Jets: manifestations of quarks and gluons

Quarks, gluons and antiquarks are components of protons, neutrons, and (by definition) other hadrons. An amazing physical property of our world is that when one of these particles knocks out the hadron containing it, and flies with high energy of motion, it remains unobservable macroscopically. Instead, the high-energy quark (or gluon, or antiquark) turns into a “splash” of hadrons (particles consisting of quarks, antiquarks, and gluons). These sprays are called "jet". Note that this is true for the five lightest quark colors, but not for the upper quark, which decays into a W-particle and a lower quark before a jet can appear.

In the article, I will describe approximately how and why jets appear from high-energy quarks, antiquarks and gluons.

This behavior of quarks, different from the behavior of charged leptons, neutrinos, photons, and others, comes from the fact that quarks and gluons are subject to strong nuclear interaction, while other particles are not subject to it. Most interactions between two particles become weaker with increasing distance. For example, the gravitational interaction between two planets decreases inversely with the square of the distance between them. The same is done for the electrical interaction between two charged objects; it also falls as the square of the distance. You can rub the balloon yourself, charging it with static electricity, and then bring it to your head. If you bring it closer, your hair will stand on end, but this effect quickly disappears if you move the ball further.

Strong nuclear interaction, although it grows at short distances and decreases at large (although not as fast as electricity, this property is important for understanding the history of strong interactions), however, it ceases to decrease at distances of the order of one millionth of a billionth of a meter - of the proton radius , which is 100,000 times smaller than the radius of an atom. And this is not an accident - this effect actually determines the size of the proton. This interaction generated by the gluon field becomes constant. And this means that if you try to pull the quark out of a proton, as in fig. 1, you will find that dragging it does not get easier as you move it further and further away. The feeling is roughly comparable to the stretching of a rubber band. Except that this rubber band will tear at some point. Once the tape has accumulated a lot of energy, nature will prefer to tear it in two, rather than letting you pull on. And when it breaks, instead of one hadron (proton) you will have two: a proton or a neutron plus (usually) a pion. At the moment of rupture, a quark / antiquark pair is formed in a certain way — the energy in the form of a belt tension is converted into the energy of the quark and antiquark masses, plus some additional gluons into the determined energy of motion. Energy is saved: you started with proton mass energy, added proton tensile energy, and received mass energy of two hadrons (without any stretching). The electric charge is also conserved, so you get either a neutral pion and a proton, or a positively charged pion and a neutron.

Fig. 1: if you try to pull a quark out of a proton with the help of magic tweezers, the proton will first distort and then break into two hadrons. Your attempt to release the quark will fail, and the energy expended will turn into the energy of the mass of the second hadron.

What happens when a high-energy quark is knocked out of a proton? For example, a fast-moving electron crashes into a proton, hits the quark hard, giving it a much higher driving energy than the mass energy of the entire proton?

Roughly speaking - I will say to the experts that this statement will be partially naive and a little confusing, but later I will correct it - about the same thing as shown in fig. 1, but on a larger scale. Quark moves so fast that the emerging rubber tape does not break and is stretched too much - see the middle of the figure. 2. As a result, instead of bursting in one place and forming two hadrons, it breaks in many places and forms many hadrons (mostly pions and kaons (similar to peonies, but contain a strange quark or antiquark) and this-mesons, or, less commonly, protons, neutrons, antiprotons, or antineutrons). All of them will go more or less in one direction. As a result, we will have hadron splashes, most of which will fly in the direction of the original quark. So much for the jet.

Fig. 2

The original energy of the high-energy quark is now divided between the hadrons in the jet. But for quarks of sufficiently high energies (10 GeV and more) a small fraction of the energy is involved in the formation of the mass energy of new hadrons; most of it goes into the energy of their movement. As a result, the total energy and direction of the jet are similar to the initial energy and direction of the quark. Measuring the energy and direction of motion of all hadrons of the jet, and determining the energy and direction of motion of the jet as a whole, specialists in particle physics get a good estimate of the energy and direction of motion of the initial quark.

The same is true for antiquarks, and, with a slight modification, for high-energy gluons.

I want to note that no one can calculate how this process takes place in detail. We know what I told you, as a result of a combination of decades of theoretical calculations, theoretical guesses and data - detailed data from different sources - which in general show that this story is approximately the same. And we have reason to be confident in it. Many of our high-precision tests of the theory of strong nuclear interaction would otherwise fail.

Note: this high-energy object is called high-energy physics by a QCD string (QCD, or quantum chromodynamics — these are equations that describe strong nuclear interactions). Historically, trying to understand the behavior of hadrons in nature observed by us (before physicists invented QCD and discovered gluons, and when they did not understand so well in quarks), theorists invented string theory in the late 1960s. Only later did it become clear that the string in this early string theory was a real thing, part of physics. And even later it became clear that QCD strings cannot be adequately described using standard string theory. For a time, this was considered a failure, until Scherk and Schwartz indicated that string theory could be better suited to describe quantum gravity (and probably all fundamental particles). And string theory experts set off in a different direction. And recently it became clear how you can do something unexpected with the help of standard string theory, so that it describes the QCD strings better (not perfect, but much better). Unfortunately, she still disgustingly describes the jet.

Obviously, there is still a lot to say about strong nuclear interactions.

Fig. 3

Now let me correct the inaccuracy that is allowed in fig. 2. I lowered the key stage. A hit quark, like any accelerated particle, will radiate. Suddenly, an accelerated electron will emit photons; suddenly an accelerated quark will emit gluons (and photons too, but they are much smaller). This is shown in the upper right of Fig. 3. Therefore, in fact, at the edge of the proton there appears not a fast quark (Fig. 3, left in the middle), but a set of fast gluons plus a fast quark. As a result, the process of forming a jet of hadrons (Fig. 3, below) is more complex than in Fig. 2, although the result is more or less the same. But the shape of the jet is actually determined by how gluons are emitted even before the quark leaves the proton. The process of emitting gluons by a quark can be counted! Therefore, using the equations for a strong nuclear interaction, one can calculate much more jet properties than it might seem on the basis of a naive figure. 2. These calculations are verified by data, with the result that equations have been tested to describe strong nuclear interactions.


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