The Quark-Gluon Plasma (QGP) is a phase of matter at extremely high temperature that filled the very early universe, and which is recreated and studied today using heavy-ion collisions at RHIC and the LHC. Using data from the first RHIC runs, the STAR experiment pioneered the application of “jets,” the remnants of energetic quarks and gluons which plough through the QGP and reveal how it responds to excitation (“jet quenching”) – an essential tool for understanding its structure and dynamics. Jet quenching measurements using the back-to-back correlation of a jet with an energetic photon (“gamma+jet”) were proposed long ago by theorists as a key channel for jet quenching studies, since the photon does not interact with the QGP and provides a gauge of the energy of the jet before it interacts with the QGP. The importance of measurements of gamma+jet correlations in heavy-ion collisions was highlighted in multiple NSAC Long Range Plans. In two recent papers, the STAR collaboration reports the first measurements at RHIC of jet quenching using direct photon+jet (gamma+jet) and π⁰+jet correlations in p+p and central Au+Au collisions at $\sqrt{s_{NN}} = 200$ GeV.

In Figure-1, the event display shows signals in the STAR detector of Au+Au collisions in which a photon measured in the Barrel Electromagnetic Calorimeter (red) is generated back-to-back with a spray of particles comprising a jet (blue), measured using tracks from the both Time Projection Chamber and the Calorimeter. Also shown in grey are many tracks from the large and complex underlying background in the Au+Au collision. Sophisticated analysis techniques, which were developed by STAR in previous publications, were required to distinguish the gamma+jet signal from this background.

These new STAR papers measure the frequency of recoil jets correlated with a "trigger" photon or neutral pion, as a function of both the jet momentum and the aperture within which jet particles are collected, with radius R. The comparison of these distributions in p+p collisions, in which the QGP is not formed, and in Au+Au collisions in which the QGP is created, provide important new insight into the excitation of the QGP and its response. Figure 2 shows the measured ratio of recoil jet yield vs jet momentum for small and large aperture (R=0.2 and 0.5 radians), separately for pp and Au+Au collisions, for both π⁰+jet (upper) and gamma+jet (lower) correlations.

The ratio is seen to be smaller in pp collisions, indicating that the recoil jets are substantially broadened due to their interaction in the QGP. The comparison of π⁰+jet and gamma+jet correlations in the same measurement is of especial importance, since the π⁰ and photon triggers are generated by very different physical mechanisms, and their comparison to theoretical calculations will provide new and unique insight into the dynamics of the QGP-jet interaction and the response of the QGP to excitation. Current theoretical models incorporating jet quenching do not describe these new data well, challenging our current description of QGP dynamics. These findings were published in Physical Review Letters (Phys. Rev. Lett. 134, 232301 (2025)) and Physical Review C(Phys. Rev. C 111, 064907 (2025)) as a joint submission to the American Physical Society.

Posted July 21, 2025

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One of the ultimate goals of heavy-ion collision experiments is to map the phase diagram of nuclear matter, the so-called QCD phase diagram, with respect to temperature (T) and baryon chemical potential ($µ_{B}$). Higher-order cumulants of net-baryon multiplicity distributions are expected to be sensitive to the QCD phase structure. The STAR experiment observed that the $6^{th}$-order fluctuation of the net-proton multiplicity distribution becomes progressively negative systematically from peripheral to central Au+Au 200 GeV collisions, which hints at a smooth crossover transition at $µ_{B}$=25 MeV.

Recently, the STAR experiment published the net-proton higher-order cumulants from p+p 200 GeV collisions. Experimental results from the LHC, such as collectivity and strangeness enhancement, suggest that the Quark Gluon Plasma (QGP) could be formed in high-multiplicity p+p collisions, although it does not necessarily indicate that thermalized QCD drops of matter have been created. This can be tested by higher-order correlations in p+p collisions at top RHIC energy and LHC energies. The figure below (Phys.Lett.B 857 (2024) 138966 ) shows the charged-particle multiplicity dependence of the cumulant ratios, (a) $C_4$/$C_2$, (b) $C_5$/$C_1$, and (c) $C_6$/$C_2$ of net-proton multiplicity distributions. Results from p+p 200 GeV minimum bias collisions are close to those from peripheral Au+Au 200 GeV collisions. On the other hand, the ratios measured at each charged-particle multiplicity bin in p+p collisions decrease with increasing the multiplicity more rapidly than Au+Au collisions, and $C_5$/$C_1$ and $C_6$/$C_2$ values reach zero at the highest multiplicity bin. The results favor the negative values of the ratios at higher multiplicity events. Although part of the decrease as a function of the charged particle multiplicity in a given event is due to baryon conservation, the negative ratios in the context of lattice QCD calculations imply that even in the small 𝑝+𝑝 system, thermalized QCD matter may be created in the highest-multiplicity collisions. Further, the steeper slope indicates that thermalized QCD matter is created more efficiently in high multiplicity 𝑝+𝑝 collisions than that in heavy-ion collisions. Systematic measurements of the cumulant ratios from higher collision energies and in larger colliding systems will provide important information on the underlying dynamics of thermalization in high-energy collisions.

