24 Search Results

Here, we report the first evidence of spin interference in exclusive 𝐽/𝜓 → 𝑒⁺⁢𝑒⁻ photoproduction in ultraperipheral heavy-ion collisions at STAR at $$\sqrt{𝑠_{𝑁⁢𝑁}}$$ = 200  GeV. In Au + Au collisions, a negative cos⁡(2⁢𝜙) modulation is found for 𝑝_𝑇 < 120  MeV/𝑐 with a significance of 3.2⁢𝜎, while the isobar data (Ru + Ru, Zr + Zr) show a consistent negative modulation with a significance of 1.9⁢𝜎, opposite in sign to that in 𝜌⁰ → 𝜋⁺⁢𝜋⁻ photoproduction. This establishes for the first time that the interference sign is controlled by the spin structure of the final-state daughters, resolving the ambiguity present inmore » the all-boson 𝜌⁰ channel. The compact 𝐽/𝜓 probes gluon distributions at perturbative scales, resulting in a weaker modulation and providing stringent constraints on color glass condensate calculations. These findings demonstrate that spin-dependent interference in heavy vector mesons provides a new, experimentally accessible handle on gluon structure beyond traditional cross-section measurements.« less

Using a sample of ( 2712.4 ± 14.3 ) × 10 6     ψ ( 3686 ) events collected with the BESIII detector, we perform a search for the isospin-violating decays χ c J → Λ Σ ¯   0 + c . c . ( J = 0 , 1 , 2 ) and η c → Λ Σ ¯   0 + c . c . No significant signal for χ c J or η c is observed in the Λ Σ ¯   0 invariant mass distribution. The upper limits on the branching fractions at the 90% confidencemore » level are set to be B ( χ c 0 → Λ Σ ¯   0 + c . c . ) < 1.5 × 10 − 6 , B ( χ c 1 → Λ Σ ¯   0 + c . c . ) < 1.6 × 10 − 6 , B ( χ c 2 → Λ Σ ¯   0 + c . c . ) < 1.7 × 10 − 6 , and B ( η c → Λ Σ ¯   0 + c . c . ) < 6.2 × 10 − 5 for the first time.« less

Rapidity-odd directed flow v₁ measurements are presented for $K^±$ and $$K^0_S$$ in Au + Au collisions for $$\sqrt{s_{NN}}$$ from 3.0 to 3.9 GeV with the STAR experiment. For comparison, v₁ of π^±, protons, and Λ from the same collisions are also discussed. The mid-rapidity v₁ slope dv₁/dy|_y=0 for protons and Λ is positive in these collisions. On the other hand, v₁ slope of kaons exhibits a strong dependence: negative at p_T < 0.6 GeV/c and positive at higher p_T. A similar p_T dependence is also evident for the v₁ slope of charged pions. Compared to the spectator-removed calculations in Au+Aumore » collisions at $$\sqrt{s_{NN}}$$ = 3.0–3.9 GeV, the JAM model demonstrates a pronounced shift of the v₁ slopes of mesons towards the negative direction. It suggests that the shadowing effect of the spectators plays an important role in the observed kaon anti-flow at low p_T in the high baryon density region of non-central collisions.« less

The correlation between the mean transverse momentum, [p_T], and the squared anisotropic flow, $$v^2_n$$, on an event-by-event basis has been suggested to be influenced by the initial conditions in heavy-ion collisions. We present measurements of the variances and covariance of [p_T] and $$v^2_n$$, along with their dimensionless ratio, for Au+Au collisions at various beam energies: $$\sqrt{s_{NN}}$$ = 14.6, 19.6, 27, 54.4, and 200 GeV. Our measurements reveal a distinct energy-dependent behavior in the variances and covariances. In addition, the dimensionless ratio displays a similar behavior across different beam energies. We compare our measurements with hydrodynamic models and similar measurements frommore » Pb+Pb collisions at the Large Hadron Collider (LHC). These findings provide valuable insights into the beam energy dependence of the specific shear viscosity (η/s) and initial-state effects, allowing for differentiating between different initial-state models.« less

We report measurements of charmonium sequential suppression in Ru+Ru and Zr+Zr collisions at $$\sqrt{s_{NN}}$$ =200  GeV with the STAR experiment at the Relativistic Heavy Ion Collider (RHIC). The inclusive yield ratio of 𝜓⁡(2⁢S) to J/𝜓 as a function of transverse momentum is reported, along with the centrality dependence of the double ratio, defined as the 𝜓⁡(2⁢S) to J/𝜓 ratio in heavy-ion collisions relative to that in 𝑝 +𝑝 collisions. In the 0–80% centrality class, the double ratio is found to be 0.41±0.10 (stat)±0.03 (syst)±0.02 (ref), lower than unity with a significance of 5.6 standard deviations. This provides experimental evidence that 𝜓⁡(2⁢S) is significantly moremore » suppressed than J/𝜓 in heavy-ion collisions at RHIC. This sequential suppression pattern seems to increase from peripheral to central collisions, but with no significant dependence on the transverse momentum.« less

