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The measurement of an alignment between the angular momentum of a non-central collision between heavy ions and the spin of emitted particles reveals that the fluid produced in the collision is extremely vortical. Colliding ions go into a vortex When heavy ions such as gold collide in a particle collider, they form exotic states of matter that are similar to fluids. If the particles hit non-centrally, then the fluid is predicted to have vortices. However, these vortices have not yet been observed in an experiment. Here, the STAR Collaboration shows that during gold–gold collisions, spin alignment of Λ hyperons with the angular momentum of the fluid occurs. This is experimental evidence of the formation of vortices. They also show that the fluid produced in heavy-ion collisions has the highest vorticity ever observed. The results could provide general insights into how vortices form in ideal liquids. The extreme energy densities generated by ultra-relativistic collisions between heavy atomic nuclei produce a state of matter that behaves surprisingly like a fluid, with exceptionally high temperature and low viscosity 1 . Non-central collisions have angular momenta of the order of 1,000 ћ , and the resulting fluid may have a strong vortical structure 2 , 3 , 4 that must be understood to describe the fluid properly. The vortical structure is also of particular interest because the restoration of fundamental symmetries of quantum chromodynamics is expected to produce novel physical effects in the presence of strong vorticity 5 . However, no experimental indications of fluid vorticity in heavy ion collisions have yet been found. Since vorticity represents a local rotational structure of the fluid, spin–orbit coupling can lead to preferential orientation of particle spins along the direction of rotation. Here we present measurements of an alignment between the global angular momentum of a non-central collision and the spin of emitted particles (in this case the collision occurs between gold nuclei and produces Λ baryons), revealing that the fluid produced in heavy ion collisions is the most vortical system so far observed. (At high energies, this fluid is a quark–gluon plasma.) We find that Λ and hyperons show a positive polarization of the order of a few per cent, consistent with some hydrodynamic predictions 6 . (A hyperon is a particle composed of three quarks, at least one of which is a strange quark; the remainder are up and down quarks, found in protons and neutrons.) A previous measurement 7 that reported a null result, that is, zero polarization, at higher collision energies is seen to be consistent with the trend of our observations, though with larger statistical uncertainties. These data provide experimental access to the vortical structure of the nearly ideal liquid 8 created in a heavy ion collision and should prove valuable in the development of hydrodynamic models that quantitatively connect observations to the theory of the strong force.

Atomic nuclei are self-organized, many-body quantum systems bound by strong nuclear forces within femtometre-scale space. These complex systems manifest a variety of shapes 1 – 3 , traditionally explored using non-invasive spectroscopic techniques at low energies 4 , 5 . However, at these energies, their instantaneous shapes are obscured by long-timescale quantum fluctuations, making direct observation challenging. Here we introduce the collective-flow-assisted nuclear shape-imaging method, which images the nuclear global shape by colliding them at ultrarelativistic speeds and analysing the collective response of outgoing debris. This technique captures a collision-specific snapshot of the spatial matter distribution within the nuclei, which, through the hydrodynamic expansion, imprints patterns on the particle momentum distribution observed in detectors 6 , 7 . We benchmark this method in collisions of ground-state uranium-238 nuclei, known for their elongated, axial-symmetric shape. Our findings show a large deformation with a slight deviation from axial symmetry in the nuclear ground state, aligning broadly with previous low-energy experiments. This approach offers a new method for imaging nuclear shapes, enhances our understanding of the initial conditions in high-energy collisions and addresses the important issue of nuclear structure evolution across energy scales. The collective-flow-assisted nuclear shape-imaging method images the nuclear global shape by colliding them at ultrarelativistic speeds and analysing the collective response of outgoing debris.

According to the CPT theorem, which states that the combined operation of charge conjugation, parity transformation and time reversal must be conserved, particles and their antiparticles should have the same mass and lifetime but opposite charge and magnetic moment. Here, we test CPT symmetry in a nucleus containing a strange quark, more specifically in the hypertriton. This hypernucleus is the lightest one yet discovered and consists of a proton, a neutron and a Λ hyperon. With data recorded by the STAR detector 1 – 3 at the Relativistic Heavy Ion Collider, we measure the Λ hyperon binding energy B Λ for the hypertriton, and find that it differs from the widely used value 4 and from predictions 5 – 8 , where the hypertriton is treated as a weakly bound system. Our results place stringent constraints on the hyperon–nucleon interaction 9 , 10 and have implications for understanding neutron star interiors, where strange matter may be present 11 . A precise comparison of the masses of the hypertriton and the antihypertriton allows us to test CPT symmetry in a nucleus with strangeness, and we observe no deviation from the expected exact symmetry. The STAR collaboration reports a measurement of the mass difference and binding energy of the hypertriton and its antiparticle. This work constrains the hyperon–nucleon interaction and allows us to test the CPT theorem in a nucleus with strangeness.

