In particle physics, what is a jet?
In: Physics
~50 years ago we learned that protons and neutrons are made up of smaller constituents called “quarks” and “gluons”.
However, these constituents have a weird property that they cannot exist on their own.
As a consequence, if you separate two quarks (which are together stable), you will create two new quarks in the distance between the two, resulting in a total of four quarks.
Imagine blowing air into a large bubble which breaks into two smaller bubbles at some point instead.
If this process is repeated, more and more quarks are created in a cascade.
You can do this by colliding two protons (as is done at LHC). Through this if a quark is pushed out of the proton, new quarks are created along the direction of the quark that was pushed out. This creates a cascade as described above which is called a jet.
A jet is basically made up of a “spray” of quarks. Quarks are the particles that make up protons and neutrons. Quarks have a special property called “confinement”. It means you only ever find quarks in pairs, or triples, or more exotic larger combinations, but never alone. The reason their never alone is that if you try to pull two quarks apart you have to provide enough energy to produce two more quarks. Those two new quarks will pop into existence and be paired up with the quarks you were pulling apart.
[This comic demostrates the basic idea](https://qph.fs.quoracdn.net/main-qimg-5f56c16ebb060ddd52eaf60dd456b527-c).
For high energy particle physics experiments a quark can get hit really hard and it’ll be given enough energy to repeat that process many/several times. So a quark (in a proton say) gets hit and ejected from the proton, it becomes a quark pair, then becomes 2 pairs, then 4, then 8 etc. The result is basically a big spray of quark pairs, all of which are going in basically the same direction, that’s a jet.
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