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ATLAS Highlights from Quark Matter 2025

14 April 2025 | By

Collaboration,Conferences,ATLAS — Members of the ATLAS Heavy Ion team at the Quark Matter 2025 conference. (Image: A. Dimri/ATLAS Collaboration)

Last week, the ATLAS Collaboration presented new results at the Quark Matter 2025 conference, held in Frankfurt am Main (Germany). The conference gathered more than a thousand physicists from around the world to share the latest developments in high-energy heavy-ion physics. This year’s edition featured a wealth of new experimental results from the Large Hadron Collider (LHC) experiments as well as new theoretical work.

ATLAS physicists showcased a wide range of new measurements based on heavy-ion data from LHC Run 2 (2015–2018) and Run 3 (2022–ongoing). From detailed studies of the quark-gluon plasma’s collective motion to new measurements in ultra-peripheral collisions, ATLAS is providing new insight into the fascinating behaviour of heavy-ion collisions and the quark–gluon structure of nuclei. Key new ATLAS results are described below, addressing some of the biggest questions in the field.

How does the QGP flow radially?

When heavy ions collide at the LHC, they produce quark-gluon plasma (QGP) – a hot, dense state of matter that existed shortly after the Big Bang. As this primordial soup expands and cools, it exhibits unique fluid characteristics. While one of these characteristics – anisotropic flow, based on the dynamics of the original heavy-ion collision and the shape of the region where the ions overlap – has been studied for years, another – radial flow, based on the collective expansion of the QGP – has received less attention.

The ATLAS Collaboration presented the first measurements of radial flow in lead-lead collisions. Researchers performed differential measurements looking at the transverse momentum (p_T) and pseudorapidity (η) of charged particles produced in those collisions. They found radial flow to be largely independent of particle transverse momentum and centrality of the lead-lead collisions, with a long-range correlation in pseudorapidity – all key features that point to its collective nature (see Figure 1). By comparing the measurements to a hydrodynamic model, researchers also demonstrated their sensitivity to the bulk viscosity of the QGP. The bulk viscosity is a measure of how a moving fragment of QGP liquid pulls the liquid behind it. These findings establish a new, powerful tool for probing the collective dynamics and properties of the QGP.

Physics,ATLAS — Figure 1: The pT-differential radial flow, v0(p_T), measured for different momenta of reference particles (left), different pseudorapidity gap (middle) in the 0-5% most central collisions, and the shape of v0(p_T) in various centrality ranges, obtained by normalizing with p_T integrated value v0 (right). These three scaling behaviours at low p_T (< 4 GeV) are hallmarks suggesting that radial flow is a type of global, collective phenomenon. (Image: ATLAS Collaboration/CERN)

What can near-miss collisions tell us about the strong force?

Not all particles collide head on. Interesting phenomena can also occur when lead-nuclei narrowly miss each other and do not create a QGP. These ultra-peripheral collisions typically involve photons pulled from the intense electromagnetic fields surrounding each nucleus. They were used to observe light-by-light scattering in 2019 and the production of tau pairs in 2022, and are now shedding light on the strong force.

Using the lead-lead collision data collected in 2023, ATLAS researchers reported the creation of J/ψ mesons in ultra-peripheral collisions. ATLAS physicists searched for events where a photon from one nucleus creates a quark-antiquark pair, which gently scatters off several virtual gluons in the second nucleus in a special way that creates a single J/ψ meson, without breaking up the nearby nucleus. As this process involves multiple gluons interacting with each other, it allows physicists to study the nonlinear nature of Quantum Chromodynamics (QCD, the theory that describes the strong force).

The data studied used an innovative event selection strategy focusing on signals from the ATLAS Transition Radiation Tracker. Researchers measured the kinematic distributions of J/ψ mesons over the widest rapidity region covered by a single experiment (Figure 2), and found that nonlinear QCD dynamics with additional gluonic fluctuations provide a good description of the data.

These results highlight the power and versatility of the ATLAS experiment in exploring the physics of heavy-ion collisions.

How do particles lose energy in the QGP?

One of the most intriguing features of the QGP is how high-energy “jets” of quarks and gluons lose energy as they pass through and interact with it. Previous studies have found that this jet quenching depends on the opening angle of the jet and the ability of the QGP to resolve that structure.

At this year’s Quark Matter conference, ATLAS physicists presented a new measurement of jet quenching, making significant refinements over previous measurements. To achieve unprecedented resolution, they studied large-radius jets made of clusters of smaller-radius jets. They then considered two variables of jet substructure – the angular separation (dR₁₂) and momentum sharing ( √d₁₂) – evaluated with charged tracks. Figure 3 shows the energy loss-induced suppression in the jet yield as a function of dR₁₂.

They found that jets with wider angular separation are more strongly suppressed. Interestingly, there are hints that this suppression changes behavior near dR₁₂ ≈ 0.1, suggesting an interplay of competing mechanisms that govern how the QGP resolves jet substructure. These precise data will provide new constraints on theoretical models of how particles lose energy in this extreme environment.