Measurements of _{Λ}^{3}H and _{Λ}^{4}H Lifetimes and Yields in Au+Au Collisions in the High Baryon Density Region

Observation of the antimatter hypernucleus H ¯ Λ ¯ 4

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 antimatter1-6. 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 properties7-14, hoping to shed some light on the existing questions on the asymmetry between matter and antimatter. Here we report the observation of the antimatter hypernucleusH ¯ Λ ¯ 4 , composed of aΛ ¯ , 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 Collider15,16. In total, 15.6 candidateH ¯ Λ ¯ 4 antimatter hypernuclei are obtained with an estimated background count of 6.4. The lifetimes of the antihypernucleiH ¯ Λ ¯ 3 andH ¯ Λ ¯ 4 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.

Nuclear structure opportunities with GeV radioactive beams at FAIR

The Facility for Antiproton and Ion Research (FAIR) is in its final construction stage next to the campus of the Gesellschaft für Schwerionenforschung Helmholtzzentrum for heavy-ion research in Darmstadt, Germany. Once it starts its operation, it will be the main nuclear physics research facility in many basic sciences and their applications in Europe for the coming decades. Owing to the ability of the new fragment separator, Super-FRagment Separator, to produce high-intensity radioactive ion beams in the energy range up to about 2 GeV/nucleon, these can be used in various nuclear reactions. This opens a unique opportunity for various nuclear structure studies across a range of fields and scales: from low-energy physics via the investigation of multi-neutron systems and halos to high-density nuclear matter and the equation of state, following heavy-ion collisions, fission and study of short-range correlations in nuclei and hypernuclei. The newly developed reactions with relativistic radioactive beams (R3B) set up at FAIR would be the most suitable and versatile for such studies. An overview of highlighted physics cases foreseen at R3B is given, along with possible future opportunities, at FAIR. This article is part of the theme issue 'The liminal position of Nuclear Physics: from hadrons to neutron stars'.

Measurement of the Lifetime and Λ Separation Energy of _{Λ}^{3}H

The most precise measurements to date of the _{Λ}^{3}H lifetime τ and Λ separation energy B_{Λ} are obtained using the data sample of Pb-Pb collisions at sqrt[s_{NN}]=5.02  TeV collected by ALICE at the LHC. The _{Λ}^{3}H is reconstructed via its charged two-body mesonic decay channel (_{Λ}^{3}H→^{3}He+π^{-} and the charge-conjugate process). The measured values τ=[253±11(stat)±6(syst)]  ps and B_{Λ}=[102±63(stat)±67(syst)]  keV are compatible with predictions from effective field theories and confirm that the _{Λ}^{3}H structure is consistent with a weakly bound system.

Observation of Directed Flow of Hypernuclei _{Λ}^{3}H and _{Λ}^{4}H in sqrt[s_{NN}]=3 GeV Au+Au Collisions at RHIC

We report here the first observation of directed flow (v_{1}) of the hypernuclei _{Λ}^{3}H and _{Λ}^{4}H in mid-central Au+Au collisions at sqrt[s_{NN}]=3  GeV at RHIC. These data are taken as part of the beam energy scan program carried out by the STAR experiment. From 165×10^{6} events in 5%-40% centrality, about 8400 _{Λ}^{3}H and 5200 _{Λ}^{4}H candidates are reconstructed through two- and three-body decay channels. We observe that these hypernuclei exhibit significant directed flow. Comparing to that of light nuclei, it is found that the midrapidity v_{1} slopes of _{Λ}^{3}H and _{Λ}^{4}H follow baryon number scaling, implying that the coalescence is the dominant mechanism for these hypernuclei production in the 3 GeV Au+Au collisions.

Light hypernuclei in heavy-ion collisions

Prediction for hyper nuclei multiplicities from GSI to LHC energies from the Ultra-relativistic Quantum Molecular Dynamics (UrQMD) model combined with a final state coalescence approach is presented and compared to the thermal model. The influence of the coalescence radius on the collision energy and centrality dependence of the Λ3H/Λ ratio is discussed.

