Probing jet quenching via the correlation of groomed jet substructure observables in Pb$-$Pb and pp collisions
This ALICE collaboration study reports the first measurement of the correlation between the Soft Drop grooming splitting angle and momentum-sharing fraction in 5 TeV Pb-Pb and pp collisions, revealing that jets in heavy-ion collisions are narrower and potentially less balanced at large opening angles, a phenomenon best described by models incorporating elastic scatterings like the HYBRID model.
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In the heart of matter, where temperatures soar beyond anything found in the stars, protons and neutrons melt away their individual identities. They dissolve into a seething, ultra-hot soup of their smallest constituents: quarks and gluons. This state of matter, known as the quark-gluon plasma, existed only fractions of a second after the Big Bang, but scientists can recreate it today by smashing heavy atomic nuclei together at nearly the speed of light. When these collisions occur, they do not just produce a uniform fireball; they also fire off narrow, high-speed sprays of particles called jets. These jets are like messengers from the very moment of impact, carrying information about the extreme conditions they were born into. As they race through the hot plasma, the particles inside the jets interact with the surrounding soup, losing energy and changing their shape in a process physicists call "jet quenching." By studying how these jets are altered, researchers hope to understand the fundamental rules governing how energy moves through this exotic state of matter.
A team of scientists using the ALICE detector at the Large Hadron Collider has taken a fresh look at this phenomenon by examining the internal structure of these jets with unprecedented precision. Instead of just measuring how much energy a jet loses, they focused on the very first time a particle inside the jet splits into two. They looked at two specific features of this split: the angle at which the two new paths diverge and how the momentum is shared between them. In simple terms, they asked whether the two new paths fly off at a wide angle or a narrow one, and whether they carry equal amounts of energy or if one is much stronger than the other. By comparing these splits in ordinary proton collisions with those in collisions of heavy lead nuclei, where the quark-gluon plasma is formed, the researchers could isolate exactly how the hot medium affects the way particles break apart.
The researchers analyzed millions of collision events, focusing on jets with a specific range of high energy. They used a sophisticated filtering technique to strip away the soft, wide-angle noise that often obscures the true signal, allowing them to see the hard, clean splits that define the jet's core. In the collisions of lead nuclei, they found that the jets were noticeably narrower than those in ordinary proton collisions. The particles inside the lead collisions tended to stay closer together, flying off at smaller angles. This narrowing effect was particularly strong for jets where the two split paths shared the energy equally. However, the story was different for the balance of energy itself. For jets that split at a very wide angle, there was a hint that the energy sharing became less equal in the lead collisions compared to the proton ones, suggesting that the wider jets might be losing energy in a more uneven way.
To understand what was causing these changes, the team compared their real-world data against several computer simulations that model how jets interact with the plasma. Some of these models predicted that the hot medium would act like a collective fluid, creating a wake behind the jet that would alter its structure. The data showed that the experiment was largely insensitive to this kind of fluid-like response, likely because the jets being studied were so narrow and the filtering so strict that these subtle effects were washed out. Instead, the results pointed toward a different mechanism: elastic scattering. This is a process where the particles in the jet bounce off the particles in the plasma, much like billiard balls colliding. The data suggested that including these bounces in the models was necessary to explain why the wide-angle jets in the lead collisions appeared less balanced in their energy sharing.
The findings offer a clearer picture of how different types of jets behave in extreme heat. While the narrowing of the jets was a clear and consistent observation across the board, the changes in how energy was shared depended heavily on the angle of the split. Narrow jets seemed to survive the journey through the plasma with their internal balance largely intact, while wider jets showed signs of being disturbed more significantly. This distinction helps scientists separate the effects of the medium from other factors, such as the simple fact that wider jets are more likely to lose so much energy that they disappear from the detector entirely. The study confirms that while the plasma does reshape the jets, the specific way it does so depends on the geometry of the jet itself. By peeling back the layers of these particle sprays, the researchers have provided a new, detailed view of the forces at play in the earliest moments of the universe, showing that even in a chaotic, super-hot soup, the rules of particle interaction leave a distinct and measurable signature.
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