Tracking the baryon number with nuclear collisions
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73 NUCLEAR PHYSICS AND RADIATION PHYSICSβ
Engineering topics
Guryn, W.
Publications and source records attributed to Guryn, W..
Measuring spin correlation between quarks during QCD confinement
The vacuum is now understood to have a rich and complex structure, characterized by fluctuating energy fields and a condensate of virtual quarkβantiquark pairs. The spontaneous breaking of the approximate chiral symmetry, signalled by the nonvanishing quark condensate $\langle$$q\bar{q}$$\rangle$, is dynamically generated through topologically nontrivial gauge configurations such as instantons. The precise mechanism linking the chiral symmetry breaking to the mass generation associated with quark confinement remains a profound open question in quantum chromodynamics (QCD)βthe fundamental theory of strong interaction. High-energy protonβproton collisions could liberate virtual quarkβantiquark pairs from the vacuum that subsequently undergo confinement to form hadrons, whose properties could serve as probes into QCD confinement and the quark condensate. Here we report evidence of spin correlations in $Ξ\bar{Ξ}$ hyperon pairs inherited from spin-correlated strange quarkβantiquark virtual pairs. Measurements by the STAR experiment at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory reveal a relative polarization signal of (18 Β± 4)% that links the virtual spin-correlated quark pairs from the QCD vacuum to their final-state hadron counterparts. Crucially, this correlation vanishes when the hyperon pairs are widely separated in angle, consistent with the decoherence of the quantum system. Our findings provide a new experimental model for exploring the dynamics and interplay of quark confinement and entanglement.
73 NUCLEAR PHYSICS AND RADIATION PHYSICSβ
Temperature measurement of Quark-Gluon plasma at different stages
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 β and ΞΌ + ΞΌ β ) 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+eβ production at the Relativistic Heavy Ion Collider (RHIC). The average temperature from the low-mass region (in-medium Ο 0 vector-meson dominant) is (2.01 Β± 0.23) Γ 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 Ο0 mass, QGP dominant) is significantly higher at (3.25 Β± 0.60) Γ 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.
73 NUCLEAR PHYSICS AND RADIATION PHYSICSβ
Hyperon global polarization in isobar Ru+Ru and Zr+Zr collisions at $\sqrt{s_{NN}}$ = 200Ge V
The polarization of Ξ, $\overline{Ξ}$, $Ξ$ β , and $\overline{Ξ}$ + hyperons along the angular momentum of the system has been measured in isobar collisions of Ru+Ru and Zr+Zr at $\sqrt{s_{NN}}$ = 200 GeV with the STAR detector at RHIC. The polarization dependence on collision centrality exhibits an increasing trend in more peripheral collisions. Ξ and $\overline{Ξ}$ polarization dependence on the transverse momentum and pseudorapidity have been investigated, but no significant dependence was observed. The polarizations of Ξ and $\overline{Ξ}$ are found to be consistent with each other, indicating little contribution of the spin-magnetic coupling to the measured polarization. Comparison to previously measured polarization in Au+Au collisions show no obvious system size dependence. The results are qualitatively consistent with hydrodynamic calculations including contributions from shear-induced polarization and thermal vorticity. For the first time in heavy-ion collisions, the dependence of the global polarization on the hyperonβs emission azimuthal angle relative to the second-order event plane has been measured, indicating stronger polarization for the in-plane emitted hyperons at the level of 2.4 Ο significance in 20β50 % centrality. The $Ξ$ hyperon polarization measurements via polarization transfer analysis yield finite positive values with 2.9 Ο significance in 20β50 % centrality, slightly larger compared to the inclusive Ξ polarization.