Measurements on the production and properties of light hypernuclei at STAR

The hyperon-nucleon (Y-N) interaction, an important ingredient for the nuclear equation-of-state (EoS), remains poorly constrained. Precise measurements of hypernucei intrinsic properties and production yields in heavy-ion collisions are crucial for the understanding of their production mechanisms and the strength of the Y-N interaction. Thanks to the high statistics data taken from the STAR BES II program, a series of hypernuclei measurements are carried out at low energies. In these proceedings, we present the kinematic and centrality dependence of light hypernuclei production yields and strangeness population factor (S3, S4) in Au+Au collisions at √SNN = 3 GeV. We also report the energy dependence of Λ3H yields and S3 at mid-rapidity from 3 to 27 GeV Au+Au collisions. Precise measurements of Λ4He lifetime and Λ3H branching ratio are also reported. These results are compared with model calculations and physics implications are discussed.

Midrapidity cluster formation in heavy-ion collisions

We study the production of clusters and hypernuclei at midrapidity employing the Parton-Hadron- Quantum-Molecular-Dynamics (PHQMD) approach, a microscopic n-body transport model based on the QMD propagation of the baryonic degrees of freedom with density dependent 2-body potential interactions. In PHQMD the cluster formation occurs dynamically, caused by the interactions. The clusters are recognized by the Minimum Spanning Tree (MST) algorithm. We present the PHQMD results for cluster and hypernuclei formation in comparison with the available experimental data at relativistic energies. PHQMD allows to study the time evolution of formed clusters and the origin of their production, which helps to understand how such weakly bound objects are formed and survive in the rather dense and hot environment created in heavy-ion collisions. It offers therefore an explanation of the ’ice in the fire’ puzzle. To investigate whether this explanation of the ’ice in the fire’ puzzle applies only to the MST results we study also the deuterons production by coalescence. We embed MST and coalescence in the PHQMD and UrQMD transport approaches in order to obtain model independent results. We find that both clustering procedures give very similar results for the deuteron observables in the UrQMD as well as in the PHQMD environment. This confirms that our solution for the ’ice in the fire’ puzzle is common to MST and coalescence and independent of the transport approach.

Studies of hypernuclei with heavy-ion beams, nuclear emulsions and machine learning

The lightest hypernucleus, the hypertriton, has been a benchmark in the field of hypernuclear physics. However, some of recent experiments employing energetic heavy-ion beams have revealed that the hypertriton lifetime is significantly shorter than 263 ps which is expected by considering the known weakly binding nature of the hypertriton. The STAR collaboration has also measured the hypertriton binding energy, and the deduced value is contradicting to its formerly known small binding energy. These measurements have indicated that the fundamental physics quantities of the hypertriton such as its lifetime and binding energy have not been understood, therefore, they have to be measured very precisely. Furthermore, an unprecedented Λnn bound state observed by the HypHI collaboration has to be studied in order to draw a conclusion whether or not such a bound state exists. These three-body hypernuclear states are studied by the heavy-ion beam data in theWASA-FRS experiment and by analysing J-PARC E07 nuclear emulsion data with machine learning.

Recent studies on hypernuclei lifetimes from STAR

The hyperon-nucleon (Y-N) interaction is an essential ingredient in the description of the equation-of-state of high-baryon-density matter. Light hypernuclei (A = 3, 4), being simple Y-N bound states, serve as cornerstones of our understanding of the Y-N interaction. Thus, precise measurements of their lifetimes are important, as they provide stringent tests to hyperon-nucleon interaction models.

The yields of light hypernuclei are expected to increase from high to low energy heavy-ion collisions due to the increase in baryon density. As a result, the STAR Beam Energy Scan II program, which spans an energy range of √sNN = 3.0 − 27.0 GeV, is particularly suited for hypernuclei studies. In these proceedings, recent results on the lifetimes of light hypernuclei (3ΛH, 4ΛH, 4ΛHe) measured in √sNN = 3.0 and 7.2 GeV Au+Au collisions are presented. The relative branching ratio R3 of the 3ΛH is intimately related to its lifetime. A new R3 measurement using data from √sNN = 3.0 GeV Au+Au collisions is reported. These results will be compared to previous measurements and theoretical calculations, and the physics implications will be discussed.

