particle accelerators (1, 2). Collisions of heavy nuclei at the Relativistic Heavy-Ion Collider (RHIC) at Brookhaven National Laboratory (BNL) briefly produce hot and dense matter that has been interpreted as a quark gluon plasma (QGP) (3, 4) with an energy density similar to that of the universe a few microseconds after the Big Bang. This plasma contains roughly equal numbers of quarks and antiquarks. As a result of the high energy density of the QGP phase, many strange-antistrange (ss) quark pairs 1 are liberated from the quantum vacuum. The plasma cools and transitions into a hadron gas, producing nucleons, hyperons, mesons, and their antiparticles.Nucleons (protons and neutrons) contain only up and down valence quarks, while hyperons (Λ, Σ, Ξ, Ω) contain at least one strange quark in its 3-quark valence set. A hypernucleus is a nucleus that contains at least one hyperon in addition to nucleons. All hyperons are unstable, even when bound in nuclei. The lightest bound hypernucleus is the hypertriton ( 3 Λ H), which consists of a Λ hyperon, a proton, and a neutron. The first observation of any hypernucleus was made in 1952 using a nuclear emulsion cosmic ray detector (5). Here, we present the observation of an antimatter hypernucleus. Production of antinuclei:Models of heavy-ion collisions have had good success in explaining the production of nuclei by assuming that a statistical coalescence mechanism is in effect during the late stage of the collision evolution (4, 6). Antinuclei can be produced through the same coalescence mechanism, and are predicted to be present in cosmic rays. An observed high yield could be interpreted as an indirect signature of new physics, such as Dark Matter (7, 8). Heavy-ion collisions at RHIC provide an opportunity for the discovery and study of many antinuclei and antihypernuclei.The ability to produce antihypernuclei allows the study of all populated regions in the 3-dimensional chart of the nuclides. The conventional 2-dimensional chart of the nuclides organizes nuclear isotopes in the (N, Z) plane, where N is the number of neutrons and the Z is the number of protons in the nucleus. This chart can be extended to the negative sector in the (N, Z) plane by including antimatter nuclei. Hypernuclei bring a third dimension into play, based on the strangeness quantum number of the nucleus. The present study probes the territory of antinuclei with non-zero strangeness ( Fig. 1), where proposed ideas (9-12) related to t...

Production of (Anti-)(Hyper)nuclei in Pb–Pb Collisions Measured with ALICE at the LHC

“…A detailed description of the ALICE detector can be found in [10] and references therein. The 3 Λ H was identified via the invariant mass reconstruction of the products of its mesonic weak decay 3 Λ H → ( 3 He + π − ) [11]. Since its mean lifetime is similar to that of the free Λ (cτ ∼ 8 cm), in most cases it is possible to identify its decay up to a few cm away from the primary vertex.…”

“…The decay vertex was determined by studying track pairs that pass a set of cuts. Selections on the decay topology include cuts on the Distance of Closest Approach (DCA) between the two particle tracks identified as 3 He and π using the information on the specific energy loss dE/dx in the TPC, on the DCA of the π ± tracks from the primary vertex, and on the cosine of the angle between the total momentum of the decay pair and the vector connecting the primary and the secondary vertex (pointing angle). A selection on the proper lifetime (ct < 30 cm) of the candidate was made.…”

“…The simulated particles were propagated through the detector using the GEANT3 transport code [14] and then processed through the same reconstruction chain as for the real data. Since the absorption of (anti-)(hyper)nuclei is not properly implemented in GEANT3, a correction based on the measured p (p) absorption [15] was applied in order to take into account the absorption of 3 Λ H ( 3 Λ H) and 3 He ( 3 He) in the material of the ALICE detector. In this approach, the 3 He and 3 Λ H were treated as states of three independent p (p) by means of a proper model, which was tested with deuterons.…”

“…The signal was extracted in the intervals 1 ≤ ct <4 cm, 4 ≤ ct < 7 cm, 7 ≤ ct < 10 cm and 10 ≤ ct < 28 cm, and was corrected by the detector acceptance, the reconstruction efficiency and the absorption of 3 Λ H ( 3 Λ H) in the material. The detector acceptance and reconstruction efficiency were evaluated using a dedicated HI-JING [13] Monte Carlo simulation, where the only allowed hypertriton decay mode was in two charged particles, 3 Λ H → 3 He + π − and 3 Λ H → 3 He + π + . The simulated particles were propagated through the detector using the GEANT3 transport code [14] and then processed through the same reconstruction chain as for the real data.…”

“…Their production in heavy-ion collisions was proposed long ago and has been studied ever since [1,2]; moreover, at ultrarelativistic energies, it is also possible to produce anti-hypernuclei, inaccessible with other reactions. In fact, while many Λ-hypernuclei have been observed so far, the first observation of an anti-hypernucleus is rather recent and was reported in the analysis of Au-Au collisions at √ s NN = 200 GeV by the STAR Collaboration at RHIC [3]. Since light hypernuclei are weakly bound nuclear systems, they are sensitive probes of the final stages of the evolution of the fireball formed in the heavy-ion collisions [4].…”