Notwithstanding decades of progress since Yukawa first developed a description of the force between nucleons in terms of meson exchange 1 , a full understanding of the strong interaction remains a considerable challenge in modern science. One remaining difficulty arises from the non-perturbative nature of the strong force, which leads to the phenomenon of quark confinement at distances on the order of the size of the proton. Here we show that, in relativistic heavy-ion collisions, in which quarks and gluons are set free over an extended volume, two species of produced vector (spin-1) mesons, namely ϕ and K *0 , emerge with a surprising pattern of global spin alignment. In particular, the global spin alignment for ϕ is unexpectedly large, whereas that for K *0 is consistent with zero. The observed spin-alignment pattern and magnitude for ϕ cannot be explained by conventional mechanisms, whereas a model with a connection to strong force fields 2 – 6 , that is, an effective proxy description within the standard model and quantum chromodynamics, accommodates the current data. This connection, if fully established, will open a potential new avenue for studying the behaviour of strong force fields. At the Relativistic Heavy Ion Collider, observations of two meson species produced by heavy-ion collisions, ϕ and K *0 , show surprising patterns of global spin alignment, being unexpectedly large and consistent with zero, respectively.

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 H ¯ Λ ¯ 4 , composed of a Λ ¯ , 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 H ¯ Λ ¯ 4 antimatter hypernuclei are obtained with an estimated background count of 6.4. The lifetimes of the antihypernuclei H ¯ Λ ¯ 3 and H ¯ 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. An antimatter hypernucleus formed by an anti-lambda hadron, an antiproton and two antineutrons was observed through its two-body decay after production in ultrarelativistic heavy-ion collisions.

In a Quark-Gluon Plasma (QGP), the fundamental building blocks of matter, quarks and gluons, are under extreme conditions of temperature and density. A QGP could exist in the early stages of the Universe, and in various objects and events in the cosmos. The thermodynamic and hydrodynamic properties of the QGP are described by Quantum Chromodynamics (QCD) and can be studied in heavy-ion collisions. Despite being a key thermodynamic parameter, the QGP temperature is still poorly known. Thermal lepton pairs ( e + e − and μ + μ − ) are ideal penetrating probes of the true temperature of the emitting source, since their invariant-mass spectra suffer neither from strong final-state interactions nor from blue-shift effects due to rapid expansion. Here we measure the QGP temperature using thermal e + e − production at the Relativistic Heavy Ion Collider (RHIC). The average temperature from the low-mass region (in-medium ρ 0 vector-meson dominant) is (2.01 ± 0.23) × 10 12 K, consistent with the chemical freeze-out temperature from statistical models and the phase transition temperature from Lattice QCD. The average temperature from the intermediate mass region (above the ρ 0 mass, QGP dominant) is significantly higher at (3.25 ± 0.60) × 10 12 K. This work provides essential experimental thermodynamic measurements to map out the QCD phase diagram and understand the properties of matter under extreme conditions. Thermal lepton pairs are ideal probes for the temperature of quark-gluon plasma. Here, the STAR Collaboration uses thermal electron-positron pair production to measure quark-gluon plasma average temperature at different stages of the evolution.

A bstract We report multi-differential measurements of strange hadron production ranging from mid- to target-rapidity in Au+Au collisions at a center-of-momentum energy per nucleon pair of s NN = 3 GeV with the STAR experiment at RHIC. K S 0 meson and Λ hyperon yields are measured via their weak decay channels. Collision centrality and rapidity dependences of the transverse momentum spectra and particle ratios are presented. Particle mass and centrality dependence of the average transverse momenta of Λ and K S 0 are compared with other strange particles, providing evidence of the development of hadronic rescattering in such collisions. The 4 π yields of each of these strange hadrons show a consistent centrality dependence. Discussions on radial flow, the strange hadron production mechanism, and properties of the medium created in such collisions are presented together with results from hadronic transport and thermal model calculations.