73 NUCLEAR PHYSICS AND RADIATION PHYSICSβ
Semi-inclusive direct photon + jet and π 0 + jet correlations measured in π + π and central Au + Au collisions at $\sqrt{s_{NN}}$ = 200GeV
The STAR experiment at RHIC reports new measurements of jet quenching based on the semi- inclusive distribution of charged-particle jets recoiling from direct photon (Ξ³ dir ) and neutral pion (Ο 0 ) triggers in pp and central Au + Au collisions at $\sqrt{s_{NN}}$ = 200 GeV, for triggers in the range 9 < $E$$^{trig}_{T}$ < 20 GeV. The datasets have integrated luminosities of 3.9nb β1 for Au + Au and 23pb β1 for ππ collisions. Jets are reconstructed using the anti-π π algorithm with resolution parameters π = 0.2 and 0.5. The large uncorrelated jet background in central Au + Au collisions is corrected using a mixed-event approach, which enables precise charged-particle jet measurements at low transverse momentum π$^{ch}_{π,jet}$ and large π . Recoil-jet distributions are reported in the range π$^{ch}_{π,jet}$ < 25 GeV/π. Comparison of the distributions measured in ππ and Au + Au collisions reveals strong medium-induced jet yield suppression for π = 0.2 with markedly less suppression for π = 0.5. Comparison is also made to theoretical models incorporating jet quenching. Furthermore, these data provide new insight into the mechanisms underlying jet quenching and the angular dependence of medium-induced jet-energy transport and provide new constraints on modeling such effects.
73 NUCLEAR PHYSICS AND RADIATION PHYSICSβ
Production of protons and light nuclei in Au +Au collisions at $\sqrt{s_{NN}}$ = 3 GeV with the STAR detector
We report the systematic measurement of protons and light nuclei production in Au +Au collisions at $\sqrt{s_{NN}}$ = 3 GeV by the STAR experiment at the Relativistic Heavy Ion Collider (RHIC). The transverse momentum (π π ) spectra of protons (π) , deuterons (π) , tritons (π‘) , 3 He , and 4 He have been measured from midrapidity to target rapidity for different collision centralities. We present the rapidity and centrality dependence of particle yields (πβ’π/πβ’π¦), average transverse momentum (β¨π π β©), yield ratios (π/π, π‘/π, 3 He/π, 4 He/π), as well as the coalescence parameters (π΅ 2 ,π΅ 3 ). The 4β’π yields for various particles are determined by utilizing the measured rapidity distributions, πβ’π/πβ’π¦. Furthermore, we present the energy, centrality, and rapidity dependence of the compound yield ratios (π π Γπ π‘ /π$^{2}_{d}$) and compare them with various model calculations. Furthermore, the physics implications of these results on the production mechanism of light nuclei and the QCD phase structure are discussed.
73 NUCLEAR PHYSICS AND RADIATION PHYSICSβ
Correlations of event activity with hard and soft processes in p + Au collisions at $\sqrt{s_{NN}}$ = 200 GeV at the RHIC STAR experiment
With the STAR experiment at the BNL Relativistic Heavy Ion Collider, we characterize $\sqrt{s_{NN}}$ = 200GeV p + Au collisions by event activity (EA) measured within the pseudorapidity range Ξ· Ο΅[β5,β3.4] in the Au-going direction and report correlations between this EA and hard- and soft-scale particle production at midrapidity (Ξ· Ο΅[β1,1]). At the soft scale, charged particle production in low-EA p + Au collisions is comparable to that in p + p collisions and increases monotonically with increasing EA. At the hard scale, we report measurements of high transverse momentum (p T ) jets in events of different EAs. In contrast with the soft particle production, high-p T particle production and EA are found to be inversely related. Furthermore, to investigate whether this is a signal of jet quenching in high-EA events, we also report ratios of p T imbalance and azimuthal separation of dijets in high- and low-EA events. Within our measurement precision, no significant differences are observed, disfavoring the presence of jet quenching in the highest 30% EA p + Au collisions at $\sqrt{s_{NN}}$ = 200GeV.
73 NUCLEAR PHYSICS AND RADIATION PHYSICSβ
Measurements of charged-particle multiplicity dependence of higher-order net-proton cumulants in p + p collisions at $\sqrt{s}$ = 200 GeV from STAR at RHIC
We report on the charged-particle multiplicity dependence of net-proton cumulant ratios up to sixth order from $\sqrt{s}$ = 200 GeV p+p collisions at the Relativistic Heavy Ion Collider (RHIC). The measured ratios C 4 /C 2 , C 5 /C 1 , and C 6 /C 2 decrease with increased charged-particle multiplicity and rapidity acceptance. Neither the Skellam baselines nor PYTHIA8 calculations account for the observed multiplicity dependence. In addition, the ratios C 5 /C 1 and C 6 /C 2 approach negative values in the highest-multiplicity events, which implies that thermalized QCD matter may be formed in p+p collisions.
