Simultaneous measurements of -subjettiness observables in jets from gluons and light-flavour quarks, and in decays of boosted W bosons and top quarks
Using 138 fb of proton-proton collision data at 13 TeV collected by the CMS experiment, this paper presents a simultaneous particle-level measurement of 25 -subjettiness observables across jets from gluons, light-flavour quarks, and boosted W bosons and top quarks to provide a detailed characterization of jet substructure for refining radiation modeling.
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 invisible world of subatomic particles, matter is not solid but a chaotic storm of energy and fleeting fragments. When protons smash together at nearly the speed of light, they shatter into a spray of new particles that fly outward in tight, focused beams. Physicists call these beams "jets." They are not single objects but rather cascades of hundreds of smaller particles, all born from the breakup of a single, high-energy piece of matter called a parton. Just as a firework shell explodes into a specific pattern of sparks, the way these jets spread out and break apart holds a secret code. The pattern of the explosion tells scientists exactly what kind of particle started the chain reaction. A jet born from a gluon looks different from one born from a quark, and both look distinct from a jet created by the decay of a heavy, unstable particle like a top quark. Understanding these patterns is crucial because it allows researchers to separate the ordinary debris of the universe from the rare, heavy particles that might reveal new laws of physics.
For decades, scientists have tried to read these patterns by looking at specific features, such as how many distinct clumps of energy appear inside a jet. A jet from a heavy particle often splits into two or three main branches, while a jet from a lighter particle usually has just one. However, the story inside a jet is far more complex than a simple count of branches. There are subtle whispers of energy, faint sprays of particles, and intricate angles between them that standard measurements often miss. To truly understand the architecture of these jets, researchers need a way to map every single detail of the radiation, not just the most obvious parts. This is the challenge that a team of scientists at the Large Hadron Collider in Switzerland recently tackled, aiming to create the most detailed map of jet structure ever produced.
The researchers, working with the CMS detector, analyzed data corresponding to an integrated luminosity of 138 fb⁻¹ recorded between 2016 and 2018. They focused on three specific types of jets: those coming from the common debris of the collision, those from the decay of a W boson (a particle that carries the weak nuclear force), and those from the decay of a top quark, the heaviest known elementary particle. Because the W bosons and top quarks in these collisions were moving so fast, their decay products were squashed together into single, large jets. The team wanted to measure 25 different characteristics of these jets simultaneously. These characteristics, known as N-subjettiness observables, act like a set of rulers measuring how well the energy inside a jet aligns with one, two, three, or more central axes. By measuring all of them at once, the team could reconstruct the full three-dimensional shape of the energy flow inside the jet, capturing details that previous studies had to ignore.
To make sense of the data, the team had to account for the fact that their detectors are not perfect. The instruments blur the sharp edges of the particle showers and sometimes miss faint signals. They used a sophisticated mathematical process to "unfold" the data, effectively peeling back the layers of detector noise to reveal what the particles looked like when they were first created. This process allowed them to present a clean, corrected picture of the jet substructure, free from the distortions of the machine itself. They also calculated how the different measurements relate to one another, ensuring that the final results could be used by other scientists to test and improve the computer simulations that model how particles interact.
The results revealed a clear limit to how much information can be extracted from a jet. The team found that for distinguishing a W boson jet from a standard quark jet, the most useful information is captured once the jet is resolved into four distinct parts. For the heavier top quark jets, the discrimination power saturates once five parts are resolved. Beyond these points, adding more detailed measurements does not significantly improve the ability to tell the particles apart. This suggests that the essential "fingerprint" of these heavy particles is contained within a relatively small number of emissions. However, the researchers went further and measured a complete set of 25 observables to ensure they captured every possible nuance, creating a robust dataset that overconstrains the physics of the jet. This means they have measured more than is strictly necessary to identify the particle, providing a safety net of redundant information that makes the results incredibly reliable.
When they compared their measurements to the predictions made by current computer models of particle physics, they found that the models were not perfect. While the simulations generally captured the broad shapes of the jets, there were consistent differences in the details. For the common jets from quarks and gluons, the models tended to predict slightly different energy patterns than what was observed in the real data. For the jets from W bosons and top quarks, the discrepancies were even more pronounced, particularly in how the energy was distributed among the different branches of the jet. The study showed that no single computer model could perfectly describe all the different types of jets at the same time. Some models worked better for the common jets, while others were closer to the truth for the heavy particles, but none were universally accurate.
This work does not claim to have solved the mystery of jet formation, but it provides a much sharper tool for doing so. By providing a simultaneous measurement of 25 different observables, along with a detailed map of how they correlate with each other, the researchers have handed the physics community a comprehensive set of instructions for refining their simulations. The data shows that the current understanding of how particles shower and break apart is incomplete, especially when it comes to the complex interplay of forces inside a high-energy jet. The findings serve as a rigorous test for future theories, forcing modelers to adjust their calculations to match the precise, unfolded reality of the data. In doing so, this study lays a firmer foundation for future searches, ensuring that when scientists look for new, rare particles in the future, they will be comparing them against a much more accurate picture of the ordinary background.
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