Unique approach for precise determination of binding energies of hypernuclei with nuclear emulsion and machine learning

Hypertriton is the lightest hypernucleus and a benchmark in hypernuclear physics. However, it has recently been suggested that its lifetime and binding energy values may differ from the established values. To solve this puzzle, it is necessary to measure both values with a higher precision. For the precise measurement of the binding energy, we are aiming at developing a novel technique to measure the hypertriton binding energy with unprecedented accuracy by combining nuclear emulsion data and machine learning techniques. The analysis will be based on the J-PARC E07 nuclear emulsion data. Furthermore, a machine-learning model is being developed to identify other single and double-strangeness hypernuclei.

WASA-FRS HypHI experiment at GSI for studying light hypernuclei

From January till March 2022, the WASA-FRS HypHI experiment performed a precise measurement of the hypertriton and the 4ΛH hypernucleus lifetime at GSI. The data collected should also confirm whether or not the nnΛ bound state can exist. The experiment were carried out with the WASA central detector with a complex of additional dedicated detectors mounted together at the mid-focal plane of the high-momentum-resolution forward spectrometer, the so-called fragment separator FRS. Hypernuclei of interest were produced by induced reactions of 6Li projectiles at 1.96 A GeV on a diamond target with a thickness of 9.87 g/cm2. Negatively charged π mesons from two-body decays of the hypernuclei of interest were measured by theWASA and the other detectors, and the residual nuclei after the π− decay were measured by the FRS with a momentum resolving power of 104.

Comparison of 3ΛH/4ΛH production cross-section via (K−, π0) reaction at J-PARC

Recent heavy-ion collision experiments reported a surprisingly short lifetime for the hypertriton, which has been recognized as the hypertriton lifetime puzzle. Our J-PARC E73 experiment contributes to solve this puzzle with an independent experimental method by employing 3He(K−, π0) 3ΛH reaction. In this contribution, we will demonstrate our capability to provide 3ΛH binding energy information by deriving the production cross section ratio, σ3ΛH/σ4ΛH. The production cross section data for 3ΛH and 4ΛH are already available as the pilot run of E73 experiment and data analysis is in progress.

Lifetime of the hypertriton

Conflicting values of the hypertriton lifetime τ(3ΛH) were derived in relativistic heavy ion (RHI) collision experiments over the last decade. A very recent ALICE Collaboration measurement is the only experiment where the reported τ(3ΛH) comes sufficiently close to the free-Λ lifetime τΛ, as expected naively for a very weakly bound Λ in 3ΛH. We revisited theoretically this 3ΛH lifetime puzzle [1], using 3ΛH and 3He wave functions computed within the ab initio no-core shell model employing interactions derived from chiral effective field theory to calculate the two-body decay rate Γ(3ΛH → 3He + π−). We found significant but opposing contributions arising from ΣNN admixtures in 3ΛH and from π− − 3He final-state interaction. To derive τ(3ΛH), we evaluated the inclusive π− decay rate Γπ− (3ΛH) by using the measured branching ratio Γ(3ΛH → 3He + π−)/Γπ− (3ΛH) and added the π0 contributions through the ΔI = 1/2rule. The resulting τ(3ΛH) varies strongly with the rather poorly known Λ separation energy Esep(3ΛH) and it is thus possible to associate each one of the distinct RHI τ(3ΛH) measurements with its own underlying value of Esep(3ΛH).

New results on light nuclei, hyperons and hypernuclei from HADES (HADES collaboration)

In March 2019 the HADES experiment recorded 14 billion Ag+Ag collisions at √sNN = 2.55 GeV as a part of the FAIR phase-0 physics program. In this contribution, we present and investigate our capabilities to reconstruct and analyze weakly decaying strange hadrons and hypernuclei emerging from these collisions. The focus is put on measuring the mean lifetimes of these particles.