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We report on the measurements of directed flow v1 and elliptic flow v2 for hadrons (π±, K±, KS0, p, ϕ, Λ and Ξ−) from Au+Au collisions at sNN = 3 GeV and v2 for (π±, K±, p and p‾) at 27 and 54.4 GeV with the STAR experiment. While at the two higher energy midcentral collisions the number-of-constituent-quark (NCQ) scaling holds, at 3 GeV the v2 at midrapidity is negative for all hadrons and the NCQ scaling is absent. In addition, the v1 slopes at midrapidity for almost all observed hadrons are found to be positive, implying dominant repulsive baryonic interactions. The features of negative v2 and positive v1 slope at 3 GeV can be reproduced with a baryonic mean-field in transport model calculations. These results imply that the medium in such collisions is likely characterized by baryonic interactions. © 2025 The Authors.
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Baryon quantum number is found to be conserved since baryogenesis in the early Universe. Conventionally, each fractionally charged valence quark is understood to carry one-third of a baryon number. An alternative hypothesis posits that baryon number is ...
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We report the first measurement of one- and two-point energy correlators within jets in transversely polarized proton-proton collisions at s = 200 GeV , using the STAR detector at the Relativistic Heavy Ion Collider. These observables quantify the energy-weighted angular distribution of single hadrons and hadron pairs within jets, respectively. Sizable spin-dependent asymmetries are observed for π + , π − , and π + π − pairs, revealing the onset of nonperturbative dynamics at specific angular scales. By projecting the fragmentation dynamics onto Mellin moments, these measurements provide sensitivity to the nucleon’s transversity while minimizing uncertainties from nonperturbative fragmentation functions. These results establish energy correlators as a novel and precise probe of nucleon structure and open a promising avenue for three-dimensional nucleon tomography at the future Electron-Ion Collider.
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We report a new measurement of transverse single-spin asymmetries for dijet production in collisions of polarized protons at s = 200 GeV . Correlations between the proton spin and the transverse momenta of its partons, each perpendicular to the proton momentum direction, are probed at high Q 2 ≈ 160 GeV 2 . Evidence for nonzero Sivers effects is measured for the first time in dijets from proton-proton collisions, but only when the jets are sorted by their net charge, which enhances the otherwise canceling opposite-sign u - or d -quark contributions to separate data samples. The resulting asymmetries are compared to recent theoretical calculations. Separately, the associated Sivers observable ⟨ k T ⟩ , the average parton transverse momentum, is extracted using a simple kinematics approach which further enables a determination of the individual partonic contributions to the observed asymmetries.
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Department
- Physics (165)
Resource type
- Journal Article (165)
Publication year
- Between 2000 and 2026 (165)