“Jets” is the name given to sprays of particles, headed in roughly the same direction, that appear when quarks or gluons collide. Jets turn out to be a useful way to relate experimental results on quarks and gluons with the theory of Quantum Chromodynamics (QCD) which describes the interactions of quarks and gluons. Their usefulness arises partly because the jets can be seen to emerge in a simple way. The primary particles involved in the scattering have a small probability to emit a new gluon which, it turns out, is most likely to head in the direction of the particle that emitted it. The new gluons have a small probability to emit further gluons, and so on. Iterating this process a few times gives you a jet of quarks and gluons. The gluon emission probability is small because the QCD interaction strength is fairly small in high-energy processes.
It is somewhat surprising that we can also see jets emerge in an entirely different way. It is known that QCD becomes much simpler if we imagine that the number of “colors” of quarks is a large number, N, rather than the small number, 3, that we find in our Universe. For large N, the allowed configurations have a flux-tube, or string, connecting every quark to an anti-quark (to make a meson) or have the strings of N quarks meeting at a point (to make a baryon). This “large N approximation” actually does a pretty good job of describing our world, leading to the oft repeated quasi-joke that 3 is a large number.
Of course such strings can break. This occurs when a quark and anti-quark are created at some point along the string. The energy required to produce the quark and the anti-quark can be offset by the broken string contracting. In this way we can imagine a heavy meson or baryon with a very long (excited) string decaying into lighter “daughter” mesons and baryons made of shorter strings.
Imagine producing a quark and an anti-quark in a high energy collision. The quark and the anti-quark would be flying apart in opposite directions with a string stretching between them. Starting with this very excited “meson,” we could simulate the repeated breaking of the string and see what comes out. This is a little tricky, the quarks and strings are moving in a complicated way due to all this breaking, but we were able to do it (mainly thanks to Matt Reece’s programming skills). The result is that the string tends to break into relatively short bits, which therefore have little rest mass (the mass grows like the string length) and thus lots of kinetic energy, since the total energy has to add up to the initial energy. Interestingly the string bits end up mostly going in the directions of the initial quark and anti-quark. This is because in the rest frame of one of the daughter mesons, the subsequent “grand-daughters” are equally likely to go in any direction, but in the rest frame of the lab, the daughter meson was moving rapidly in the direction of the original quark or anti-quark, and the grand-daughters are “thrown” forward in this direction. So we get something that looks very much like a jet. This is just what is shown in the picture at the top of this page.
This is very different from what happens in theories where the interaction strength and N are both large. Such theories can be approximately scale invariant in which case they are called conformal field theories (or CFT’s). CFT’s are thought to be described by almost non-interacting particles moving in a five dimensional anti-de Sitter (AdS) space. This is the basis of the AdS/CFT correspondence. It is fairly easy to work out what happens in this case, either using CFT methods or direct simulation in AdS. Each time an excited meson decays into two lighter mesons, most of the initial energy goes into the rest mass of the daughter particles, so they have very little kinetic energy. This means that there is very little difference between the rest frame of the daughter particle and the rest frame of the lab, so the grand-daughters are equally likely to go in any direction. The result of this type of process is shown below.
This raises some interesting prospects for the large hadron collider. First we need fairly precise estimates of standard QCD jets, especially those containing b quarks, so that we can separate out the “old” physics from the new physics, and the stringy picture may be helpful for improving these calculations. Second, in some scenarios the new physics does look like a CFT, in which case the standard types of analysis will not be helpful in teasing out the underlying information. In that case we will need some new ideas in order uncover the new physics.
(Technical note: in the top and bottom graphic, the length of each lines is proportional to the energy of the particle moving in that direction.)