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Hard-scattered partons ejected from high-energy proton-proton collisions undergo parton shower and hadronization, resulting in collimated collections of particles that are clustered into jets. A substructure observable that highlights the transition between the perturbative and nonperturbative regimes of jet evolution in terms of the angle between two particles is the two-point energy correlator (EEC). In this Letter, the first measurement of the EEC at RHIC is presented, using data taken from 200 GeV $p+p$collisions by the STAR experiment. The EEC is measured both for all the pairs of particles in jets and separately for pairs with like and opposite electric charges. These measurements demonstrate that the transition between perturbative and nonperturbative effects occurs within an angular region that is consistent with expectations of a universal hadronization regime that scales with jet momentum for a given initiator flavor. Additionally, a deviation from Monte Carlo predictions at small angles in the charge-selected sample could result from mechanics of hadronization not fully captured by current models. 

We report measurements of $\mathrm{\Upsilon }\left(1S\right)$, $\mathrm{\Upsilon }\left(2S\right)$and $\mathrm{\Upsilon }\left(3S\right)$production in $p+p$collisions at $\sqrt{s}=500\mathrm{GeV}$by the STAR experiment in year 2011, corresponding to an integrated luminosity ${\mathcal{L}}_{\mathrm{int}}=13{\mathrm{pb}}^{-1}$. The results provide precise cross sections, transverse momentum ( $p_{\mathrm{T}}$) and rapidity ( $y$) spectra, as well as cross section ratios for $p_{\mathrm{T}}<10\mathrm{GeV}/\mathrm{c}$and $\left|y\right|<1$. The dependence of the $\mathrm{\Upsilon }$yield on charged particle multiplicity has also been measured, offering new insights into the mechanisms of quarkonium production. The data are compared to various theoretical models: the color evaporation model (CEM) accurately describes the $\mathrm{\Upsilon }\left(1S\right)$production, while the color glass $\mathrm{condensate}+\text{nonrelativistic}$quantum chromodynamics ( $\mathrm{CGC}+\mathrm{NRQCD}$) model overestimates the data, particularly at low $p_{\mathrm{T}}$. Conversely, the color singlet model (CSM) underestimates the rapidity dependence. These discrepancies highlight the need for further development in understanding the production dynamics of heavy quarkonia in high-energy hadronic collisions. The trend in the multiplicity dependence is consistent with CGC/saturation and string percolation models or $\mathrm{\Upsilon }$production happening in multiple parton interactions modeled by 8. 

The STAR Collaboration reports precise measurements of the longitudinal double-spin asymmetry, $A_{LL}$, for dijet production with at least one jet at intermediate pseudorapidity $0.8<{\eta }_{\text{jet}}<1.8$in polarized proton-proton collisions at a center-of-mass energy of 200 GeV. This study explores partons scattered with a longitudinal momentum fraction ( $x$) from 0.01 to 0.5, which are predominantly characterized by interactions between high- $x$valence quarks and low- $x$gluons. The results are in good agreement with previous measurements at 200 GeV with improved precision and are found to be consistent with the predictions of global analyses that find the gluon polarization to be positive. In contrast, the negative gluon polarization solution from the JAM Collaboration is found to be strongly disfavored. 

Abstract 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.