# Exploring the Mysterious 8 Gluons

• Allojubrious
In summary, the reason why there are only 8 gluons is because the adjoint of SU(3), the gauge group corresponding to color, is 8-dimensional. The gluon field transforms as the adjoint of the gauge group, resulting in 8 independent types of gluons. The ninth gluon, if it existed, would be a color singlet and would not participate in the nonlinear color interaction that causes confinement. Mathematically, this can be understood by looking at the SU(3) generators and realizing that going from SU(3) to U(3) is equivalent to U(1)*SU(3), with the additional generator being the identity matrix. This means that the ninth gluon would be equivalent
Allojubrious
8 Gluons...?

I am studying quarks again, and the gluons that are exchanged, and I really want to learn about WHY ARE THERE ONLY 8 GLUONS? This has been puzzling me so I was wondering if somebody could help me!

Thanks.

The adjoint of SU(3), the gauge group corresponding to color, is 8 (=3^2-1)-dimensional. The gluon field (or any connection, for that matter) transforms as the adjoint of the gauge group. Therefore, there are 8 independent types of gluons that one could imagine exchanging.

The ninth gluon, if it existed, would be a color singlet. As a colorless particle it would not participate in the nonlinear color interaction that causes confinement of the other gluons and the quarks. It would therefore be a free massless particle with long-range interaction. Such a particle would easily be observed, and has not been.

One can understand this mathematically quite easily. You know the SU(3) generators, the so-called Gell-Mann lambda matrices. Now going from SU(3) to U(3) is the same as to U(1)*SU(3); so the additional generator is the generator of the U(1) factor. But this is nothing else but the identity matrix, i.e. 1. Of course 1 commutes with all other generators which is what Bill_K means when he is talking about a color singulet.

Thank you for the help!

Question: If I had a gauge theory U(3) is this equivalent to U(1)xSU(3)? In which case it would be QCD coupled to QED in the "9th gluon" would be the photon i.e. "a free massless particle with long-range interaction" ??

U(3) = U(1) * SU(3) is correct

## 1. What are gluons?

Gluons are subatomic particles that mediate the strong force, one of the four fundamental forces of nature. They are responsible for holding quarks together to form protons and neutrons, which make up the nucleus of an atom.

## 2. How many types of gluons are there?

There are 8 types of gluons, also known as the "eightfold way" or the "8-color state model". These include the red-antired, green-antigreen, and blue-antiblue gluons, as well as the combinations of these colors.

## 3. Can gluons exist independently?

No, gluons cannot exist independently and are always found within the nucleus of an atom. This is due to the strong force that binds them together with quarks.

## 4. How do scientists study gluons?

Scientists study gluons by using particle accelerators, which can create collisions between protons and other particles. By analyzing the particles produced in these collisions, scientists can gain insights into the behavior and properties of gluons.

## 5. What mysteries are still left to explore about gluons?

Despite significant progress in understanding gluons, there are still many mysteries that remain. Scientists are still trying to fully understand the nature of the strong force and the behavior of gluons under extreme conditions, such as in the early universe or in the presence of high energies.

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