Figure: Net-proton cumulant ratios, (a) $𝐶_{4}$∕$𝐶_{2}$, (b) $𝐶_{5}$∕$𝐶_{1}$, and (c) $𝐶_{6}$∕$𝐶_{2}$ as a function of charged-particle multiplicity for 𝑝+𝑝 collisions and Au+Au collisions at 200GeV. Cyan markers represent event averages from the 𝑝+𝑝 collisions. Results from Au+Au collisions are shown as triangles. Red and grey bands show the systematic uncertainties for 𝑝+𝑝 collisions and Au+Au collisions, respectively. The Skellam baselines are shown in dotted lines. The purple bands show corresponding susceptibility ratios of baryon number from lattice QCD calculations.

Posted Oct 22, 2024

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Recently, we measured exclusive J/ψ, ψ(2s), and di-electron pair photoproduction in Au+Au ultra-peripheral collisions at RHIC using the STAR detector. For J/ψ production, both coherent (nucleus remains intact) and incoherent (nucleus breaks up) processes were observed. For the first time, it was found that both coherent and incoherent J/ψ production are suppressed in gold nuclei compared to a free proton, with the suppression being even stronger for the incoherent process. Given the RHIC kinematics, gluon saturation is not expected to play a significant role, so the observed large nuclear suppression suggests additional effects, such as nuclear shadowing. Indeed, the Leading Twist Approximation nuclear shadowing model provides a better fit to the data, particularly for coherent J/ψ production. This research is focused on the intermediate energy range in heavy-ion UPCs, which is sensitive to the transition between regions dominated by dense gluons and those by valence quarks.

Additionally, we have made the first cross-section measurement of ψ(2s) photoproduction at RHIC and found that the ratio between ψ(2S) and J/ψ is consistent with free proton data, indicating that the nuclear suppression does not significantly differ between these two vector mesons. Finally, we reported di-electron photoproduction via the Breit-Wheeler process, extending to high invariant mass regions up to 6 GeV, which supports the nuclear breakup model involving forward neutron emission and associated photon fluxes. In summary, these UPC measurements of vector mesons and dileptons provide valuable insights into the inner structure of nucleons and nuclei, helping to pave the way for the upcoming electron-ion collider.

Posted Oct 30, 2024

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The STAR Collaboration recently published "Imaging Shapes of Atomic Nuclei in High-Energy Nuclear Collisions" in Nature. This work has been selected in Nature News & Views, highlighted in an interview for Nature Podcast, and has been selected in other Nature NEWS.

Atomic nuclei, composed of protons and neutrons, are self-organized, many-body quantum systems bound by strong-nuclear forces within femtometer-scale space. Revealing the shape of nuclei has been a scientific pursuit for over a century, beginning with Rutherford's scattering experiments. Numerous models, including the Fermi gas model, Liquid drop model, Shell model, and Collective model, have been developed to study nuclear structure, with two of these models earning Nobel Prizes. Currently, our understanding of nuclear shape largely comes from low-energy spectroscopic and scattering experiments. Investigating nuclear shape across the nuclear chart has been an important area of research and is crucial for topics such as nucleosynthesis, nuclear fission, and neutrinoless double beta decay ($0 \nu \beta \beta$).

A pertinent question arises: How do shapes observed in high-energy colliders compare to those derived from low-energy experiments? High-energy nuclear collisions, an utterly destructive process, have a much shorter timescale than nuclear rotational degrees of freedom, allowing access to many-body nucleon distribution within each nucleus. Head-on (near-zero impact parameter) collisions of prolate deformed nuclei present two extreme configurations, body-body (top) and tip-tip (bottom) collisions. The collision of two Lorentz-contracted pancake-like shapes of nuclei at high energy can generate a 3D profile of the initially produced quark-gluon plasma. The shape and size of the initial overlap region, as well as those of the final fireball after hydrodynamic expansion, differ significantly between these two collision configurations, as shown in the figure.

In this article, the STAR Collaboration developed a new method, "collective flow assisted nuclear shape imaging" method, which images the nuclear global shape by colliding them at ultrarelativistic speeds and analyzing 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 onto the particle momentum distribution observed in detectors. By taking ratios of the nearly equal mass number $^{238}$U+$^{238}$U and $^{197}$Au+$^{197}$Au collisions, final state effects are almost completely canceled, leaving model uncertainties primarily from initial conditions. Comparisons between data and IP-Glasma+MUSIC model delineate preferred $\beta_{2 \mathrm{U}}$ ranges, yielding $\beta_{2 \mathrm{U}}=0.297 \pm 0.015$ and $\gamma_{\mathrm{U}}=8.5^{\circ} \pm 4.8^{\circ}$ (mean and one standard deviation). Corresponding constraints from Trajectum are $\beta_{2{\mathrm{U}}}=0.275 \pm 0.017$ and $\gamma_{\mathrm{U}}=15.5^{\circ} \pm 7.8^{\circ}$. Combining constraints from both models yields $\beta_{2 \mathrm{U}}=0.286 \pm 0.025$ and $\gamma_{\mathrm{U}}=8.7^{\circ} \pm 4.5^{\circ}$. These values are broadly consistent with previous extractions based on low energy data.

Our approach offers a novel method for imaging nuclear shapes, enhances understanding of the initial conditions in high-energy collisions, and addresses the critical issue of nuclear structure evolution across energy scales. Future research could leverage colliders to conduct experiments with selected isobaric or isobar-like pairs, further enabling interdisciplinary research in nuclear physics.

Posted Nov. 6, 2024

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