The vacuum is now understood to have a rich and complex structure, characterized by fluctuating energy fields and a condensate of virtual quark–antiquark 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)—the fundamental theory of strong interaction. High-energy proton–proton collisions could liberate virtual quark–antiquark pairs from the vacuum that subsequently undergo confinement to form hadrons, whose propertiesmore » could serve as probes into QCD confinement and the quark condensate. Here we report evidence of spin correlations in $$Λ\bar{Λ}$$ hyperon pairs inherited from spin-correlated strange quark–antiquark 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 ± 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.« less

High-energy, heavy-ion collisions can create local domains of chirality-imbalanced quarks, reflecting the topological features of quantum chromodynamics. The chiral magnetic effect (CME) predicts an electric charge separation of quarks in such topological domains along the magnetic field ($$\vec{B}$$) generated by the passing of two high-Z nuclei. Here, we use a correlation observable Δ⁢𝛾$$^{112}$$ between charged meson pairs to detect the CME-induced charge separation and a novel event shape selection (ESS) method to mitigate the background effects related to elliptic flow (𝑣₂). The ESS method classifies events based on the emission pattern of final-state particles and determines Δ⁢𝛾$$^{112}_{ESS}$$ from the zero-flowmore » limit. We reconstruct the $$\vec{B}$$ field direction from the spectator nucleons, which minimizes backgrounds unrelated to the collective motion of the system. In this work, we report the measurements of Δ⁢𝛾$$^{112}$$ and a background indicator Δ⁢𝛾$$^{132}$$ in Au+Au collisions from the Brookhaven National Laboratory Relativistic Heavy Ion Collider (RHIC) Beam Energy Scan phase II and at the top RHIC energy. After background suppression, Δ⁢𝛾$$^{132}_{ESS}$$ aligns with zero, and Δ⁢𝛾$$^{112}_{ESS}$$ is reduced to no more than 20% of Δ⁢𝛾$$^{112}$$. We observe a finite residual charge separation with 2.5⁢𝜎, 3⁢𝜎, and 3.2⁢𝜎 significance in the 20–50% centrality range of Au + Au collisions at 11.5, 14.6, and 19.6 GeV. The results at 17.3 and 27 GeV also show positive values but with a lower significance of 1.3⁢𝜎 and 1.1⁢𝜎, respectively. The corresponding Δ⁢𝛾$$^{112}_{ESS}$$ values at 7.7, 9.2, and 200 GeV are consistent with zero within uncertainties.« less

The STAR Collaboration reports measurements of acoplanarity using semi-inclusive distributions of charged-particle jets recoiling from direct photon and 𝜋⁰ triggers, in central Au–Au and 𝑝⁢𝑝 collisions at $$\sqrt{s_{NN}}$$ = 200 GeV. Significant medium-induced acoplanarity broadening is observed for large but not small recoil jet resolution parameter, corresponding to recoil jet yield enhancement up to a factor of ≈ 20 for trigger-recoil azimuthal separation far from 𝜋. This phenomenology is indicative of the response of the quark-gluon plasma to excitation, but not the scattering of jets off of its quasiparticles. As a result, the measurements are not well described by currentmore » theoretical models which incorporate jet quenching.« less

The STAR experiment reports new, high-precision measurements of the transverse single-spin asymmetries for 𝜋^± within jets, namely the Collins asymmetries, from transversely polarized 𝑝^↑⁢𝑝 collisions at $$\sqrt{s}$$ = 510 GeV. The energy-scaled distribution of jet transverse momentum, 𝑥_T = 2⁢𝑝_T,jet/$$\sqrt{s}$$, shows a remarkable consistency for Collins asymmetries of 𝜋^± in jets between $$\sqrt{s}$$ = 200 GeV and 510 GeV. This indicates that the Collins asymmetries are nearly energy independent, with, at most, a very weak scale dependence in 𝑝^↑⁢𝑝 collisions. These results extend to high-momentum scales (𝑄² ≤ 3400 GeV²) and enable unique tests of evolution and universality in themore » transverse-momentum-dependent formalism, thus providing important constraints for the Collins fragmentation functions.« less

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 neithermore » 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.« less