The Large Hadron Collider is not just smashing atoms; it’s turning back the clock on the early universe. When heavy ions collide, they briefly melt into a quark–gluon plasma, a hot, dense soup where quarks and gluons move freely for a fleeting moment. In that microcosm, pressure gradients drive a collective expansion—the radial flow—that pushes particles outward as the fireball cools. For years, physicists have studied another kind of collective motion, the anisotropic flow, which sculpts the fireball into elliptic and triangular shapes. But radial flow has been a quieter hero, harder to pin down and understand. Now the ATLAS Collaboration at CERN has, for the first time, mapped how radial flow changes with the particles’ transverse momentum, revealing a pattern that behaves like a chorus rather than a random shuffle.
By analyzing thousands of PbPb collisions at 5.02 TeV per nucleon pair, the team measured v0(pT), the Pt differential radial flow fluctuations. The method is intricate: it looks at how the spectra in narrow Pt windows correlate with the event’s average Pt, across two separated regions in pseudorapidity. If radial flow is truly collective, the covariance should factorize into a product of v0(pT) and the total v0, and the pattern should endure across centrality classes. The results show exactly that in multiple checks, including a long-range correlation in eta, a stable shape across centralities for pT below about 2.5 GeV, and the factorization of the covariance. In other words, radial flow fluctuations behave like a global, coordinated motion of the whole fireball rather than a collection of local hiccups.
The data come from the ATLAS detector at the Large Hadron Collider, a flagship experiment at CERN. The study demonstrates that radial flow is not a niche feature but a collective property intimately linked to the bulk viscosity of the quark–gluon plasma. This matters because viscosity governs how fast the fireball can smooth out irregularities and how heat and momentum diffuse through the plasma. The measurement thus translates a messy, chaotic moment of a collision into a clean diagnostic of the plasma’s transport properties. And it does so with a kind of methodological elegance: extract a subtle, momentum-dependent signal from an ocean of particle tracks by exploiting the geometry of the collision and a simple, robust covariance relation.
What radial flow is and why it matters
Radial flow is the universal outward push of a hot, expanding medium. In the language of heavy-ion physics, it is the radial boost that tends to increase the average transverse momentum of produced particles as the fireball expands and cools. Unlike the anisotropic flow, which tells you about how the initial shape of the colliding nuclei gets converted into directional momentum, radial flow is more about the global flame that drives particles away from the center, like steam blowing off a kettle. This is where bulk viscosity enters the stage. Bulk viscosity controls how the medium resists uniform expansion or compression, effectively tuning how the pressure gradients translate into a collective outward motion. If the plasma were a perfect, frictionless fluid, the radial flow pattern would be different from what bulk viscosity allows. The study uses v0(pT) as a window into this bulk property, because the fluctuations in radial flow are most sensitive to how the whole medium responds to expansion, not just the shear that drapes anisotropic patterns on the fireball.
To see radial flow as a collective phenomenon, researchers look for three signatures that mirror the well-trodden path of anisotropic flow: long-range correlations in pseudorapidity, factorization of two-particle correlations into single-particle contributions, and a centrality-insensitive shape of the momentum dependence when you scale out the overall size of the overlapped region. Put simply, if radial flow is a genuine collective motion, the same basic pattern should repeat from the most head-on to more glancing collisions, once you account for how many particles you produced. The ATLAS results deliver on all three fronts, painting a picture of a fluid-like expansion that is global in character, not the sum of many independent pockets reacting locally.
How ATLAS peeled back the signal
The measurement used a data sample of Pb+Pb collisions at 5.02 TeV recorded by the ATLAS detector in 2015, focusing on charged tracks with 0.5 < pT < 10 GeV and pseudorapidity |η| < 2.5. The analysis hinges on a two-subevent method: particles are gathered from two η regions separated by a gap in between, which suppresses short-range, non-flow correlations that could masquerade as collective behavior. Each track is weighted to correct for reconstruction efficiency and residual fake tracks, so the final correlations aren’t biased by detector quirks. The team then constructs a fractional spectrum n(pT) within each event and examines how fluctuations in n(pT) covary with the event’s average pT, across the two subevents. The central quantity is a normalized covariance that, when the flow is collective, factorizes into a product of v0(pT) and the integrated v0. This is the crux: a robust, data-driven way to isolate a momentum-dependent radial-flow signal from a background of trivial fluctuations.
To ensure the signal wasn’t a trick of a particular momentum window, the ATLAS team repeated the measurement for three different reference pT ranges: 0.5–2 GeV, 0.5–5 GeV, and 1–5 GeV. The resulting v0(pT) curves track each other well, especially below a few GeV, and they satisfy a sum rule that ties v0(pT) to the overall v0, within a few percent. Across centralities—from most central to mid-peripheral—the v0(pT) signal remains remarkably stable in shape, even as its magnitude changes. The upshot is clear: the radial-flow fluctuations are not a collection of random fluctuations; they behave as a single, flowing entity, responsive to the overall geometry and density of the overlapping nuclei.