Antimatter worth the weight The α-particle — the helium nucleus consisting of two protons and two neutrons — was identified a century ago by Ernest Rutherford. Its antimatter counterpart of two antiprotons and two antineutrons has now been detected by the Relativistic Heavy Ion Collider at Brookhaven National Laboratory in Upton, New York. The STAR Collaboration has detected anti-α-particles — the heaviest antinuclei observed to date — at a yield that is consistent with expectations from thermodynamic and coalescent nucleosynthesis models. This discovery provides an indication of the likely production rates of even heavier antimatter nuclei, and serves as a benchmark for possible future observations of anti-α-particles in the cosmos. High-energy nuclear collisions create an energy density similar to that of the Universe microseconds after the Big Bang 1 ; in both cases, matter and antimatter are formed with comparable abundance. However, the relatively short-lived expansion in nuclear collisions allows antimatter to decouple quickly from matter, and avoid annihilation. Thus, a high-energy accelerator of heavy nuclei provides an efficient means of producing and studying antimatter. The antimatter helium-4 nucleus ( ), also known as the anti-α ( ), consists of two antiprotons and two antineutrons (baryon number B = −4). It has not been observed previously, although the α-particle was identified a century ago by Rutherford and is present in cosmic radiation at the ten per cent level 2 . Antimatter nuclei with B  < −1 have been observed only as rare products of interactions at particle accelerators, where the rate of antinucleus production in high-energy collisions decreases by a factor of about 1,000 with each additional antinucleon 3 , 4 , 5 . Here we report the observation of , the heaviest observed antinucleus to date. In total, 18 counts were detected at the STAR experiment at the Relativistic Heavy Ion Collider (RHIC; ref. 6 ) in 10 9 recorded gold-on-gold (Au+Au) collisions at centre-of-mass energies of 200 GeV and 62 GeV per nucleon–nucleon pair. The yield is consistent with expectations from thermodynamic 7 and coalescent nucleosynthesis 8 models, providing an indication of the production rate of even heavier antimatter nuclei and a benchmark for possible future observations of in cosmic radiation.

A bstract A surprisingly large transverse polarization of Λ hyperons in unpolarized hadron-nucleon/nucleus collisions has been observed for 50 years, and the origin of this polarization remains an important open question. Recently, theoretical frameworks have advanced in describing this puzzle with the polarizing fragmentation function (PFF). We report the first measurement of Λ and transverse polarization inside jets in unpolarized proton-proton collisions, which is directly attributed to the PFF. The polarization is measured as a function of the jet transverse momentum, the fraction of the jet momentum carried by hyperons, and the transverse momentum of hyperons relative to the jet axis. Covering a wide jet-energy range, these data provide the first constraints on the gluon PFF and allow tests of TMD evolution and its universality.

In a Quark-Gluon Plasma (QGP), the fundamental building blocks of matter, quarks and gluons, are under extreme conditions of temperature and density. A QGP could exist in the early stages of the Universe, and in various objects and events in the cosmos. The thermodynamic and hydrodynamic properties of the QGP are described by Quantum Chromodynamics (QCD) and can be studied in heavy-ion collisions. Despite being a key thermodynamic parameter, the QGP temperature is still poorly known. Thermal lepton pairs (e+e− and μ+μ−) are ideal penetrating probes of the true temperature of the emitting source, since their invariant-mass spectra suffer neither from strong final-state interactions nor from blue-shift effects due to rapid expansion. Here we measure the QGP temperature using thermal e+e− production at the Relativistic Heavy Ion Collider (RHIC). The average temperature from the low-mass region (in-medium ρ0 vector-meson dominant) is (2.01 ± 0.23) × 1012 K, consistent with the chemical freeze-out temperature from statistical models and the phase transition temperature from Lattice QCD. The average temperature from the intermediate mass region (above the ρ0 mass, QGP dominant) is significantly higher at (3.25 ± 0.60) × 1012 K. This work provides essential experimental thermodynamic measurements to map out the QCD phase diagram and understand the properties of matter under extreme conditions.