What is a jet?

In particle collisions like the one shown above, it is common for debris to be grouped into clumps known as jets rather than uniformly distributed in a circle. This event has about 10 jets, but most have only two or three. Image courtesy of ATLAS

In these articles, I often get stuck when I need to describe jets. Jets are complicated yet ubiquitous in particle physics — they’re hard to avoid and hard to explain. In this article, I intend to give jets the space they deserve.

Whenever a single quark or gluon flies off on its own, it pulls new particles out of the vacuum and becomes a cloud of particles, flying in roughly the same direction. This is a jet. If physicists want to know how many quarks or gluons were emitted by an interaction, they have to disentangle the (sometimes overlapping) jets.

This phenomenon is unlike any other in particle physics. There are other processes that create particle-antiparticle pairs, but free quarks spontaneously generate more quarks until they are all are bound, either in pairs or triples. Isolated quarks cannot exist because the force between them doesn’t fall off to zero with distance. Gravity, electric forces and the weak force all become negligible as objects separate, but the strong force, which governs the behavior of quarks, increases as quarks separate, leveling off to a constant — approximately 14 tons.

When a quark is far enough from its neighbors, the energy becomes large enough to become the mass of a new quark-antiquark pair. (Distance times force is energy, and energy can be converted into mass.) This is why an isolated quark doesn’t stay isolated: It costs less energy to create more quarks than to be alone. For a highly energetic quark or gluon flying away from a collision, this process happens several times, resulting in a jet.

Although jets are interesting on their own, they’re present in any interaction that produces quarks or gluons. Often, they act as a smokescreen, hiding information about the primary interaction. For instance, more than half of Higgs bosons decay to a b quark and a b antiquark, but they appear in a particle detector as dozens of particles in two rough bundles. It’s hard to tell exactly which particles came from each quark and which are from other debris. This ambiguity is a source of uncertainty in the energy of the original b quarks — so much so that the Higgs mass peak has never been observed in this decay mode, despite it being the most frequent.

To combat the uncertainties due to jets, physicists have developed sophisticated jet finding algorithms. These algorithms are related to clustering, a machine learning technique that lets computers discover patterns on their own, but jet finders are more highly specialized. The latest generation of algorithms peers inside the jet and identifies individual particles (particle flow algorithms) and even jets within jets (jet substructure algorithms). It is now possible to do precise experiments with jets, despite their apparent messiness.

Jim Pivarski

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