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Measurement of the angle between jet axes in Pb$-$Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV

This paper presents the first measurement of the angle between jet axes in central Pb-Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV, revealing a distribution narrowing that suggests quark-initiated jets survive the quark-gluon plasma more readily than gluon-initiated ones and favoring incoherent energy loss mechanisms over simple intra-jet $p_{\rm T}$ broadening.

Original authors: ALICE Collaboration

Published 2026-09-25

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CC BY 4.0

Original authors: ALICE Collaboration

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

In the first fleeting moments after the Big Bang, the universe was not made of atoms, but of a seething, super-hot soup of fundamental particles called quarks and gluons. This state of matter, known as the quark-gluon plasma, is so energetic that the particles within it move freely rather than being locked together inside protons and neutrons. Today, physicists recreate this primordial state for a fraction of a second by smashing heavy atomic nuclei together at nearly the speed of light. Inside the resulting fireball, the quark-gluon plasma acts as a dense medium that interacts with any particle passing through it. To study this invisible, short-lived substance, scientists use high-energy particles as probes. When two particles inside the colliding nuclei crash into each other, they can scatter apart with tremendous force, creating sprays of new particles called jets. As these jets travel through the plasma, they lose energy and change shape, leaving behind clues about the properties of the medium they traversed.

A researcher using the ALICE detector at the Large Hadron Collider has now taken a fresh look at how these jets behave in the heart of a heavy-ion collision. They focused on measuring the angle between different internal directions within a single jet. Imagine a jet not as a single beam, but as a complex structure with a main path and several branching offshoots. The researcher defined three different ways to locate the "center" or axis of this structure: one that accounts for every particle in the spray, one that ignores the soft, wispy edges, and one that follows only the most energetic core. By calculating the angle between these different axes, they could determine how much the jet's internal structure had been disturbed by the plasma. The study analyzed data from lead-lead collisions at an energy of 5.02 TeV, specifically looking at the most violent, central collisions where the plasma is densest.

The researcher found that in these heavy-ion collisions, the different axes of the jet tend to line up more closely with each other than they do in ordinary proton-proton collisions. In simpler terms, the jet appears narrower and more focused when it emerges from the plasma. This observation suggests that the plasma is acting as a filter. Because jets created by gluons (a type of force-carrying particle) interact more strongly with the plasma than those created by quarks, the gluon-driven jets are more likely to lose so much energy that they disappear or become too faint to detect. The jets that survive the journey are therefore more likely to be the quark-driven ones, which have a naturally tighter structure. This selection effect explains why the surviving jets look more aligned and narrower than expected.

The study also tested specific theories about how the plasma disrupts the internal structure of these jets. One idea was that the plasma causes the particles inside the jet to scatter sideways, effectively widening the jet's internal angle. The new measurements argue against this specific mechanism. The data does not support the simple model where the plasma acts like a fog that blurs the jet's internal lines. Instead, the results favor a scenario where the plasma can distinguish between the different parts of the jet almost immediately after they split apart. This implies that the plasma is capable of resolving the fine details of the jet's structure very quickly, causing the energy loss to happen in a way that preserves the jet's narrow core while stripping away the outer layers.

By comparing the results from collisions of different sizes and with different levels of background noise, the researcher confirmed that their findings are robust. They observed that the narrowing effect is consistent across various energy ranges and jet sizes. The data aligns well with complex computer simulations that account for the different ways quarks and gluons lose energy, but it clashes with simpler models that assume a uniform broadening effect. This work provides a clearer picture of how the quark-gluon plasma interacts with high-energy particles, revealing that the medium is not just a passive obstacle but an active filter that shapes the very structure of the particles passing through it. The findings help refine our understanding of the fundamental forces that govern the behavior of matter under the most extreme conditions in the universe.

Technical Summary: Measurement of the Angle Between Jet Axes in Pb–Pb Collisions at $\sqrt{s_{NN}} = 5.02$ TeV

Problem and Motivation
This paper addresses the challenge of characterizing the microscopic structure of the Quark-Gluon Plasma (QGP) created in ultrarelativistic heavy-ion collisions. While jets of energetic hadrons serve as auto-generated probes of the QGP, their modification as they traverse the medium is complex. A key open question is how the QGP resolves the angular structure of parton showers and how different mechanisms of jet energy loss—such as incoherent multiple scattering versus coherent radiation—manifest in observables. Specifically, the paper investigates the angular separation between different definitions of a jet's axis ( $\Delta R_{\text{axis}}$ ) to probe the medium's resolution length ( $L_{\text{res}}$ ) and the interplay between soft and hard radiation within the jet.