The three hallmarks of collectivity in radial flow
Three features emerge as the paper’s central claims about collectivity. First, the correlations show long-range behavior in pseudorapidity, indicating that the same physical mechanism links particles that were produced far apart in angle. That is a signature of a common, bulk-driven process rather than a patchwork of independent sources. The long-range nature of the signal also helps rule out lingering non-flow effects that can creep in from jet fragments or resonance decays. Second, the covariance factorizes into a product of a momentum-dependent v0(pT) and an event-wide v0, reinforcing the idea that the fluctuations are governed by a global radial expansion. Finally, the shape of v0(pT) is nearly independent of centrality for pT below about 2–3 GeV, suggesting a universal hydrodynamic response to radial expansion that is set by the properties of the QGP rather than the particular geometry of the collision. In the language of the analysts, the pT dependence is a fingerprint of a bulk, collective fluid, not a patchwork of local effects.
These hallmarks were tested in several ways. The study varied the η gap between the two subevents to check non-flow contributions; increasing the gap leaves the v0(pT) signal largely unchanged in central collisions, while peripheral collisions show a small drop at higher pT consistent with residual non-flow effects. They also compared the data with a hadronic model that contains only non-flow correlations. That model underpredicts the measured magnitudes at pT below 6 GeV, implying that collective radial flow is indeed the dominant driver of the observed correlations in the bulk of the momentum range. The conclusion is robust: radial flow fluctuations are global in nature and thus carry real information about the QGP’s collective expansion.
Bulk viscosity and what v0(pT) tells us about the QGP’s inner life
The comparison with hydrodynamic models is where the new data become truly informative. The ATLAS results are contrasted with Trento+MUSIC simulations that implement either a constant shear viscosity or a temperature-dependent bulk viscosity. What emerges is a clear sensitivity of the Pt-differential radial-flow fluctuations to the bulk viscosity, a transport coefficient that governs how the plasma responds to uniform expansion. Models that include bulk viscosity tend to align better with the observed v0(pT) patterns, especially at higher pT, and they shift the zero-crossing point where v0(pT) changes sign. This is not just a niche detail; it is a direct, quantitative probe of how viscous the QGP is when it expands and cools.
To make the comparison robust against initial-state uncertainties, researchers also look at the ratio v0(pT)/v0 as a function of pT/ pT_mean. This scaling helps separate the roles of the initial geometry from the medium’s dynamic response. The data show that the shape is nearly centrality-independent for pT up to roughly 2–2.5 GeV, while the magnitude follows the expected centrality trend. It is in this dance between magnitude and shape that the bulk viscosity leaves its imprint, offering a pathway to constrain one of the most elusive properties of QCD matter.
What this means for the future of heavy-ion physics
The discovery of a clean, Pt-differential radial-flow signal with three confirmation checks is more than a technical triumph. It provides a new, powerful handle on the bulk properties of the quark–gluon plasma that many years of flow studies could only hint at. If v0(pT) is as sensitive to bulk viscosity as the data suggest, then this observable becomes a crucial testbed for refining our hydrodynamic descriptions of QCD matter. The result helps close the loop between macroscopic fluid behavior and microscopic transport coefficients, anchoring a bridge between the fireball’s outward push and the microphysical interactions among quarks and gluons.
Beyond pure theory, the work underscores a broader message: even the most violent, high-energy events can reveal universal patterns. The radial flow signal, with its centrality-independent shape and its long-range correlations, hints at a common hydrodynamic language that transcends the messy, event-by-event complexity of the collisions. In that sense, the QGP acts like a cosmic laboratory for studying how complex systems—from plasmas to early-universe matter—respond to stress and stretch, with bulk viscosity as a central dial. The ATLAS measurement thus adds a new, elegant verse to the ongoing story of how nuclear matter behaves under extreme conditions.
All of this comes from CERN, the home of the ATLAS experiment, and the ATLAS Collaboration itself. The paper is a collective achievement, a snapshot written by a global team of scientists, with leadership shared across institutions rather than concentrated in a single lab or individual. In practice, the story behind this work is the story of the collaboration, a sprawling enterprise that stitches together dozens of universities and national labs into a single instrument capable of parsing the faint murmurs of a tiny, fleeting universe in a detector the size of a cathedral. The lead voice, in this sense, is the collaboration itself, with hundreds of co-authors contributing to data, simulations, and interpretation. The result is a reminder that in modern physics, the most precise windows onto nature often come from collective effort at an unprecedented scale.
From quark soup to universal insights
So what should you take away from this latest piece of the heavy-ion puzzle? First, radial flow is not a peripheral curiosity; it is a central piece of how a quark–gluon plasma expands, and its fluctuations carry information about the medium’s bulk viscosity. Second, the signal is robust against several potential confounds, thanks to clever analysis choices like the two-subevent method and cross-checks with non-flow models. Third, the velocity fluctuations as a function of Pt reveal a universal aspect of the hydrodynamic response, at least up to a few GeV, that echoes the broader theme in heavy-ion physics: despite the chaotic beginnings of each collision, the system behaves like a fluid with predictable, testable properties when you look in the right way. Finally, the work demonstrates the value of Pt-differential observables as new levers for probing the QGP’s transport coefficients, moving us closer to a quantitative map of how QCD matter flows under extreme conditions.
In the end, the radial whispers of the quark-gluon sea carry a message about universality and plasticity under pressure. They remind us that even the shortest-lived flames can illuminate how complex matter organizes itself. The ATLAS measurement of v0(pT) closes a gap in our understanding and opens a path to tighter constraints on bulk viscosity, a property that has long haunted the theoretical limits of QCD hydrodynamics. The result is a testament to how far high-energy physics has come: from cataloging particles to reading the subtle, fluid-like heartbeat of the Universe’s most extreme matter, all through the patient work of a global scientific family operating at the edge of human knowledge.
