QCD IN STRONG INTERACTION AND NUCLEAR PHYSICS
QCD, often known as quantum chromodynamics, is the current strong interaction theory. 1 Its origins can be traced back to nuclear physics and the explanation of common stuff, specifically the nature of protons and neutrons and their interactions. Today, the majority of what occurs at high-energy accelerators is described using QCD.
It was typical to refer to this process as "testing QCD" twenty or even fifteen years ago. The idea has been so successful that we no longer refer to "calculating QCD backgrounds" when looking into more speculative phenomena. For instance, without a clear, trustworthy understanding of the more prevalent processes governed by QCD, it would have been considerably more challenging and uncertain to identify the top quark or the heavy W and Z bosons that mediate the weak interaction. Search tactics for the Higgs particle and for super-symmetry manifestations depend on a thorough grasp of production mechanisms and backgrounds predicted using QCD with regard to items that are still undiscovered.
The theory of quantum chromodynamics is elegant and exact. The fact that the essence of QCD can be depicted in the few straightforward images at the bottom of the box on the following page without much distortion is one example of its elegance. But first, for the sake of comparison, let me remind you that the single image at the top of the box, which represents the interaction vertex at which a photon reacts to the presence or motion of electric charge, can be used to depict the essence of quantum electrodynamics (QED), which is a generation older than QCD. 2 It's not simply a metaphor, either. The Feynman diagrams of QED, built by joining such such graphs, have very specific and exact algorithms for calculating physical processes related to them and vertices of interaction.
QCD appears as an enlarged version of QED in the same graphical language. In contrast to QED, which only has one type of charge, QCD has three different types of charge, each identified by a different colour. We might decide on the colours red, green, and blue to avoid chauvinism. However, it goes without saying that the colour charges in QCD have nothing to do with actual colours.
Instead, they possess characteristics similar to electric charge.
The colour charges are particularly conserved in all physical processes, and there are massless particles that resemble photons called colour gluons that react appropriately to the presence or mobility of colour charge in a manner very similar to how photons react to electric charge.
Quarks and gluons
The quarks are one type of particle that has a colour charge. We are aware of six distinct quark types, or "flavours," which are designated by the letters u, d, s, c, b, and t, respectively, for up, down, strange, charmed, bottom, and top. Only the u and d quarks have a substantial role in the composition of ordinary stuff. All of the additional, considerably heavier quarks are unstable. A unit of any one of the three colour charges can be carried by a quark of any one of the six flavours. Despite the fact that each quark flavour has a unique mass, the theory is perfectly symmetrical when it comes to the three hues. The Lie group SU describes this colour symmetry (3).
Like electrons, quarks are point particles with a spin of 1/2. However, they contain colour charge rather than electric energy. To be more precise, quarks carry fractional elec- tric charge in addition to their colour charge (+ 2e/3 for the u, c, and t quarks and - e/3 for the d, s, and b quarks).
Despite their many similarities, QCD and QED differ significantly in a few key ways. First of all, the QCD coupling constant shows that the response of gluons to colour charge is substantially stronger than the response of photons to electric charge. Second, as depicted in the box, gluons can convert one colour charge into another in addition to simply responding to colour charge. All such adjustments are permitted while maintaining the colour charge. Gluons must therefore be able to carry imbalanced colour charges themselves. For instance, if a blue quark is transformed into a red quark by the absorption of a gluon, the gluon itself must have carried one unit of red charge and minus one unit of blue charge.
It would appear that this would require 3 x 3 = 9 different colour gluons. The color-SU(3) singlet, a special set of gluons that reacts to all charges equally, stands out from the others. If we want a theory that is perfectly color-symmetric, we must get rid of it. Only 8 physical gluon states (creating a color-SU(3) octet) are then remaining. Fortunately, an experiment supports this conclusion!
The most significant distinction between QCD and QED, the third, comes after the second. It follows that gluons, unlike photons, respond directly to one another because they carry unbalanced colour charge and respond to the existence and motion of colour charge. Naturally, photons have no electrical charge. As a result, the Star Wars-style laser sword battles would not be possible. But since the movie is set in the future, it's possible that colour gluon lasers are being used.
In terms of its fundamental equations, we can display QCD even more succinctly (the below picture). That shouldn't necessarily make you feel impressed. After all, Richard Feynman demonstrated that the universe's equation might be expressed in a single line: U, the entire unworldliness, is a discrete function, and U = 0. It is the result of the combined effects of all physical laws:
As a result, one single, comprehensive equation can encompass both the physical principles that are known and those that are still to be found. The only usable procedure for unpacking U is to return to its constituent components, hence it is obviously a complete cheat. Feynman's sarcastic unification is quite dissimilar from the equations for QCD shown in the above picture. Their entire content is displayed up front, and the procedures used to unpack them come directly from the clear mathematics of symmetry.
The above picture illustrates a noteworthy characteristic of QCD. It demonstrates how few movable parameters the theory requires. There only has six quark-mass and one overall coupling constant, g. for the six quark flavour parameters, mj. Considering that coupling strength is a relative notion, there there are numerous instances where the mass parameters are not important As an illustration, the heavier quarks only a minor part in the composition of common stuff. Hence, QCD meets the theoretical ideal as closely as possible: conceptual components, it creates an abundance of physical effects that accurately reflect nature.