Methodology
The analysis utilizes data from Pb–Pb collisions at a center-of-mass energy per nucleon pair of $\sqrt{s_{NN}} = 5.02$ TeV, collected by the ALICE detector in 2018. The study focuses on the 0–10% most-central events. Charged-particle jets are reconstructed using the anti- $k_T$ algorithm with resolution parameters $R=0.2$ and $R=0.4$ , covering transverse momentum intervals of $40 < p_T^{\text{ch jet}} < 140$ GeV/ $c$ and $80 < p_T^{\text{ch jet}} < 140$ GeV/ $c$ .

The core observable is $\Delta R_{\text{axis}}$ , defined as the angular distance in the rapidity-azimuth ( $y-\phi$ ) plane between two distinct jet axes:

Standard Axis: The 4-vector resulting from clustering all jet constituents (including soft radiation) using the anti- $k_T$ algorithm with the E recombination scheme.
Soft Drop (SD) Axis: The axis of the jet after removing soft, wide-angle radiation via the Soft Drop grooming procedure.
Winner-Takes-All (WTA) Axis: The axis determined by reclustering constituents with the WTA recombination scheme, which aligns with the hardest constituent and is insensitive to soft radiation.

The analysis compares the $\Delta R_{\text{axis}}$ distributions in Pb–Pb collisions against a reference from $pp$ collisions at $\sqrt{s} = 5.02$ TeV. To ensure precision, the measured distributions are unfolded to correct for detector effects and residual underlying event (UE) fluctuations using an iterative Bayesian procedure with a 4D response matrix. Systematic uncertainties are evaluated regarding tracking efficiency, unfolding models, UE subtraction, and combinatorial jets.

Key Results

Narrowing of the Distribution: The study observes a narrowing of the $\Delta R_{\text{axis}}$ distribution in Pb–Pb collisions relative to $pp$ collisions. This effect is seen for both ungroomed (Standard–WTA) and groomed (SD–WTA) jets.
Quark vs. Gluon Composition: The observed narrowing is quantitatively consistent with models suggesting that the surviving jet population in Pb–Pb collisions is enriched with quark-initiated jets relative to gluon-initiated jets. Since gluon-initiated jets couple more strongly to the medium and lose more energy, their suppression leads to a sample dominated by quark jets, which exhibit narrower angular structures.
Disfavoring Simple $p_T$ Broadening: The results disfavor models implementing intra-jet transverse momentum ( $p_T$ ) broadening via a simple BDMPS formalism with a single $\hat{q}$ value. Such models predict a broadening of the distribution that is not observed in the data.
Medium Resolution and Incoherence: The data show sensitivity to the medium resolution length ( $L_{\text{res}}$ ). The results favor mechanisms incorporating incoherent energy loss (where the medium resolves splittings immediately, $L_{\text{res}} = 0$ ) over fully coherent energy loss ( $L_{\text{res}} = \infty$ ).
Grooming Effects: Grooming improves the agreement between theoretical models and data, suggesting that ungroomed observables capture significant non-perturbative aspects of jet quenching. However, even with grooming, fully reproducing the data requires models that include medium response effects (e.g., the "recoils on" scenario in JEWEL).
Jet Radius Dependence: For high- $p_T$ jets ($100-140 $GeV/$ c$), jets with $R=0.4$ show their Standard axis more aligned with the WTA axis compared to $R=0.2$ jets. This alignment is further enhanced by grooming.

Significance and Claims
The paper claims to present the first measurement of the angle between different jet axes in heavy-ion collisions. Its primary significance lies in providing a new observable that is sensitive to the angular scale at which the QGP can resolve independent parton splittings.

The author asserts that these measurements:

Offer evidence that the narrowing of jet substructure in the medium is driven by a selection bias toward quark-initiated jets rather than simple intra-jet broadening.
Demonstrate that the $\Delta R_{\text{axis}}$ observable is a powerful tool for distinguishing between coherent and incoherent energy loss mechanisms.
Highlight the necessity of including medium response (recoiling partons) in theoretical models to accurately describe jet substructure in Pb–Pb collisions.
Suggest that future measurements using photon- or Z-boson-tagged jets could further isolate quark vs. gluon contributions to explore the relationship between the WTA axis and the initial parton direction.

The paper concludes that while perturbative QCD calculations agree with $pp$ data, the modification of jet axes in the QGP requires a nuanced understanding of medium-induced effects, specifically the differential energy loss of quark and gluon jets and the resolution properties of the plasma.

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