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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.

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.

A key requirement for the growth of the wide bandgap nitrides, GaN and InGaN, is providing a sufficient supply of atomic nitrogen from the vapor phase during growth. In order to prevent a high concentration of nitrogen vacancies, especially for GaInN, one strategy is to use a precursor with a low pyrolysis temperature, thus yielding a high concentration of atomic nitrogen in the vapor phase. 1,1 dimethylhydrazine (DMHy), with an appropriate vapor pressure of 157 Torr at 25°C and a low pyrolysis temperature (T50∼420°C), is a promising candidate to replace NH3, the most common nitrogen source. The pyrolysis studies of DMHy suggest the rate limiting step for decomposition is a heterogeneous, unimolecular process that is independent of both input concentration and ambient. Radical reactions are also involved as indicated by the products. In He they include CH4, NH3, N2, H2, HCN, C2H6, CH3NCH2, and (CH3)2NH. Experiments on the copyrolysis of DMHy and trimethylgallium show that an adduct is formed at room temperature followed by CH4 elimination, presumably forming (CH3)2GaNHN(CH3)2. At higher temperatures, the products indicate that the covalent Ga-N bond doesn’t dissociate during pyrolysis.

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 We report on the measurement of the Central Exclusive Production of charged particle pairs h + h − ( h = π, K, p ) with the STAR detector at RHIC in proton-proton collisions at s = 200 GeV. The charged particle pairs produced in the reaction pp → p ′ + h + h − + p ′ are reconstructed from the tracks in the central detector and identified using the specific energy loss and the time of flight method, while the forward-scattered protons are measured in the Roman Pot system. Exclusivity of the event is guaranteed by requiring the transverse momentum balance of all four final-state particles. Differential cross sections are measured as functions of observables related to the central hadronic final state and to the forward-scattered protons. They are measured in a fiducial region corresponding to the acceptance of the STAR detector and determined by the central particles’ transverse momenta and pseudorapidities as well as by the forward-scattered protons’ momenta. This fiducial region roughly corresponds to the square of the four-momentum transfers at the proton vertices in the range 0 . 04 GeV 2 < −t 1 , −t 2 < 0 . 2 GeV 2 , invariant masses of the charged particle pairs up to a few GeV and pseudorapidities of the centrally-produced hadrons in the range |η| < 0 . 7. The measured cross sections are compared to phenomenological predictions based on the Double Pomeron Exchange (DPE) model. Structures observed in the mass spectra of π + π − and K + K − pairs are consistent with the DPE model, while angular distributions of pions suggest a dominant spin-0 contribution to π + π − production. For π + π − production, the fiducial cross section is extrapolated to the Lorentz-invariant region, which allows decomposition of the invariant mass spectrum into continuum and resonant contributions. The extrapolated cross section is well described by the continuum production and at least three resonances, the f 0 (980), f 2 (1270) and f 0 (1500), with a possible small contribution from the f 0 (1370). Fits to the extrapolated differential cross section as a function of t 1 and t 2 enable extraction of the exponential slope parameters in several bins of the invariant mass of π + π − pairs. These parameters are sensitive to the size of the interaction region.

A bstract We report a new measurement of the production of electrons from open heavy-flavor hadron decays (HFEs) at mid-rapidity (| y | < 0.7) in Au+Au collisions at s NN = 200 GeV. Invariant yields of HFEs are measured for the transverse momentum range of 3 . 5 < p T < 9 GeV/ c in various configurations of the collision geometry. The HFE yields in head-on Au+Au collisions are suppressed by approximately a factor of 2 compared to that in p + p collisions scaled by the average number of binary collisions, indicating strong interactions between heavy quarks and the hot and dense medium created in heavy-ion collisions. Comparison of these results with models provides additional tests of theoretical calculations of heavy quark energy loss in the quark-gluon plasma.

Transmission electron microscopy (TEM) and atomic force microscopy (AFM) studies have been performed on organometallic vapor phase epitaxial GaInP heterostructures grown on (001) GaAs singular and vicinal substrates to investigate nitrogen doping effect on the ordering and domain structures. TEM results show that well-defined order–disorder–order heterostructures are formed when nitrogen doping level is high. This indicates that nitrogen hinders the occurrence of ordering. For the singular samples, ordered domain structures are found to be dependent on the nitrogen doping level of the underlying layer, on which they are grown. The doping dependence of ordered structures and the formation of anti-phase boundaries are described based on surface undulations (i.e., hillocks) and step configuration.

We report a new measurement of the production of electrons from open heavy-flavor hadron decays (HFEs) at mid-rapidity (| y | < 0.7) in Au+Au collisions at$$ \\sqrt{s_{\\textrm{NN}}} $$s NN = 200 GeV. Invariant yields of HFEs are measured for the transverse momentum range of 3 . 5 < p T < 9 GeV/ c in various configurations of the collision geometry. The HFE yields in head-on Au+Au collisions are suppressed by approximately a factor of 2 compared to that in p + p collisions scaled by the average number of binary collisions, indicating strong interactions between heavy quarks and the hot and dense medium created in heavy-ion collisions. Comparison of these results with models provides additional tests of theoretical calculations of heavy quark energy loss in the quark-gluon plasma.

We report a new measurement of the production of electrons from open heavy-flavor hadron decays (HFEs) at mid-rapidity (|y| < 0.7) in Au+Au collisions at $ \\sqrt{s_{\\textrm{NN}}} $ = 200 GeV. Invariant yields of HFEs are measured for the transverse momentum range of 3.5 < pT < 9 GeV/c in various configurations of the collision geometry. The HFE yields in head-on Au+Au collisions are suppressed by approximately a factor of 2 compared to that in p + p collisions scaled by the average number of binary collisions, indicating strong interactions between heavy quarks and the hot and dense medium created in heavy-ion collisions. Comparison of these results with models provides additional tests of theoretical calculations of heavy quark energy loss in the quark-gluon plasma.

Gallium indium phosphide (GaxIn1−xP) epitaxial layers were grown on GaAs substrates by chemical beam epitaxy (CBE) without thermally precracking the group V precursor. Trisdimethylaminophosphine (TDMAP), triisopropylgallium (TIPGa), and ethyldimethylindium (EDMIn) were used as the phosphorus, gallium and indium sources, respectively. GaxIn1−xP was grown without group V precracking for substrate temperatures in the range of 400–520 °C. Above 500 °C, the epilayers had a hazy appearance presumably due to being phosphorus deficit. A strong solid composition dependence on substrate temperature was observed. The samples were In-rich at low growth temperatures and Ga-rich at high growth temperatures. It was possible to grow the GaxIn1−xP epilayers over a large composition range with good morphology and strong photoluminescence. Values of full width at half maximum were as low as 45 meV at 14 K photoluminescence measurements.