73 NUCLEAR PHYSICS AND RADIATION PHYSICSβ
Observation of the antimatter hypernucleus $^4_{\bar{\Lambda}}\overline{\hbox{H}}$
Matter-antimatter asymmetry is a research topic of fundamental interest, as it is the basis for the existence of the matter world, which survived annihilation with antimatter in the early Universe. High energy nuclear collisions create conditions similar to the Universe microseconds after the Big Bang, with comparable amounts of matter and antimatter. Much of the antimatter created escapes the rapidly expanding fireball without annihilation, making such collisions an effective experimental tool to create heavy antimatter nuclear objects and study their properties. In this paper, we report the first observation of the antimatter hypernucleus $^4_{\bar{\Lambda}}\overline{\hbox{H}}$, composed of an $\bar{\Lambda}$, 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. In total, 15.6 candidate $^4_{\bar{\Lambda}}\overline{\hbox{H}}$ antimatter hypernuclei are obtained with an estimated background count of 6.4. Lifetimes of the antihypernuclei $^3_{\bar{\Lambda}}\overline{\hbox{H}}$ and $^4_{\bar{\Lambda}}\overline{\hbox{H}}$ are measured and compared with lifetimes of their corresponding hypernuclei, testing the symmetry between matter and antimatter. Various production yield ratios among (anti)hypernuclei and (anti)nuclei are also measured and compared with theoretical model predictions, shedding light on their production mechanism.
11 NUCLEAR FUEL CYCLE AND FUEL MATERIALSβ
Exclusive J / Ο , Ο ( 2 s ) , and e + e - pair production in Au + Au ultraperipheral collisions at the BNL Relativistic Heavy Ion Collider
Measurements of exclusive J/Ο, Ο(2β’s), and electron-positron (e + β’e - ) pair photoproduction in Au+Au ultraperipheral collisions are reported by the STAR experiment at $\sqrt{s_{NN}}$ = 200GeV. Here, we report several first measurements at the BNL Relativistic Heavy Ion Collider, which are (i) J/Ο photoproduction with large momentum transfer up to 2.2β’(GeV/c) 2 , (ii) coherent J/Ο photoproduction associated with neutron emissions from nuclear breakup, (iii) the rapidity dependence of incoherent J/Ο photoproduction, (iv) the Οβ‘(2β’s) photoproduction cross section at midrapidity, and (v) e + β’e - pair photoproduction up to high invariant mass of 6GeV/c 2 . For measurement (ii), the coherent J/Ο total cross section of Ξ³ + Au β J/Ο + Au as a function of the center-of-mass energy W Ξ³N has been obtained without photon energy ambiguities. The data are quantitatively compared with the Monte Carlo models STARlight, Sartre, BeAGLE, and theoretical calculations of gluon saturation with color glass condensate, nuclear shadowing with leading twist approximation, quantum electrodynamics, and the next-to-leading-order perturbative QCD. At the photon-nucleon center-of-mass energy of 25.0 GeV, the coherent and incoherent J/Ο cross sections of Au nuclei are found to be 71% Β± 10% and 36% Β± 7%, respectively, of that of free protons. These data provide an important experimental constraint for nuclear parton distribution functions and a unique opportunity to advance the understanding of the nuclear modification effect at the top RHIC energy.
73 NUCLEAR PHYSICS AND RADIATION PHYSICSβ
Observation of Strong Nuclear Suppression in Exclusive J/Ο Photoproduction in Au + Au Ultraperipheral Collisions at RHIC
We report a measurement of exclusive J/Ο and Οβ‘(2β’s) photoproduction in Au+Au ultraperipheral collisions at βs NN = 200 GeV using the STAR detector. For the first time, (i) the Οβ‘(2β’s) photoproduction in midrapidity at the Relativistic Heavy-Ion Collider has been experimentally measured; (ii) nuclear suppression factors are measured for both the coherent and incoherent J/Ο production. At average photon-nucleon center-of-mass energy of 25.0 GeV, the coherent and incoherent J/Ο cross sections of Au nuclei are found to be 71 Β±10% and 36 Β±7%, respectively, of that of free protons. The stronger suppression observed in the incoherent production provides a new experimental handle to study the initial-state parton density in heavy nuclei. As a result, data are compared with theoretical models quantitatively.
73 NUCLEAR PHYSICS AND RADIATION PHYSICSβ